Journal of Petroleum Geology, Vol. 32(2), April 2009, pp 129-156
129
BACH HO FIELD, A FRACTURED GRANITIC
BASEMENT RESERVOIR, CUU LONG BASIN,
OFFSHORE SE VIETNAM: A “BURIED-HILL” PLAY
Trinh Xuan Cuong* and J. K. Warren**
A combination of seismic, wireline, FMI and core data shows that Bach Ho field in the Cuu Long
Basin, offshore SE Vietnam, is an unusual “buried hill” reservoir.There is little or no production from
associated siliciclastic “grus” or granite wash, and the fractured reservoir matrix is largely made up
of unaltered acid igneous lithologies (mostly granites and granodiorites). A major NE-SW late
Oligocene reverse fault system cross-cuts the field, with about 2000 m of lateral displacement in
the highly productive Central Block. The associated fracture meshwork greatly enhances reservoir
quality. Transpressional wrench faulting in the late Oligocene in this part of the field emplaced a
block of brittle granitic rock on top of organic-rich Eocene – Oligocene mudstones, and facilitated
the early migration of hydrocarbons into the fracture network.
Structure, not erosion, set up the 1000 m column of liquids in the fractured granodiorites
which form the reservoir at Bach Ho. Faulted intervals with associated damage zones create an
enhanced secondary porosity system in the granodiorite; effective porosities range from 3-5% and
occasionally up to 20%. Some associated fractures are partially blocked by authigenic calcite and
kaolinite.
Features that degrade reservoir quality at Bach Ho include: (i) a thin, low-permeability clayplugged “rind” created by surface-related (meteoric) Eocene – Oligocene weathering — this rind
variably overprints the uppermost 10-40 m of exposed basement throughout the Cuu Long Basin;
and (ii) widespread hydrothermal cements which largely predate late Oligocene wrench faulting;
cementation mostly took place during post-magmatic cooling and precipitated zeolites, carbonates
and silica in fractures which cut across both the igneous and the country rocks.
Porosity-occluding hydrothermal and authigenic precipitates developed in pre-existing fractures
in the Bach Ho granodiorite. These pre– late Oligocene mineral-filled fractures acted as zones of
structural weakness during and after subsequent late Oligocene structural deformation. Together
with new fractures formed during thrusting, the older fractures may have reopened during thrust
emplacement, and subsequent gravitational settling of, the Central Block.
INTRODUCTION
Vietnam has produced almost 1 billion brls of crude
oil and 300 billion cu. ft of natural gas. The country’s
*Exploration and Production Dept, Vietnam National
Oil Group, Petrovietnam, 22 Nyo Quyem St, Hanoi,
Vietnam.
** Shell Chair, Sultan Qaboos University, Muscat,
Sultanate of Oman. previously: Dept. Petroleum
Geoscience, Universiti Brunei. Present address:
Chulalongkorn University, Bangkok, Thailand. author for
correspondence: [email protected]
estimated resources, both on- and offshore, total some
6.5-8.5 billion brls of oil and 75-100 trillion cu. ft of
gas (AAPG Explorer, February 2005). Most of the
production in the Cuu Long Basin, offshore SE
Vietnam, has comes from fractured and weathered
granitic basement reservoirs at fields including Bach
Ho (White Tiger) (the largest field), Rang Dong
(Aurora), Rong Tay (Dragon West) and Hong Ngoc
(Ruby). Use of the term “basement” in this paper
Key words: Bach Ho, Cuu Long Basin, Vietnam, fractured
basement, buried hill, basement high, Vung Tau, granite.
© 2009 The Authors. Journal compilation © 2009 Scientific Press Ltd
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
130
Re
d
Ri
ve
N
rF
atu
l
ng
Ho
ng sni
S o Ba
Basin boundary
Major fault
reading ridge
Extensional crustal margin
Do
0
200
ng
Cuu Long
Basin
Fa
lt
ng
au
Pi
iF
ae
Na
M
km
ut l
Ho Chi
Minh City
3P
Vung Tau High
on
nS
o
C
ag
od
Nam Con Son
Basin
as
Fa
ut l
Fig. 1. Generalized map of the major Cenozoic sedimentary basins and current tectonic elements in the
South China (East Vietnam) Sea.
follows that of Landes (1960) who defined it as any
combination of metamorphic or igneous rocks
(regardless of age), which are unconformably overlain
by a sedimentary sequence. North (1990; p. 216) gave
a broader definition of “basement” that includes rocks
with a sedimentary origin, providing they have
essentially little or no matrix porosity.
The discovery well for Bach Ho was drilled in 1975
by Mobil Oil Co. but the company left Vietnam in the
same year. The Bach Ho “buried-hill” was not
developed until the mid-1980s by Vietsovpetro. Today,
the field’s average daily production (about 140,000
b/d) is declining; in 2008 it was about 25,000 b/d less
than in the previous year (www.upstreamonline.com/
live/article169696.ece). This rate of decline is
somewhat higher than that predicted by Vietsovpetro,
which envisaged an annual decline of about 20,000
b/d for the period 2010-2014, but it can perhaps be
expected in a field producing from an open fracture
system with little matrix storage.
At Bach Ho, matrix between hydrocarbon-filled
fractures is largely composed of unaltered igneous and
metamorphic rocks. Some production wells in
fractured granodiorite in the Central Block of the field
produce as much as 15,000 b/d, although rates of
2000-5000 b/d are more typical (Cuong, 2000; Dien
et al., 1998). Matrix storage in the reservoir is low to
non-existent, yet near-constant production rates have
been maintained for more than five years, implying
significant recharge to the fracture network. To date,
the fine-grained Tertiary fluvio-deltaic sedimentary
rocks which encase the productive basement block
have not produced sufficient volumes of hydrocarbons
to make them a target.
Not all basement highs in the Cuu Long Basin have
such outstanding reservoir and production
characteristics. A contrast to the success of Bach Ho
is given by the development and production history
of Dai Hung (Big Bear) field, where hydrocarbons
are hosted in a similar granite/granodiorite matrix. In
1993, BHP led a consortium that won the bid to
develop Dai Hung and predicted that the field could
produce up to 250,000 b/d. By the mid-1990s, some
25,000 b/d were being produced but then output
declined rapidly and BHP left the consortium in 1997
announcing that the field was not profitable. Petronas
Trinh Xuan Cuong and J. K. Warren
107° 20’
107° 40’
131
108° 20’
108° 00’
Bach Ho
Basement
2
1.2
Isodepth (km)
1.5
1
3
Fault
15G-1x
0.5
4
1.5
10° 20’
107° 00’
2
0.5
3
1
3.5
1.5
2
1.5
3
2
2
3.5
3.2
4
1.5
5
3
2
2.5
2
1
4
3
2.5
3
3
5.5
8
3
TD
7.2
4
2.5
3
5
2
3
TD-1X
4.5 4
3.5
5
5.5
6
3
2
1.5
2
3
4
3
2
1.5
1.2
0.5
R-4
2
3
BD-1X
7
R-3
1
4.5 4.5
0.5
2
4.3
3.2
0
1.5
DD-1X
15C-1x
6
78
4
4
DD
3
3
5.5
BH-4
6.3
5
2.5
2.5
4
3.5
3.2
2
BH-1
6.2 6
3
9° 20’
6
4 3
5
6
BH-10
BH-12
6.5
BD
4
4
3
BV
6
6
5.5
5
6
7.5
8
7 15B-1x
5
5
2
2.5
4
4
4
3
2
7
3
5
6
6
4.5
2.5
2
3
2.5
3.5 4
5
3.2
15A-1x
1
4
6
3
1
10° 00’
4 5
6
BH-4
15C-1X
km
25
15G-1X
B.
0
2000
4000
6000
Middle-Late Miocene
Early Miocene
Eocene
Depth (m)
Well
9° 40’
A.
2
2
1.5
Fig. 2. Basement distribution in the Bach Ho region, Cuu Long Basin. (A) Top of basement structure map;
rectangle indicates position of the Bach Ho oilfield and the various shaded areas indicate basement highs.
(B) Regional geological section showing variation in basement depth along line indicated by dashed line in A.
(after Que, 1994).
then took over as operator but failed to raise output
beyond 10,000-15,000 b/d and left in 1999. In 2000,
Vietsovpetro (the operator of Bach Ho and Rong
fields) took over Dai Hung but was only able to
produce some 5400 b/d.
There is therefore a need to investigate the geology
of Bach Ho field and to understand why successful
economic production comes from this unusual
reservoir. Previous studies of the granitic basement
reservoirs in the Cuu Long Basin include papers by
Areshev et al. (1992), Dmitriyevskiy et al. (1993),
Tran et al. (1994), Koen (1995), Tuan et al. (1996),
Dong (1996), Dong et al. (1996, 1997, 1999), Dong
and Kireev (1998), Quy et al. (1997), Tandom et al.
(1997), Tjiia et al. (1998), and Vinh (1999). However,
these studies are based on relatively old data or
focused on particular areas of research. In this paper,
we attempt to integrate available data in order to
compile a predictive exploration model for the Bach
Ho type of “buried-hill” play. We attempt to evaluate
fracture distribution in the basement reservoir and to
assess factors influencing reservoir quality.
REGIONAL STRUCTURE AND
STRATIGRAPHY OF BACH HO FIELD
The Cuu Long Basin is filled with 6 to 8 km of
sedimentary rocks which overlie pre-Tertiary
basement. The sedimentary fill rests on an undulating
basement surface which divides the basin into a series
of sub-basins trending parallel to the main basin axis
(Fig. 2) (Hall and Morley, 2004; Lee et al., 2001).
The pre-Tertiary basement is mostly composed of
Upper Triassic – Cretaceous plutonic rocks whose
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
EARLY
BACH
HO
LATE
23.3
TRA
TAN
29.3
35.4
EOCENE
TRA
CU
BASEMENT
COMPLEX
56.5
600 -
500 600
Mainly shallow marine with a strong fluvial
influence in its lower part. Mostly sandstones,
interbedded with grey, green or brown
mudstones. It is absent in the west of the
basin. Its upper part (the Rotalia Bed) is
composed of green mudstones.
800 -
Predominantly lacustrine sediments, mostly
in the centre and the east of the basin.
Sediments are mostly black mudstones
intercalated with grey-brown, fine to
medium-grained sandstones. In the west,
where fluvial deposition dominated,
sediments show a higher percentage of sand
and coarser grain sizes. Volcaniclastics are
also common in this unit.
200 -
Only in the western part,dominated by interbedded conglomerates and medium-grained
sandstones, minor mudstones.
B3
900
1400
B2
B1
C
2700
REACTIVE
Seal
Thermal sag
CON
SON
16.3
EARLY
B4
Postrift tr ough
10.4
Coastal environment to the west, river-mouth
and shallow marine sedimentation in the basin
center and to the east. Mostly fine to
medium-grained sandstones, intercalated with
grey siltstones and mudstones.
Coastal setting with fluvial dominance. Mostly
in the basin centre and in east where coarse
and medium-gained sandstones alternate with
grey mudstones and several interbeds of
lignite.
Structure Tectonics Source Reserrock
voir
RIFTED SEDIMENTARY BASIN
LATE
DONG
NAI
MIDDLE
MIOCENE
5.2
OLIGOCENE
Lacustrine
700
PLIOCENE
LATE
MESOZOIC
400 -
Open
marine
Rifting pull apart
BIEN
DONG
Fine-grained marine sands interbedded with
siltstones
Thick Seis.
(m) Hor.
DIVERGENCE
1.64
Lithology
100 300
Mostly hydrothermally-altered Late
Cretaceous granites, lesser Late Triassic &
Late Jurassic intrusives (basic and weakly
acidic). Together with the generation of the
Late Cretaceous granite, some andesitic and
dacitic dykes were emplaced with patterns
controlled by conjugate faults and fractures.
Country rocks were metamorphosed by both
mechanical and thermal processes especially
during Late Cretaceous.
PRERIFT
CONVERGENCE
OF CONTINENTS
QUAT.
Description
Continent.
shelf cover
Formation
Synrift graben
Ma
CONTINENTAL
ENVIRONMENT
Period
Hydrocarbon
Associations
Palaeobathymetric
Change
Stratigraphy
Geologic Age
HETEROGENOUS
BASEMENT
132
Fig. 3. Cuu Long Basin stratigraphy (after Dien et al., 1998).
composition ranges form basic (diorite) to acidic
(granite) (Table 1; Dong and Kireev, 1998).
The shallowest part of the highly fractured “buriedhill” at Bach Ho lies at a depth of 3050 m (Fig. 2),
while the effective base for liquids production in the
field is typically shallower than 4650 m. However, the
deepest producing interval in the field is a fractured
zone at 5013 m, with a flow rate around 100 b/d at a
temperature of 162oC. Productive intervals in Bach Ho
wells are invariably fractured zones.
Production data indicates that Bach Ho field is made
up of a number of separate basement blocks with
differing hydrodynamic properties. For production
purposes, the field is divided into the Northern, Central
and Southern regions. Communication between
fractured basement and onlapping sandstones is not
seen in current production and pressure data.
The current geothermal gradient at Bach Ho field
increases with formation age. It is 2.2-2.5oC/100m in
Quaternary – middle Miocene sediments; 3.4oC/100m
in the lower Miocene and Oligocene; and 3.9oC/100m
in the basement. Temperatures in the basement at
depths of 3-4 km range from 120 to 165oC. It has been
observed that 90°C formation waters from Bach Ho
contain thermophilic sulphate-reducing archaea/
bacteria (Rozanova et al., 2001). Geothermal gradients
were locally higher in the Mesozoic and early Tertiary
during magmatism.
Tectonic evolution
The structural evolution of the extensional Cuu Long
Basin can be explained in terms of (Figs 1, 2): (i) the
propagation of the South China (East Vietnam) Sea
rift around 17-32 Ma (Taylor and Hayes, 1983;
Hutchison, 2004), and (ii) the collision of India and
Eurasia beginning in the Palaeogene, which drove
the extrusion of Indochina along the Mae Ping and
Red River Faults (Tapponier et al., 1982, 1986),
although more recent studies have shown that
collision-driven strike-slip extrusion, and its
influence on basement tectonics throughout
Indochina (including offshore Vietnam) has probably
been over-emphasized (Morley, 2004; Hall and
Morley, 2004; Searle, 2006). Local variations in
tectonic activity have had a significant influence on
basement structure in the Cuu Long and Nam Con
Son Basins, resulting in subsidence and uplift with
different intensities and styles at various times since
the Cretaceous. Hall and Morley (2204.) concluded
that the basement “grain” in northern Sundaland
(including offshore Vietnam) is not related to the
Himalayan collision but is due to localised collisional
Trinh Xuan Cuong and J. K. Warren
133
Fig. 4. Character of Bach Ho basement.
(A, above). Classification of acid igneous rocks in the
basement of well BH-12 (see Fig. 6 for well location).
Open circles indicate core samples, solid circles
indicates side-wall core samples.
(B, right). Cores of typical fractured granodiorite
basement. Sub-vertical tectonic fractures (0.1-1.5
cm wide) are filled with zeolites, silica and calcite.
There is residual oil in some of the fractures
(appears brown and is indicated by red arrow).
Drilling-induced fractures (light blue arrow) are also
present. Scale bars are 5 cm long. These cores come
from deeper intervals with few open fractures,
hence the higher recovery efficiency.
events which preceded the main India–Asia collision.
These smaller-scale events included left-lateral
movements on the Mae Ping and Three Pagodas
Faults. Initial movement on some of the major faults
which are still active at the present day therefore took
place in the Cretaceous, and not all movements were
driven by the mid- to late Tertiary uplift of the Tibetan
Plateau.
The Cuu Long Basin is located along-strike from
the youngest spreading centre in the South China (East
Vietnam) Sea. Since spreading began at around 32
Ma, its dominantly SW propagation has influenced
structural grain in the Cuu Long Basin basement (Fig.
2; Hutchison 2004). The regional structural grain is
NW–SE to north-south (Hall and Morley, 2004;
Morley, 2004). Extension-derived stresses from this
spreading centre may still result in fault movements
in the Cuu Long Basin (Shi et al., 2003).
Stratigraphic evolution of the Bach Ho cover
The sedimentary succession at Bach Ho comprises
Eocene, Oligocene and lower Miocene deposits (Fig.
3). Sands in the mud-dominated Eocene and lower
Oligocene succession are mostly immature arkoses,
with lesser lithic-arkoses and some feldspathic
litharenites deposited in alluvial/fluvial and lacustrine
environments. Upper Oligocene sediments are finegrained lacustrine muds, while the overlying lower
Miocene succession passes from muddy delta-front
deposits up into marine shales (Que, 1994).
Sandstones in the lower Oligocene and lower Miocene
successions are possible secondary reservoir targets
at Bach Ho. Late Oligocene lacustrine and lower
Miocene marine shales constitute regional seals. Oils
throughout the Cuu Long Basin are sourced from
lacustrine muds (ten Haven, 1996; Reid, 1997). Source
rocks in Bach Ho are present in the Eocene and
Oligocene intervals (Canh et al., 1994; Areshev et al.,
1992), with the upper Oligocene being the most
prolific (Hung et al., 2003). None of the hydrocarbons
offshore Vietnam are derived from a deep mantle
source, contrary to a suggestion which was made
during early field development studies (Dmitriyevskiy
et al., 1993).
134
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Table 1. Primary minerals, composition and K-Ar age ranges in igneous rocks that constitute the reservoir in
Bach Ho field, offshore Vietnam (After Dong and Kireev, 1998). See Fig. 6 for the location of the Central and
Northern blocks.
Central
Block
Ca-Na
complex
(108-115
Ma)
Granite
Centre of Northern Block
Eastern part of the
Northern Block
Dinh Quan complex
Hon Khoai complex
(135-154 Ma)
(194-261 Ma)
Biotite
Quartz
Quartz
Biotite quartz
Granodiorite
quartz
biotite
biotite
monzonite
monzonite monzonite
monzonite
Plagioclase
25-40
40-54
47-58
30-40
43-52
62-68
K-feldspar
20-40
14-28
17-28
30-45
Oct-27
03-Dec
Quartz
25-35
18-24
Dec-17
May-18
May-17
<5
Biotite
02-Oct
06-Oct
05-Oct
Oct-13
May-13
13-15
Muscovite
0-3
-
-
-
-
-
Hornblende
-
0-3
0-2
0-2
0-2
-
Hastingsite
-
-
-
Jun-18
Jun-18
05-Oct
Dominant type
of plagioclase
Oligoclase
Oligoclase
andesine
Oligoclase
andesine
Oligoclase
andesine
Oligoclase
andesine
Andesine
Igneous and hydrothermal evolution
Widespread Mesozoic plutonism on- and offshore SE
Vietnam was dominated by Jurassic – Late Cretaceous
granites and granodiorites (Hall and Morley, 2004).
At a regional scale, plutonism was driven by the
NWward subduction of the Proto-Pacific Plate beneath
East Asia. Lateral forces associated with the northward
movement of India gave rise to stress changes within
Indo-china which resulted in crustal thickening and
pluton emplacement in Vietnam and Thailand (Morley,
2004; Hutchinson, 2004).
At Bach Ho, three phases of igneous intrusion are
recognised (Late Triassic, Late Jurassic and Late
Cretaceous) and correspond to the Hon Khoai, Dinh
Quan and Ca Na complexes, respectively (Table 1)
(Areshev et al. 1992; Dong and Kireev, 1998; Morley,
2004). Late Triassic and Late Jurassic intrusives consist
of basic and weakly acidic rocks. These are a minor
component of the Bach Ho basement and occur mainly
in the northern part of the field. Elsewhere, the
basement is composed of Late Cretaceous acidic rocks
(mostly granitic) which are characterised by their brittle
behaviour under stress (Fig. 4a). Andesitic and dacitic
dykes were emplaced with orientations controlled by
conjugate faults and fractures. Thermal haloes
associated with each phase of magma emplacement
altered country rocks into greenschist meta-sediments.
Cements precipitated during associated hydrothermal
circulation tended to occlude any porosity in the
granites and adjacent Mesozoic sediments (Fig. 4b).
During subsequent episodes of tectonism, regions of
hydrothermally-cemented and fractured zones tended
to be weaker than adjacent zones dominated by
unaltered and unfractured granites.
Bach Ho Basement – seismic character
Seismic responses can be used to divide the Bach Ho
basement into three depth-related seismic zones (Fig.
5, line positions shown in Fig. 6; see also the
schematic diagram in Fig. 16). Interval velocities in
the upper, middle and lower zones typically range
from 3500 to 3800 m/s, 3900 to 5200 m/s and 5000
to 5500m/s, respectively.
The upper zone is characterized by high continuity
of the basement-top reflector immediately below the
purple reflector in all three lines in Fig. 5. Its character
is created by the sharp acoustic impedance contrast
between the weathered top of the basement, the
overlying fine-grained sediments and the underlying
fractured igneous basement. This upper zone
signature is best developed at the highest parts of the
Bach Ho structure and loses thickness and definition
away from the crest.
Below is a zone characterized by groups of lower
frequency, higher amplitude and lower continuity
signatures within a chaotic reflection background (the
approximate vertical extent of this zone is indicated
by the yellow arrows in Fig. 5a-c). Its presence in all
the Bach Ho seismic lines implies that there are
laterally-developed velocity contrasts between the
relatively brecciated and altered zones (or dykes) in
the upper part of the basement high and the
Fig. 5. Seismic profiles across Bach Ho field (see Fig. 6 for line locations).
Top of basement shows high continuity and amplitude in all three lines,
this interval is referred to as Seismic Zone 1. Below this, Seismic Zone 2
(yellow arrows) is made up of low frequency, high and low amplitude
reflections with a background of chaotic reflectors. Seismic Zone 3 is a
zone of free reflection. Note the low-angle reverse fault and its
associated alteration zone present in all three lines, which continues to
both north and south (see Fig. 6) Some high-angle basement faults are
also visible. Variations in reflection intensity within the basement block
indicate, in part, variations in acoustic impedance related to variations in
the intensity of alteration. A schematic interpretation based on lines 1
and 2 is shown in Fig. 16.
Trinh Xuan Cuong and J. K. Warren
135
136
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Fig. 6. Main fault and fracture system in Bach Ho field (after Cuong, 2000). The highest basement, shown by
yellow and green shading, is in the Central Block adjacent to the region with most obvious displacement on
the reverse fault (approx. 2000 m; see line 1 in Fig. 5). The Central Block shows the most intense deformation
and wells here have higher production capacities.
surrounding granitic rocks. There is a suggestion of
pervasive faulting in this interval (indicated by the
red lines in Fig. 5). but individual faults cannot be
located accurately due to the relatively low resolution
of the seismic data.
Seismic attribute contrasts in lines 1 and 2, and
comparison with outcrop analogues at Vung Tau (see
below), imply that the upper part of the Bach Ho high
is more altered and brecciated compared to the
underlying zones. Cores and sidewall cores show the
lower part of the high are more weakly altered and,
although fractures and breccias are still common, they
are typically occluded and cemented by hydrothermal
minerals (Fig. 4b). An underlying third zone (beneath
the yellow arrows in Fig. 5a-c) is characterized by
chaotic reflections and is thought to possess few open
fractures; it is not considered to be economic.
Seismic, image logs and core data can be used to
divide the Bach Ho basement into three blocks (North,
Central, and South), each bounded NE-SW fault
segments (Fig. 6). The reverse fault to NW of the
Central Block locally has a lateral displacement of up
to 2000 m (Dong, 1996). The “chaotic” seismic zone,
described above as indicating a high degree of
fracturing and deformation, is particularly associated
with this reverse fault (see Figs 5a, b). The reverse
fault dies out along strike and passes into a linked
normal fault to both NE and SW (Fig. 6). This
relationship implies that the reverse-faulted margin
of the Central Block is an inverted portion of a pre-
Trinh Xuan Cuong and J. K. Warren
existing normal fault. FMI analyses (see below) show
that fracture intensity is greatest in the Central Block.
Production wells in the Central Block consistently
show production rates as high as 4000-6000 b/d.
By contrast, the seismic profiles indicate the
presence of normal faults along the SE flank of the
field, dipping to the south and SE. Seismic character
of the basement in this part of the field is characterized
by lower amplitudes, and reservoir quality is lower
compared to the NW margin of the Central Block.
Most fault sets at Bach Ho, as interpreted from
seismic profiles, are confined to the basement and do
not appear to continue into the overlying sedimentary
cover; nor can they be resolved in deeper parts of the
basement (Fig. 5a-c). The lower transitional zones of
both the weathering profile and the faulted interval in
the basement are difficult to define on seismic profiles.
Comparison with the Vung Tau outcrops (see below)
suggest that the lower transition of the basement
reservoir zone follows an irregular boundary, probably
controlled by variations in fracture intensity in the
basement.
Seismic analyses indicates a significant association
between late Oligocene reverse faulting and the Bach
Ho Central Block. NW displacement of up to 2000 m
on this fault emplaced a block of igneous basement
on top of Tertiary mudstones. Subsequent gravitational
collapse of the block and stress relaxation served both
to open uncemented fractures within the block and to
reopen pre-existing zeolite- and kaolinite-filled
fractures. Intense fracturing of basement rocks in the
Central Block and enhanced reservoir characteristics
are reflected in the high well outputs.
THE VUNG TAU GRANITES:
ANALOGUES FOR BACH HO BASEMENT
Triassic-Cretaceous granites and granodiorites outcrop
on the coast near Vung Tau, about 100 km SE of Ho
Chi Minh City (Fig. 1). Weathered outliers can be
found up 0.5-1km away from the main outcrops which
extend for several km along the coast. A number of
granite quarries and road cuts were visited, mapped
and photographed in order to better understand
fracture style, timing and host lithologies. The outcrop
studies defined a number of fracture styles and
exposure-related features which are discussed in the
following paragraphs.
Contractional and surface-related fractures
Stress-release sheet fractures (exfoliation fractures)
in granodiorites at the Big Mountain location, Vung
Tau, are sub-parallel to the topography of the basement
block. Dip angles tend to be low – sub-horizontal to
10-15° (Fig. 7a). Near the present-day land surface,
the spacing between stress-release sheet fractures
137
ranges from several centimetres to 3 m and increases
progressively with depth.
Exfoliation fractures form in the shallow
subsurface as a result of stress release as a pluton
exhumes and associated overburden stresses first
decrease then disappear (e.g. Younes et al., 1998).
The fractures are formed due to changes in thermal
stress during annual or daily ambient temperature
cycles in combination with meteoric-driven
weakening of matrix strength due to mineral
transformations (Holzhausen, 1989). The lateral extent
of individual sheet fractures could not be measured at
the Vung Tau exposures. Nermat-Nasser and Horri
(1992) noted that open sheet fractures in granites can
reach widths of 200m. Tandom et al. (1997) showed
that at depths of up to 200-250 m, exfoliation sheet
fractures in an igneous host can be 25-30 m or more
apart. By analogy, sub-horizontal exfoliation fractures
may therefore be present at Bach Ho in the top 50100 m of basement rocks.
At the Vung Tau outcrops, exfoliation fractures
were observed to become blocked at depths of 10-30
m below the surface with fine clay soils and other
weathering products including kaolin and Fe-oxides.
The contribution of these types of fracture to reservoir
porosity at Bach Ho will therefore probably be low.
Sub-horizontal features (“temperature reduction
fractures”) can also form as a magma body cools and
contracts. The intensity of this type of fracturing varies
with the relative proportions of quartz, K-feldspar and
plagioclase minerals. Contractional fractures are more
likely to occur in acidic rocks with elevated
proportions of quartz (Osipove, 1974). Rocks with
minor quartz but more feldspar (quartz-monzonites
or diorites) show less volume shrinkage and hence
fewer contractional fractures. The acidic lithologies
which dominate the Central Block at Bach Ho will
be susceptible to contractional fracturing. However,
the contribution of early thermal reduction fractures
to effective primary porosity will probably be low. At
the Vung Tau outcrops, these fracture types were
observed to be filled by hydrothermal mineral cements
such as zeolites or coarse calcite spar.
Tectonic fractures
Tectonic fractures in the Vung Tau outcrops dip at
angles of 50-90o and are typically composed of crosscutting conjugate sets (Fig. 7a, b). Dominant
orientations are NE-SW and north-south; the NE-SW
trend is aligned with the trends of the regional Dong
Nai and Mae Ping faults, and parallels the regional
structural “grain” associated with opening of the South
China Sea (Morley, 2004; Hall and Morley, 2004; Tjiia
et al., 1998; Dong et al., 1999).
At a quarry at Round Mountain near Vung Tau,
tectonically-induced fracture apertures are mostly mm-
138
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Fig. 7. Field photographs of basement outcrops at Vung Tau (see Fig. 1 for location)
(A) A weathered zone, 2-5 m thick, overlies the top of basement rocks at Small Mountain, Vung Tau. Subvertical (red) and sub-horizontal (green) fractures with varying densities occur beneath the weathered zone.
Sub-horizontal fractures are a combination of sheet and exfoliation fractures with fracture spacing varying
from 0.05-0.50m (yellow arrows) and 0.5-3.0m (light blue arrows).
(B) Basement rock (at Small Mountain, Vung Tau) is deformed by tectonic fractures showing high dip angles
(red). A narrow igneous dyke (ca.0.8m wide) cuts across the basement. The dyke runs along a fault and
separates hanging- and foot-walls. Near the dyke, fractures are mostly parallel to the dyke margin and there
are vugs or aperture fractures at the contact with the country rocks. Rocks to the left of the dyke are more
highly fractured than those to the right.
(C) Plan view of basement (at Small Mountain, Vung Tau) showing conjugate, relatively steeply-inclined
fractures.
(D) A weathered zone (0.5 to 2.0 m thick) caps the basement rocks which are cross-cut by conjugate, steeply
dipping fractures and faults (Small Mountain, Vung Tau).
(E) Close-up of Fig. 7d showing deformation and brecciation in the fault damage zone, especially at the
intersection of two faults where a cave has formed (indicated by white arrow). Numerous steeply inclined
fractures are associated with the breccia. There are two fracture sets: a tectonic (sub-vertical) set and a
release (sub-horizontal) set.
(F) Close-up of breccia in Fig. 7e, showing composition of broken basement fragments (0.3 to 3.0 cm across)
in a matrix dominated by the products of weathering and diagenesis. There are two sets of fractures: a steeply
inclined tectonic set and a set of sub-horizontal release fractures.
Trinh Xuan Cuong and J. K. Warren
139
Fig. 8. Fault-damage fracture map showing distribution of fracture density in a brecciated fault zone, based on
measured fractured separations collected from the Vung Tau outcrop at sites shown in Figs 7e, f. The fault
centre consists of a breccia (0.1 – 10m wide; Fig. 7f), while zones of high fracture density occur adjacent to the
brecciated zone (10-35 m; Fig. 7e). Mineral precipitates, and less commonly sediments, fill or partially fill
dissolution voids equivalent to those in the subsurface but more labile minerals are leached at outcrop.
scale but range up to 1-2 cm wide, with rare apertures
up to tens of cm across. About 30% of measured
fracture apertures were between 0.2 cm and 1.0 cm.
Most fractures were filled, or partially filled, by
varying combinations of modified magma and
hydrothermal precipitates and clays. They are
interpreted as outcrop analogues of the mineral-filled
fractures illustrated in Fig. 4b.
Fault damage zone widths ranged from tens of
decimetres to several metres, depending on the
associated fault displacement (Fig. 7b, f). The intensity
of fault-related fracturing varies from very high near
to major faults (100-150 fractures/metre), and
decreases at greater distances (e.g. 35-50 fractures/
metre at a distance of 1.5 m from the fault in Fig. 8).
Major shear zones with high associated fracture
intensities are up to 35 m or more wide at Vung Tau
(Tjiia et al., 1998).
Igneous dykes
Andesitic and rhyolitic dykes cut across the granites
and granodiorites exposed along the Vung Tau coast
(Fig. 7b). The dykes are in general oriented parallel
to nearby faults and fractures, with widths varying
from 0.5m to several metres. The dykes are
characterized by higher densities of fractures (about
5-10 cm spacing) than the surrounding granites (1050 cm spacing). Fracturing in the dykes is typically
oriented parallel to the dyke axis. Mineral-filled (clay
and zeolite) mm-cm scale vugs and contractional
fractures occurred at contacts between dykes and
adjacent granitic rocks. Analogous features in the
Bach Ho subsurface contained zeolites and other
hydrothermal products. Oil staining in such fractures
indicated later re-opening. The mineral-filled fracture
constituted zones of relative structural weakness
compared to the adjacent granodiorite (Fig. 4b).
Weathered zones
A weathered and lateritized soil profile blankets the
Vung Tau outcrops, and soil-related muds infiltrate
exfoliation, tectonic and contractional fractures (Fig.
7a, d). Soil penetrations into open fractures, and
associated weathering haloes, constitute a zone up to
40-50 m thick on the flanks of Vung Tau outcrop
blocks; the zone is 10-20 m thick over the crests of
the blocks. Inland from the coast, in areas where soils
are less well developed, weathered and bleached
granite domes up to tens of metres across are exposed
subaerially. Basment blocks are Bach Ho were
likewise subaerially exposed. By analogy with the
Vung Tau outcrops, weathering and the infiltration of
clay-sized fines will have occluded open fractures in
the subsurface (Vinh, 1999).
FRACTURE CHARACTER IN BACH HO
BASED ON IMAGE LOGS
Image log analysis at Bach Ho detected several
fracture types and their interpretation was aided by
calibration against outcrop analogues (Fig. 7) and
core. An interpreted image suite from well BH-12 is
140
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
shown in Fig. 9 (fracture identification follows the
classification scheme of Schlumberger, 1992, 1997a,
b). This interval illustrates the utility of running a
combination of imaging tools (DSI, FMI, UBI, ARI)
when defining the nature of fractures intersecting a
borehole.
The FMI (Fullbore Formation MicroImager) tool
produces a high-resolution microresistivity image of
the borehole wall, covering 80% of the wall in an 8.5inch borehole. Each microconductivity button can read
up to 1 cm into the formation and in combination with
adjacent buttons is capable of distinguishing
conductivity anomalies smaller than the tool’s
effective resolution of 0.2 cm. Of currently available
imaging tools, the FMI tool provides the most detail
for analysis of the Bach Ho reservoir. It sees most
natural and induced open fractures intersecting the
borehole wall together with matrix variations in bulk
formation textures, but is has problems seeing finer
open fractures and in separating open from closed
mineral-lined fractures at the sub-millimetre scale.
The DSI (Dipole Shear Sonic Imager or Stoneley
wave) tool, in contrast to the FMI tool, has a 15 cm
(6-inch) depth of investigation and a vertical resolution
of 1 m. The presence of open fractures strongly affects
the waveform, so this tool is better suited to locating
broader-scale intervals (m-scale) characterized by
numerous open fractures.
The ARI (Azimuthal Resistivity Imager) tool
records lateral resistivity up to 60 cm (24 inches) into
the formation. The increased depth of investigation
attained when using a combination of DSI and ARI
images allows for intervals with numerous open
fractures to be distinguished from intervals with
numerous but perhaps smaller, probably cemented and
unproductive fractures. As with the FMI images, it is
possible to orient fractures picked from ARI images
(Fig. 10).
The UBI (Ultrasonic Borehole Imager) tool is an
acoustic imaging log providing a 360° scan of the
borehole wall. Although it has a lower resolution than
the FMI tool, the full-bore coverage of the UBI tool
provides a detailed caliper of the borehole, and can
separate larger mineral-lined and closed from open
fractures. This is invaluable in confirming fractured
intervals seen in FMI, and is also useful for stress
analysis and in confirming good hole conditions for
setting MDT packers.
The following discussion, together with the results
shown in Fig. 10 and the individual well fracture roses
illustrated in Fig. 6, are based on the application of
these imaging tools to the Bach Ho reservoir. Much
of the image-to-rock calibration was based on a
detailed core study of well BH 12 (Fig. 11), this was
also the well where a complete multi-image log suite
was run across the reservoir (Fig. 9).
Natural fractures
Continuous fractures
These fractures show a continuous conductive
response in the electrical image (e.g. 3705 m in well
BH-12: Fig. 9). They also have a strong corresponding
Stoneley wave reflection coefficient. The amount of
energy lost in the fracture is thought to correspond to
how open the fracture is (Horby and Luthi, 1992).
However, zones with extremely rugose or washedout profiles can generate similar acoustic loss
anomalies (above 3590 m).
Discontinuous fractures
Partly-filled mineralized fractures generate
discontinuities which in the image log appear as only
partly conductive intervals (e.g. 3715m in Fig. 9). This
style of fracture development is not laterally extensive
and does not drain as effectively as a grouping of
continuous fractures intersecting the well bore.
Boundary fractures
These fractures are normally associated with changes
in the lithology and/or texture of the rock (e.g. 3696
m, 3719.5 m, 3722 m in Fig. 9).
Brecciated fractures
These are commonly associated with major faults,
creating brecciated zones with polygonal textures on
the electrical image and more conductive signatures
in the deep resistivity log (e.g. 3690-3696m in Fig.
9). The thickness of the brecciated zones can vary
from several decimetres to tens of metres. In the
Stoneley waveform image, these zones show obvious
intersections of downgoing and upgoing wave fields
(e.g. 3719.5-3722m in Fig. 9). Ties to resistivity logs
and production data show that these brecciated zones
within the basement reservoir act as significant
hydrocarbon storage systems and have high
permeability. In well BH-12, major breccia intervals
occur at 3597-3604 m, 3617-3625 m and 3704-3723
m.
Vuggy fractures
Strong Stoneley wave and corresponding low
resistivity responses can be used to indicate the likely
presence of vugs in the reservoir. They can occur in
unaltered granite but are more common within
fractured and altered zones (e.g. 3705m in Fig. 9).
Typically they form the large cm-scale “cavities,” as
seen in the FMI and UBI, but these logs indicate
resistivity or acoustic contrast and so are indirect
indicators of fracture style. The logs may be
responding to mineral fills, not fluid content. Even if
the vugs are fluid-filled, they may be isolated and such
a vuggy zone may not be capable of generating
substantial fluid interconnection to the well bore.
Trinh Xuan Cuong and J. K. Warren
141
Fig. 9. Interpreted image log suite from 3690 – 3730 m in well BH-12, showing naturally induced fractures and
breakouts, togetgher with dykes and brecciated zones (see Fig. 6 for well location).
Faults
Faults are often indicated by associated rapid increases
in fracture intensity, truncation features or brecciated
zones on the image log, as can be expected by
anaology with observations from Vung Tau (Fig. 7).
Drilling induced fractures:
Borehole enlargements are a common feature along
the axis in well BH-12 and other wells at Bach Ho
and are obvious in the caliper response. They are
related to stress release failure, and FMI shows they
are typically parallel to the least principal horizontal
stress. Long, straight drill-induced fractures are
usually perpendicular to the direction of borehole
enlargement, as seen in the image logs (e.g. 37253730m in Fig. 9). Recognizing induced fractures and
borehole breakout is invaluable in determining the
current orientation of the principal stress field. Sudden
rotations of the image tool travelling across basement
sections indicates that principal stress directions vary
with reservoir depth and reflect the complex tectonic
history of the basement.
142
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Fig. 10. Fracture pattern and distribution from FMI/FMS interpretations of wells in Bach Ho field. Dip angles
vary from 20 to 45o in Group 1 fractures (release and exfoliation) and 45 to 75o in Group 2 fractures
(tectonic). Dominant strike angles vary from block to block across the field (see Fig. 6 for fracture roses for
this data).
Enhanced fractures
In addition to natural fractures, pre-existing fractures
which are extended or reopened in the borehole by
drilling are classified as “enhanced fractures”
(Standen, 1991). It is difficult to differentiate enhanced
from other fractures using image and conventional
logs. For example, possible enhanced fractures may
occur below a depth of 3750m in FMI images in well
BH-12, but production test data shows they do not
affect the volume or rate of hydrocarbon production
in this interval.
Fracture identification using conventional logs
Fracture identification using conventional logs is
important in Bach Ho wells where there are no image
logs. Many authors have discussed the use of
conventional logs for fracture detection (e.g. Stearns
and Friedman, 1972; Aguilera, 1993; Ershaghi, 1995;
Schlumberger, 1997b). In well BH-12, image logs
could be used for fracture calibration and verification
of the interpretations arising on conventional log
approaches (Figs 11, 12, 13). The vertical resolution
of conventional logs is typically lower than that of
Trinh Xuan Cuong and J. K. Warren
Schist: dark grey, friable, soft
thin schistose layering.
Abundant black biotite,
common pyrite, trace whitish
zeolite veins (>1 mm in width)
developed parallel to
schistosity. The rock has been
strongly deformed, compacted,
fractured and locally
weathered.
Schistose
Sample
Massive
Good
Moderate
Poor
Fine
Very fine
Poor fracture porosity
3831.0
Depth
(m)
Granite: similar to above
but is medium to coarse
grained. Bulk of rock is
crosscut by a fine
fracture meshwork
generally filled with
zeolite(?) and quartz
(white grayish). Some
caverns 5 mm wide with
grey white euhedral
crystals of 1 mm to 2 mm
in diameter growing in
some cavern pores.
Poor to moderate fracture
porosity.
Permeability
(Kair md)
Max. 90° Vert.
Grain density
(gm/cc)
Schist, similar to above, with
medium sized crystals (3600 to
3600.1 m) and whitish grey
patches (>1 cm diameter) of
possible relict lithology.
Medium
Lithology
Granite: White and very
light grey with dark
grayish olive speckles,
very hard, medium
grained, slightly coarser
locally at 3830.80 m,
massive structure.
Abundance of quartz and
feldspar crystals, minor
black mica, trace of
yellowish pyrite, trace of
fractures.
3832.0
Schist with the same
features as above, intensely
fractured. Common zeolite
veining (0.5-2.5 mm wide,
locally 2.5 cm). Some open
fractures and caverns more
than 1 mm in width. Fair to
good fracture porosity. Pore
connectivity is good.
Description
Porosity
(helium % )
Schist: Dark grey to light
grey, strong schistose
layering, very fine to finely
crystalline, hard. Bulk of the
rock is crosscut by fracture
meshworks, usually filled by
zeolites, quartz, and calcite.
White to white pinkish grey,
friable. Locally, there are
some caverns of 1 mm to 3
mm width, which are partially
filled by zeolites and other
vein minerals. Abundant
whitish feldspar, black biotite.
Very finely crystalline
yellowish pyrite is common.
Intense fracturing. Size of
fractures is variable, ranging
from 0.05 mm to 1-2 mm in
width and commonly > 1-5
mm in length. Moderate to
fair fracture porosity.
Coarse
Schistose
Sample
Poor
Good
Moderate
Fine
Very fine
Medium
Lithology
Depth
3830.0
3599.0
3600.0
Struct.
Crystal size Porosity
Description
Core 2
Core 1
3598.0
Coarse
3597.0
Massive
Struct.
Crystal size Porosity
Depth
143
3598.49 0.68 0.09 0.13 1.37 2.73
3598.55 1.70 0.16 0.37 1.71 2.72
3599.83 0.06 0.01 0.01 0.91 2.75
3599.70 to base core highly fragmented
3830.25 0.25 0.09 0.01 0.72 2.65
3830.65 0.25 0.01 0.02 1.22 2.66
2831.92 0.35 0.19 5.10 0.77 2.63
3601.0
Fig. 11. Core descriptions of cores 1 and 2 from well BH-12. Core 1 sampled a strongly altered rock (SAR
electrofacies), while core 2 sampled the unaltered granite (FR electrofacies) from 3830-3830.8m and the
moderately altered rock (MAR electrofacies) from 3830.8-3832m. Inset at lower right shows standard coreplug derived permeability, porosity and grain density. The listed plug permeabilities are maximum (plug
drilled parallel to mineral-filled microfracture), at 90° to microfracture and vertical (with respect to core
axis). Definitions of the various electrofacies were given by Cuong (2000).
Fig. 12. Composite log of well BH-12 summarizing reservoir zones based on wireline logs, oil flow rates and core analysis data. Wireline logs indicate lithological
properties in each zone. Note the rotation of the FMI tool in fractured intervals, this is a zone of prominent secondary porosity. Flow rates indicate Zones III and
perhaps IV are effective reservoir. Pores in Zones I and II are poorly connected and are sub-economic. Within the Bach Ho reservoir interval, the caliper log is one of
the best direct indicators of reservoir quality. Definitions of the various electrofacies are given by Cuong (2000).
144
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Trinh Xuan Cuong and J. K. Warren
image logs and the depth of investigations is broader,
so that conventional logs define fractured intervals
rather than individual fractures.
A higher gamma signal is typical of fractured
intervals in Bach Ho and reflects precipitation of
radiogenic hydrothermal minerals along fractures
(although these fractures only contribute to reservoir
porosity when re-opened as a result of later
deformation) Electric logs, especially the
microlaterlog and the proximal log, can indicate
fractures in Bach Ho basement. However, conductive
minerals, such as pyrite and other metal sulphides,
and drilling-induced fractures, reduce the accuracy
of natural fracture determination using these logs.
Porosity-logging tools, neutron, density and sonic,
are the most useful conventional logs when defining
fractured zones. Our studies show the sonic log is the
most useful porosity log in Bach Ho field as it not
only indicates fractures by elevated values on shear
and compressional responses, but also differentiates
secondary and primary porosity. Caliper logs reliably
define fractured intervals by enlargements of the
wellbore (e.g. 3600m, 3626m, 3705m, 3720m, and
3721m in well BH 12: Fig. 12), while associated hole
rugosity tends to create anomalous values in most
conventional logs, especially pad-mounted porosity
logs. This is an important factor in deriving reliable
poroperm core-to-wireline transforms (see below).
In general, individual wireline logs do not provide
reliable definitions of fractured intervals at Bach Ho.
Logs in combination, such as cross-plots, give more
reliable results but still show variable quality outputs
that are subject to error. Based on conventional log
and cross-plot approaches, we attempted to define
fractured zones in well BH-12. We then compared
these outputs to image-log based fracture
interpretations (compare Figs 12 and 13). The results
were variable and only the M-N, M-PEF and DT4SDT4P cross-plots could be considered partially
reliable fracture indicators. The M-N, M-PEF crossplots detect fracture zones through the presence of
hydrothermal minerals in the fractures. They do not
clearly distinguish between zones with open versus
closed fracture networks. Comparison of the fractured
zones defined by the three cross-plots is shown in Fig.
13. These zones are generally coincident and given
the resolution problems of conventional logs to image
logs, the outputs are considered to reasonably indicate
the intensity of fracturing in Bach Ho basement.
Fracture presence or absence can be estimated from
the wireline determined compressional and shear
velocities (Sibbit, 1996), but again the result is not
quantifiable in Bach Ho due to washout in almost all
the fracture zones. This also strongly influences the
reliability of sonic readings (e.g. 3617-3628m, 37043723m in well BH-12).
145
PROBLEMS WITH OBTAINING
REPRESENTATIVE POROSITY
MEASUREMENTS IN CORES
During core-based petrophysical studies, it was not
possible to quantify porosity distribution in the
fractured basement at Bach Ho, even using whole core
determinations. Core and plugs of suitable quality for
core analysis tended to come from relatively
unfractured intervals, while fractured zones tended
to be recovered as rubble without little coherency.
Studies showed that neither water saturation nor
permeability could reliably be estimated from either
logs or core (Fig. 14). This reflects both the inherent
limits of conventional wireline log interpretation in
the high resistivity environment of the Bach Ho
igneous and metamorphic basement, and also the
limited and biased core recovery.
Total porosities in Bach Ho basement rocks are
low, typically between 0 and 3%, this is less than the
standard deviation or error range of tool-based
porosity estimates (e.g. inset in Fig. 11). Therefore,
calculated values of porosity from conventional
wireline logs, even in fractured intervals, are at best
semi-quantitative at Bach Ho. Simple equations of
wireline porosity calculation were applied and
appeared to give consistent results, but are influenced
by poorly controlled variations in matrix mineralogy
and fracture intensity. An assumption was made that
unaltered rocks have zero effective porosity. Therefore
porosities calculated by different logging tools had to
be corrected to zero for intervals of unaltered matrix,
even where apparent porosity is listed in log outputs
due to lithology effects within these intervals. Multiple
mineral models can remove some of these lithological
effects, but are only accurate when volumetric inputs
for each mineral are known. Wireline porosities
illustrated in Fig. 12 were calculated as best-estimates
based on combinations of neutron, density and sonic
logs, and verified against core data and image logs.
In many fractured reservoirs, especially in
carbonates, the difference between porosities obtained
from neutron/density logs (i.e. total porosity) and sonic
porosity can be used to indicate intervals of secondary
porosity. But such wireline-determined intervals of
secondary porosity in Bach Ho basement typically did
not correspond to open fractures or vugs. The intervals
also encompassed zones of non-effective porosity due
to hydrogenation by authigenic minerals (read as
porosity by the neutron log) or microporosity in
authigenic clays and zeolites. These features were
created during hydrothermal alteration and episodes
of pore occlusion, and were not tied to effective
porosity in the reservoir.
The matrix in Bach Ho has extremely high
resistivities as it is composed of non-porous igneous
146
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Fig. 13. Fracture interpretation using
conventional logs in well BH-12. Likely
fracture zones interpreted from
conventional cross-plots are indicated by
red highlights in the column to the left of
each of the three conventional log tracks.
(A) is based on an M-N cross-plot,
(B) uses M-PEF, while (C) is based on a
DT4S-DT4P cross-plot. All three show
only moderate agreement and matches
to fracture interpretation based on FMI
(see Fig. 12).
rock. This means a formation factor exponent “m”
could not be measured accurately in the laboratory,
making water saturation calculations (via Archies
Equation) tentative.
FRACTURE DISTRIBUTION AT BACH HO
Fracture analysis shows that there is no consistent
field-wide relationship between the faults and
fractures mapped by seismic methods and the natural
fracture orientation defined by image logs (Figs 6 and
15). In the Southern Block (well BH8), the strike of
the FMI-defined fractures are sub-perpendicular or at
a high angle to strikes of seismically-mapped faults
(Fig. 6). FMI-defined fractures in the Central Block
(wells BH2, BH3, BH4) show a less consistent set of
strike orientations. In wells to the north of the Central
Block (wells BH1, BH5 and BH6), and wells to the
east but downdip of the Central Block (well BH9),
the strike of the FMI-defined fractures is less well
oriented than in wells in the Southern Block, but again
is sub-perpendicular to the general NNE-SSW
orientation of the seismically-defined faults (Fig. 6).
The lack of a general trend in fracture strike is most
marked in wells in the Central Block compared with
the surrounding regions (Fig. 6).
Regions to the north, south and east of the central
high show a reasonably consistent tie between seismic
and image-log data, whereby FMI fracture strikes are
oriented sub-perpendicular to seismically-defined
fault trends. This signature is consistent with the
regional-scale early-mid Tertiary extension tied to the
opening of the proto-South China Sea (see Hall and
Morley, 2004). The region with the poorest tie between
FMI- and seismically-defined fault orientation is the
Central Block. This is the structurally highest block
in the field and unlike the adjacent blocks, was
effected by late Oligocene thrusting. Blocks to the
north, south and east of the Central Block show
evidence of extensional block faulting but no reverse
faulting (Fig. 6). As discussed above, seismic analysis
shows the Central Block was translated by late
Oligocene reverse faulting, which laterally displaced
it by up to 2 km to the WNW.
Trinh Xuan Cuong and J. K. Warren
147
70
Percentage (%)
60
Permeability from different diameter plugs
and whole core in Bach Ho basement
50
40
30
20
10
0
30 mm 0-1
50 mm
Full core
1-10
10-100 100-1000
>1000
Gas permeability (md)
Fig. 14. Fragment size controls the scale of measurement volume in a fractured basement sample in Bach Ho
and so influence measured permeability (after Tuan et al., 1996). Standard plug values for porosity and
permeability in well BH-12 are listed in Fig. 11.
The less ordered orientation of fracture strike in
the Central Block is perhaps related to a combination
of buckling and subsequent stress release.
Compression-driven fracturing has partially
overprinted pre-existing extensional fractures in the
block but also opened new set of compression-related
fractures. During the Miocene, the thrust-emplaced
Central Block was subject to gravity-induced
extension, a process that opened pre-existing fractures.
Fracture sets in Bach Ho exhibit structurally
consistent variations in dip angle; dominant dips of
fractures in all wells are 50-75° (Fig. 10). This is
consistent with stress patterns created both by
extension or compression, and the associated fractures
cannot be differentiated in the FMI data (see
discussion in Aydin, 2000). A subordinate set of
fractures have moderate dips of less than 40°. By
analogy with the Vung Tau outcrops, these fractures
are interpreted as exfoliation fractures. They are most
obvious in wells located at some distance from the
most highly tectonised part of the field (the Central
Block); compare, for example, the dip angles of well
BH 3 with those of other wells listed in Fig. 10.
Across Bach Ho field the fracture density recorded
in FMI and core data shows two trends: (i) porosity
tends to decline with depth (Fig. 15a). and (ii) porosity
tends to increase in zones of faulting and fracture (Fig.
15b). These trends outline the most highly deformed
upper portion of the basement. Alhough the basement
reservoir is a granodiorite, it can be modelled as a
fracture system with the greatest flow potential in the
uppermost interval. High flow will also occur if a well
intersects an open fracture system tied to a deeperseated; as study of the Vung Tau outcrops showed,
fracture density and porosity increase significantly in
the damage zone (Figs 7, 15b).
In summary, open fractures tend to be most
significant in the Central Block of Bach Ho, where
thrust faulting emplaced a brittle basement block on
top of Eocene-Oligocene mudstones. Thrusting,
together with subsequent gravity-driven extension,
served to open or re-open tectonic fracture networks.
Cross-linkage of such fractures was probably aided
by the formation of exfoliation fractures.
RESERVOIR ZONATION TIED TO
GEOLOGIC HISTORY
Combining all the available geological, petrophysical
and seismic data, the Bach Ho basement can be
divided into three reservoir zones, A-C (Fig. 16). Zone
boundaries are not horizontal, but are probably
irregular and fracture-controlled.
Zone A is strongly altered and fractured due to a
combination of tectonism and fluid flushing. It is
capped by a relatively thin, intensely weathered and
clay-plugged interval, similar to that observed at
outcrop at Vung Tau, which varies in thickness and
can be unconsolidated, even at depths of 4000 m. This
interval can be considered as a regional-scale seal
capping the underlying basement reservoirs.
The remainder of Zone A is characterized by the
presence of open to partially-open conjugate fractures
and brecciated zones, and has good but variable
900
600
300
0
This well
penetrated
the main
fault
Layer 5: 100m
Layer 6: 100m
Layer
7: 125 m
Layer 1: 0-60m
Layer
8:
125m
(Weathered zone)
Layer
9:
250m
Layer 2: 25m
Layer 10: 500m
Layer 3: 50m
Layer 4: 75m
(unaltered rock)
10
9
8
7
6
5
1
2
3
4
FMI fracture Density (#/m)
3
4
5
6
1
2
7
B.
200
240
DT
(μs/m)
X420
X410
X400
Depth
(m)
0
5
X393.5
10
NPHI (% )
X415
1
2
3
4
5
Core porosity (% )
Well BH-4 2 0
6
Breccia from logs
(fault zone)
Fracture density
(Fractures/m)
10 20 30 40
Fig. 15. Fracture distribution variations with depth and faulting at Bach Ho field. (A) Fracture density in Bach Ho wells changes with depth below top-basement.
Fractures tend to be most common in the upper part of the basement block (Zone A) but also increase in the vicinity of faults in deep wells. Layering (1-10) is based
on FMI. For reasons of confidentiality the well locations are not given, but all wells are located in the region shown in Fig. 6.
(B) Fracture density and porosity distribution increases in the cored damage zone near a fault. There was no core recovery in the fault zone itself. The figure is based
on core in well BH-420 but for reasons of confidentiality the location of this well is not given. This trend of fracture increasing toward a brecciated zone is similar to
the trend seen in outcrop near Vung Tau (see Fig. 7).
1500
1200
Depth into reservoir (m)
A.
Cored interval (97.6 %)
148
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
Trinh Xuan Cuong and J. K. Warren
Weathered rind (zone IV in BH-12 basement,
and from outcrop, seismic data)
Early Oligocene shales (source rock)
Late Cretaceous granite (from core analyses,
BH-12 wireline logs and seismic data)
Eocene lacustrine shales (source rock)
149
Eocene and Oligocene sands (immature and cemented - zeolites and calcite)
Triassic - Jurassic intrusions (from core
analyses, BH-12 wireline logs and seismic data)
Release, exfoliation and contractional sheet fractures (from core
analyses, BH-12 wireline logs and seismic data)
Conjugate faults, associated fractures and breccias (from FMI data,
seismic data and outcrop analogues)
Late Oligocene reverse fault in Central Block, associated fractures and breccias
(from seismic data)
Andesite - Dacite dykes (from core analyses,
BH-12 wireline logs and seismic data)
Metasediments (Pre Late Triassic)
Zone A-B boundary (coincides with boundary of wireline -defined
zones II and III in BH-12 and the lower part of the upper second
seismic zone and is around 3800-3900m in Central Block)
in
BH-12 and the top of the third seismic zone)
A
B
C
Not to scale
Fig. 16. Reservoir model for the Central Block in Bach Ho field based on the integration of outcrop, seismic,
wireline and core data. Reservoir zones A, B and C are discussed in the text.
reservoir qualities. Zone A forms the principal
reservoir interval at Bach Ho. Pores and fractures may
have been locally enlarged by meteoric dissolution.
However, most of the effective porosity is associated
with fractures of tectonic origin. New fractures were
formed, and pre-existing zeolite-filled fractures were
re-opened, during late Oligocene thrust-related
emplacement of the Central Block. This combination
of fracture origins explains the wide spread of fracture
orientations seen in FMI data (Fig. 6).
Regionally, the base of the fractured interval in
Zone A at Bach Ho is at a depth of around 3800-3900
m. This depth is approximately equivalent to the top
of the Eocene sedimentary interval where it onlaps
the basement block. The top-Eocene also corresponds
regionally to the base of the upper part of the second
seismic zone (roughly at the level of the blue reflector
in Fig. 5 and part way into the zone defined by yellow
arrows). In well BH-12, the boundary between Zones
A and B is coincident with the wireline-defined
boundary separating Zones II and III.
Zone B is less intensely fractured and altered than
Zone A, and many fractures are closed or filled by
authigenic or hydrothermal precipitates. The base of
the zone lies at a depth of around 4000-4600m and
probably coincides with the lowermost extent of
effective production. Zone B reservoir quality is
variable and patchy. The zone corresponds to the
wireline-defined Zone II in well BH-12, and the lower
part of the second seismic zone in Fig. 5.
Zone C is weakly altered and fractured, and almost
all pores and fractures are filled by zeolites and other
hydrothermal minerals. Reservoir quality is very poor
and the zone is not therefore considered an exploration
target. Zone C is equivalent to wireline-defined Zone
I in well BH-12 and to the third seismic zone located
below the yellow arrows in Fig. 5.
DISCUSSION: BACH HO AND OTHER
“BURIED HILL” PLAYS
In the oil-industry the term “buried hill” is widely used
to describe a reservoir associated with a
palaeotopographic high, where onlap by subsequent
sedimentary layers indicate that the feature was once
exposed at the landsurface (e.g. Luo et al., 2005).
Reservoirs are in general located in the uppermost
(fractured and weathered) parts of the “hill” or in the
immediately surrounding sediment apron. Table 2 and
Fig. 17 summarise how the reservoir characteristics
of the Bach Ho structure compare with those at other
“buried-hill” type traps. Table 2 does not list discovery
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
S
Panhandle Field
Palo Duro
Basin
gas
After Sorenson, 2005
3 km
Oil
H2O
mud
granite (fresh)
80 km
0
B NW
SE
Sea level
S
N
Cenozoic
Paleogene
congl.(wash)
Oil & gas
Oil & gas
Cretaceous
Palaeozoic
Brec
Tertiary lake
muds (seal)
cia
Basement
(Proterozoic granite)
0
2
C
Basement
Clair Field,
Offshore Shetland
After Ridd, 1981
km
Reservoir is tectonically
induced set of fractures,
“minor sedimentological
influence on quality
N
Granite wash play
AmarilloArch, Texas
Anadarko Basin
oil
wash
grus
Combined fracture &
reworked “grus”
Classic “Buried Hill” Analogs
A
Xinlungtai Field,
North China Basin
After Guangming &
Quanheng, 1982
Sea level
To
ene
p Eoc
Reservoir
(fractured
carbonate)
.
La Luna Fm )
(source rock
La Paz-Mara Field
Venezuela
La Paz
Field
Top E
o
Misoa
F
Top P m.
G
uaser aleo.
La Lu
a Fm
na Fm
.
.
Top Socuy
Reservoir
(fractured basement)
0.0
cene
1.0
BASEME
NT
After Nelson et al., 2000
Two-way travel time
Raservoir is mostly in reworked “grus”
with minor contribution from
fractured basement
150
2.0
Fig. 17. “Buried-hill” plays (see text and Table 2 for detail):
(A) Cross-section through the Amarillo-Kansas Arch, USA, which hosts giant oilfields (Panhandle-Hugoton) in
reefal carbonates which developed on the basement high and which are sealed by subsequent platform
evaporites. “Granite wash” derived from the eroded basement block (see inset) hosts local accumulations.
(B) Cross-sections through two “buried-hill” fields, Clair and Xinlungtai. At Clair, the reservoir is dominated by
sediments reworked from the exposed granite high; at Xinlingtai, the reservoir is a combination of a fractured
basement block, a weathered sediment “halo” and an apron of reworked grus.
(C) Cross section through La Paz Field, Venezuela, where the reservoir is hosted in a fractured thrust-block.
Hydrocarbons are sourced from the underlying La Luna Formation.
wells in fractured basement which did not pass into
production (e.g. the Lago Mercedes No. 1 well in
Chile: Wilson et al., 1993; see also wells listed by
Schutter 2003a, b); nor does it list wells and fields
producing from fractured volcaniclastics or shallow
gabbroic and serpentinite intrusives which were
synchronous with nearby sedimentation (e.g. Kalan
et al., 1994; Harrelson, 1989).
In most “buried-hill” type reservoirs within igneous
or metamorphic matrix, two main processes contribute
to effective porosity: (a) weathering and mechanical
reworking, which can produces a “granite wash”; and
(b) open fracture and joint networks (Powers, 1932).
A classic “granite wash” play occurs at Elk City field,
Oklahoma (Sneider et al., 1977). Fracture-associated
porosity tends to be tied to structurally-induced
permeability fairways, e.g at La Paz field, Venezuela,
where production comes from a combination of
fractured Mesozoic carbonates and granitic basement
(Nelson et al., 2000).
A common characteristic of all the fractured
basement fields listed in Table 2 is that the original
reservoir matrix was tight and had high resistivities,
as in Bach Ho. Effective porosity in such a reservoir
is secondary, and is related to tectonic deformation or
fluid flow. Tectonically-produced porosity is related
to stress release during faulting and uplift/unroofing,
and comprises open faults, fractures, microfractures
Trinh Xuan Cuong and J. K. Warren
and joints produced at a variety of scales ranging from
kilometres to millimetres, and with variable degrees
of matrix damage ranging across the same scales.
Secondary porosity produced by fluid circulation
includes dissolution via meteoric waters in weathering
zones, and deeper-seated alteration and leaching
produced by hydrothermal circulation. The two types
of secondary porosity development can be interrelated,
where, for example, pre-existing tectonic fractures act
as conduits for meteoric waters or hydrothermal fluids.
A combination of deeply-circulating meteoric
waters moving along tectonic fractures, and a
reworked sedimentary “halo”, characterizes riftassociated basement plays in China and the Gulf of
Suez, as well as at the Clair field, offshore Shetlands
(Fig. 17b). However, weathering or “granite wash”
associations do not occur in the Cuu Long Basin. The
limited dataset in Table 2 suggest that meteoricenhanced basement plays tend to occur at depths of
less than 3000 m, suggesting that diagenesis associated
with deeper burial causes a reduction in porosity. This
may reflect the dominance of immature first-cycle
arkoses and litharenites which weather from uplifted
basement blocks in rift settings.
An uplifted basement block in a rift setting will
subsequently undergo thermally-induced subsidence,
and can commonly be overlain by rapidly-deposited
lacustrine or marine mudstones, which can be overpressured. The mudstones may however include
source rock intervals, and a build-up of overpressure
will cause hydrocarbons, once generated, to migrate
into the sediment “halo” or into the fractured basement
(e.g. Fig. 17a).
A third group of “fractured basement” plays,
including that at Bach Ho, is characterized by a
dominance of tectonically-induced fractures with little
or no overprint by meteoric dissolution, and an
absence of sediment “haloes” associated with surfacedriven weathering. These plays tend to occur in
systems characterized by active (Neogene) tectonics
in thrust-wrench fault settings, and include the La Paz/
Mara fields of Venezuela and perhaps the
Sarkadkeresztúr field, Hungary (Fig. 17c; Table 2).
At La Paz, thrusting emplaced a block of fractured
granitic basement and associated limestone cover on
top of the prolific La Luna source rock (Nelson et al.,
2000).These plays may have effective porosity at
depths below 3000 m, related to brittle fracturing of
the recently deformed basement matrix.
Many basement highs have been drilled in the Cuu
Long Basin; none, however, have the production
capacity of Bach Ho. Historically, geologists working
in the Cuu Long Basin have used the weathered
“buried hill” plays of China as appropriate analogues.
However, the weathered crust at Bach Ho is less than
30 m thick. Also, sediment “haloes” are absent from
151
Bach Ho, and there is little evidence that surfacedriven meteoric alteration was responsible for
generation of secondary porosity. Nor can the
hydrothermal circulation, as inferred by Areshev et
al. (1992), explain the porosity. Both meteoric
leaching and hydrothermal alteration tend to degrade
effective porosity at Bach Ho, not enhance it. Rather,
porosity at Bach Ho is largely tied to zones of brittle
fracturing associated with late Oligocene thrusting and
subsequent gravity-driven extension. A more
appropriate structural analogy for Bach Ho would
therefore be the La Paz field.
CONCLUSIONS
Compared to many other “buried hill’ plays
worldwide, Bach Ho field is unusual in that the
fractured reservoir matrix is largely made up of
unaltered acid igneous lithologies (mostly granites and
granodiorites). Unlike classic “buried-hill’ plays, a
“halo” or “granite wash” of reworked basement
material is absent. A large-scale NE-SW trending, late
Oligocene reverse fault system cross-cuts the field,
with approximately 2000m of lateral displacement in
the Central Block, where a block of basement rock
was emplaced on top of organic-rich EoceneOligocene mudstones.
Using a combination of core-calibrated wireline
and seismic data, the igneous matrix at Bach Ho can
be divided vertically into three zones (A-C): the two
lower zones (below 3800-3900m) are relatively
weakly deformed and are in general sub-economic;
the upper zone, which constitutes the main reservoir
in the Central Block, is strongly fractured. Fractures
have a variety of origins, and include pre-existing
structures reopened during late Oligocene deformation
and more recent syntectonic fractures.
Future exploration offshore Vietnam should not
just define “buried hills”, but should attempt to
identify basement highs where fractured basement
blocks have been thrust onto mudstone successions. .
Postscript
This study, based on the results of more than 30 years
of exploration and development by Vietnamese and
other earth scientists, underlines a number of applied
geological principles known to explorationists for
many years.
(1) You will not fully understand a complex
reservoir before you must produce from it.
(2) Seismic collected across possible fractured
basement reservoirs gives a reliable indicator of
structural culminations in a basin and so defines the
first-order drilling targets. Subsequent higher seismic
resolution obtained from a 3D survey may better
define additional targets, and have regional
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
152
Classic “Buried-Hill” fields where the reservoir is a combination of fractured weathered basement and a variably developed clastic apron. Much of the ability of the basement to store and flow hydrocarbons is due to exposure and weathering of the fractured basement when it was subaerially exposed and supplying clastics to the adjacent sediment
apron
“Granite wash” play - most of the reservoir resides in a clastic apron
derived from weathering and mechanical reworking of material eroded
from an adjacent or underlying igneous-metamorphic high.
Table 2. Summary of oil- and gasfields known to have produced economic volumes of hydrocarbons from
fractured and variably weathered igneous and metamorphic basement highs, where the unaltered host rock
had little or no permeability.
Field and
Setting
Age and
depth
General reservoir character
Seal
References
Utikuma & Red
Earth fields,
Peace River
Arch, Alberta,
Canada
Middle to
Upper Devonian clastics
≈ 1500-2500
m
Granite wash is organized into two systems of
partially superimposed clastic wedges (alluvial fan
deltas & estuaries) that interfinger with, and are
onlapped by, the evaporitic Keg River and Muskeg
Formations. Lower and upper Granite Wash clastic
wedges reach a combined maximum thickness of
70 m. In the Devonian the Peace River Arch (Precambrian granitic/cratonic core) formed an active
series of parallel horsts (with exposed crests) and
graben depressions (basins).
Devonian
Muskeg Fm
(marine-fed
basinwide
evaporite)
Cant, 1988;
Dec et al.,
1996
Elk City, HallGurney, Gorham, Panhandle and other
fields along the
Kansas Uplift
and the
Amarillo Arch,
USA
Precambrian
basement,
Pennsylvanian clastics
(wash) and
Permian carbonates (regional reservoir)
≈ 1000 -2700
m
Minor, deeper reservoir in fractured, variablyweathered pink granites and quartzites in some
fields, but clastic production is mostly within a
drape made up of a mechanically reworked
Paleozoic grus and arkose “halo” and in syn-high
reefal build-ups. The high/arch formed in a prePennsylvanian collision event. There is some 120
m of fault displacement at the top of the
Precambrian granite on the SW flank on the
regional (10 km long) Gorham anticline.
Permian
salts and
mudstones,
(marine-fed
platform
saltern* and
mudflat)
Landes, 1960;
Sneider et al.,
1977;
Walters, 1991;
Sorenson,
2005
Clair field,
offshore
Shetlands, UK
Precambrian
(Lewisian)
basement
1850 m
Reservoir is elongate NE-SW trending, normally
faulted horst made up of a combination of
fractured Devonian-Carboniferous redbeds and
Lewisian granites. Most storage is in clastics but
basement fractures are higher permeability conduits
Cretaceous
mudstone
(marine)
Coney et al.,
1993
Xinglongtai
field,
Western depression of the
Liaohe Basin,
China
Precambrian
≈ 2700 m
Highly fractured and weathered Archean granites
and Mesozoic volcanics with a clastic halo in and
about a horst block bound by normal faults. Relief
on the Archaean-cored “hill” is 1000 m and the oil
column in the field is 700 m high.
Tertiary
mudstone
(lacustrine)
Luo et al.,
2005;
P’An, 1982
Dongshenpu
field, Da Ming
Tun Depression, China
Precambrian
≈ 2800 m
Produces from fractured and weathered Archean
metagranites and Proterozoic metasediments.
Structure is the higher part of normally faulted set
of basement blocks (edge of half graben) and is
onlapped by Tertiary sediments (source and seal).
Tertiary
mudstone
(lacustrine)
Xiaoguang &
Zuan, 1991
Yaerxia field,
Western part of
the Liuxi Basin,
China
Silurian
(Caledonian)
basement
≈ 2200 metres
Highly fractured and weathered Palaeozoic metamorphic host making up the crestal zone the
normal-faulted (wrench-faulted?) Yaerxia horst
block. Situated along trend from the thrusted
asymmetric anticline of Laojunmiao field, which is
broken up by imbricated minor thrust faults on
forelimb. Intense thrusting during Miocene.
Tertiary
mudstone
(lacustrine)
Guangming &
Jianguo, 1992;
P’An, 1982
Zeit Bay &
Geisum fields,
Gulf of Suez,
Egypt
Precambrian
basement
≈ 1200 metres
Highly fractured and weathered basement granitic
basement horst forming a fault-defined high within
Gulf of Suez rift. The 250 m thick oil column in Zeit
Bay field extends from Mesozoic sandstones and
granite wash down into the fractured and weathered basement, which contains one-third of oil in
the field.
Miocene
evaporites
(basinwide*
salt &
evaporitic
mudstone)
Salah & Alsharhan, 1998
Younes et al.,
1998
Trinh Xuan Cuong and J. K. Warren
153
Reservoir is in a thrust-associated, fracture-dominated brittle basement.
Little or no contribution from a sediment halo to oil/gas storage. Thrusting
places reservoir in proximity to source rock.
Table 2 (continued).
Augila-Nafoora
field, Sirte
Basin,
Libya
Late Precambrian or early
Cambrian
basement, ≈
2400 m
Rotated and exposed rift blocks highs with some
production from fractured and weathered granites
beneath the main Cretaceous Lower Rakb carbonate reservoir.
Late Cretaceous
mudstone
(marine)
Williams, 1972
La Paz-Mara
fields,
Venezuela
Palaeozoic
≈ 4000 m
Inverted (thrusted) wrench-fault block with fractured granite/granodiorites and schists beneath
fractured Mesozoic carbonate (initial anticlinal
reservoir prior to deepening in early 1950s).
Thrusting/inversion began prior to Eocene but
was most intense in Miocene-Pliocene. The total
hydrocarbon column is ≈ 915 m high, 305 m of
which is in a basement reservoir made up of fractured basement with little evidence of matrix
weather.
Late Cretaceous
mudstone
(marine)
Nelson et al.,
2000
Smith, 1956
ZdaniceKrystalinkum
field,
Czech Republic
Precambrian
≈ 900 m
Fractured granites and granodiorites subjected to
exposure and weathering prior to Miocene when
the Upper Eocene to Oligocene pelitic seal was
thrust into place by Zdanice-Subsilesian Nappe
event (structure is not well documented).
Eocene Oligocene
mudstone/pelite
(marine)
Krejci, 1993
Sarkadkeresztúr field, Pannonian Basin,
Hungary
Precambrian
& Tertiary?
≈ 2700 m
Majority of oil is stored in a narrow horst block
composed of fractured Precambrian
metamorphics. Oil column is 370 m thick (structure
is not well documented)
Tertiary
mudstone
(Lacustrine?)
Dank, 1988
Inverted (thrusted) rift block of fractured granite/granodiorite. Thrusting occurred mostly in the
Late Oligocene. Oil column height is around 1000
m.
Upper Oligocene
muds
(lacustrine?)
This paper
Bach Ho field
Cretaceous
(mostly)
Offshore Vietnam, Cuu Long ≈ 3500 m
Basin
significance for future exploration (e.g. the importance
of the cantilevered blocks in controlling reservoir
quality at Bach Ho), but still may not be capable of
reliably defining variation in quality in the known
reservoir blocks within the survey area.
(3) Often it is the simplest wireline tools, which
make direct physical measurements through a complex
reservoir, which are the most reliable in terms of
indicating variations in reservoir quality. Thus caliper
logs are the most reliable indicators of fracture density
and reservoir quality in wells at Bach Ho.
“Experience is a hard teacher because she gives the
test first, the lesson afterwards”
ACKNOWLEDGEMENTS
The wireline, rock property, and seismic data from
Bach Ho field was provided by Petrovietnam, as was
access to the Bach Ho core. XRD analytical services
were kindly donated by Core Laboratories, Malaysia
and the various software modules used to manipulate
the seismic and wireline data were kindly donated by
Schlumberger. Comments by J. Gutmanis (Geoscience
Ltd, UK) and G. H. Lee (Pukyong National
University) on a previous version of the MS are
acknowledged with thanks.
REFERENCES
AGUILERA, R., 1993. Formation evaluation of naturally
fractured reservoirs. In: M. D. Diana and A. M.Woods (Eds),
Development geology reference manual. AAPG, Tulsa,
Oklahoma, pp 192-193.
ARESHEV, E. G., T. L. DONG, N. T. SAN and O. A. SNIP, 1992.
Reservoirs in fractured basement on the continental shelf
of southern Vietnam. Journal of Petroleum Geology, 15, 451464.
AYDIN,A., 2000. Fractures, faults, and hydrocarbon entrapment,
migration and flow. Marine and Petroleum Geology, 17, 797-
154
Bach Ho field, offshore SE Vietnam: A fractured basement reservoir
814.
CANH, T., D. V. HA, H. CARSTENS and S. BERSTAD, 1994.
Vietnam‚ Attractive plays in a new geological province. Oil
& Gas Journal, 92, 78-83.
CANT, D. J., 1988. Regional Structure and Development of
the Peace River Arch, Alberta: A Paleozoic Failed-Rift
System? Bull. Canad. Petrol.Geol., 36, 284-295.
CONEY, D., T. B. FYEE, P. RETAIL and P. J. SMITH, 1993. Clair
Appraisal: The Benefits of a Co-operative Approach. In:
Proc. 4th Conference Petroleum Geology of Northwest
Europe, p. 1409-1420.
CUONG, T. X., 2000. Reservoir Characterisation of naturally
fractured and weathered basement in Bach Ho field,
Vietnam. Universitii Brunei Darussalam, Masters Thesis,
Bandar Seri Begawan.
DANK,V., 1988.The Pannonian Basin:A Study in Basin Evolution
(Chapter 23). AAPG Mem., 45, 319-331.
DEC, T., F. J. HEIN, and R. J. TROTTER, 1996. Granite wash
alluvial fans, fan-deltas and tidal environments,
northwestern Alberta: implications for controls on
distribution of Devonian clastic wedges associated with
the Peace River Arch. Bull. Canad. Petrol.Geol. 44, 541-565.
DIEN, P. T., P. S. TAI, L. V. DZUNG, P. V. TIEM and D. V. NHUAN,
1998. Late-Mesozoic and Early Cenozoic events and
petroleum system of the Cuu Long Basin. PetroVietnam
Review, 1-1998, 4-23.
DMITRIYEVSKIY, A. N., F. A. KIREYEV, R. A. BOCHKO, and T. A.
FEDOROVA, 1993. Hydrothermal origin of oil and gas
reservoirs in basement rock of the South Vietnam
continental shelf. International Geology Review, 35, 621-630.
DONG, T. L., 1996. Metamorphism process of magmatic rocks
in basement of Bach Ho and Rong fields. PetroVietnam
Review, 3-1996, 13-17.
DONG, T. L., and F. A. KIREEV, 1998. Igneous rocks in the Bach
Ho field and characters of reservoir in the basement.
PetroVietnam Review, 6-1998, 2-12.
DONG, T. L., P. H. LONG, H. V. QUY, P. D. HAI, and T. S. HAU,
1999. Distribution of fractures, faults and their formation
in the basement of South Vietnam continental shelf and
adjacent areas. PetroVietnam Review, 3-1999, 4-13.
DONG, T. L., H. V. QUY, and P. D. HAI, 1996. Geological model
of the basement in the Bach Ho field: PetroVietnam Review,
4-1996, 2-7.
DONG, T. L., H. V. QUY, and P. D. HAI, 1997. Distribution
characteristics of pore spaces and model of reservoir rocks
in the Bach Ho Basement. PetroVietnam Review, 3-1997, 28.
ERSHAGHI, I., 1995. Evaluation of naturally fractured reservoirs:
PE509 Petroleum Engineering. IHRDC Video library for
exploration and production specialist.
GUANGMING, Z., and S. JIANGUO, 1992. Laojunmiao Field—
People’s Republic of China Jiuquan Basin, Gansu Province.
In: E. A. Beaumont and N. H. Foster (Eds), Treatise of
Petroleum Geology; Atlas of Oil and Gas Fields. Structural
Traps VII, AAPG, Tulsa, OK, p. 159-172.
HALL, R., and C. K. MORLEY, 2004. Sundaland Basins. In: P.
Clift, P. Wang, W. Kuhnt and D. E. Hayes (Eds), ContinentOcean Interactions within the East Asian Marginal Seas.
AGU Geophysical Monograph, 149, 55-85.
HARRELSON, D. W., 1989. Hydrocarbon Occurrences in
Igneous and Metamorphic Rocks: The Plays of the 1990s.
Transactions - Gulf Coast Association of Geological Societies,
39, 85-95.
HOLZHAUSEN, G., 1989. Origin of sheet structures: 1.
Morphology and boundary condition. Engineering Geology,
27, 225-278.
HORBY, B. E., and S. M. LUTHI, 1992. An integrated
interpretation of fracture apertures computed from
electrical borehole scans and reflected Stoneley waves.
Geological Society London Special Publication, 65, 185-198.
HUNG, N. D., H. V. LE, H. VU, and H. C. MINH, 2003.
Hydrocarbon Geology of Cuu Long Basin - Offshore
Vietnam (extended Abstract). AAPG International Meeting,
Barcelona Spain. AAPG Bull., 87.
HUTCHISON, C. S., 2004. Marginal basin evolution: The
southern South China Sea. Marine and Petroleum Geology,
21, 1129-1148.
KALAN,T., H. P. SITORUS, and M. EMAN, 1994. Jatibarang Field,
Geologic Study of Volcanic Reservoir for Horizontal Well
Proposal. In: Proc. 23rd Ann. Convention, Indonesian
Petroleum Assoc., Jakarta, Paper IPA94-1.1-186, p. 229-244.
KREJCI, J. J., 1993. Zdanice-Krystalinikum field - Czechoslovakia,
Carpathian Foredeep, Moravia. In: E. A. Beaumont and N.
H. Foster (Eds), Treatise of Petroleum Geology; Atlas of
Oil and Gas Fields. Structural Traps VIII. AAPG, Tulsa, OK,
153-173.
KOEN, A. D., 1995. Discoveries, production add luster to
offshore Vietnam’s outlook. Oil & Gas Journal, 93, 17-21.
LANDES, K. K., L. J. CHARLESWORTH JR., F. HEANY, and P. J.
LESPERANCE, 1960. Petroleum Resources in Basement
Rocks. Bull. AAPG, 44, 1682-1691.
LEE, G. H., K. LEE and J. S.WATKINS, 2001. Geologic evolution
of the Cuu Long and Nam Con Son basins, offshore
southern Vietnam, South China Sea. AAPG Bull., 85, 10550182.
LUO, S., S. MORAD, Z. LIANG, and Y. ZHU, 2005. Controls on
the quality of Archean metamorphic and Jurassic volcanic
reservoir rocks from the Xinglongtai buried hill, western
depression of Liaohe basin, China. AAPG Bull. 89, 1319-46.
MORLEY, C. K., 2004. Nested strike-slip duplexes and other
evidence for Late Cretaceous-Paleogene transpressional
tectonics before and during India-Eurasia collision, in
Thailand, Myanmar and Malaysia. Journ. Geol. Soc. London,
161, 799-812.
NELSON, R.A., E. BUENO, E. P. MOLDOVANYI, C. C. MATCEK,
and I. AZPIRITXAGA, 2000. Production characteristics of
the fractured reservoirs of the La Paz Field, Maracaibo
basin, Venezuela. AAPG Bull. 84, 1791-1809.
NERMAT-NASSER, S., and H. HORRI, 1992. Compression
induced non planar crack extension with application to
splitting, exfoliation, and rockbust. Journal of Geophysical
Research, 87, 6805-6821.
NORTH, F. K., 1990. Petroleum Geology. Second Ed.:
Winchester, Mass, Unwin Hyman Ltd, 631 pp.
OSIPOVE, M. A., 1974. Granitoid contraction and endogenic
mineralogenesis. Nauka, Moscow, 158 p.
P’AN, C.-H., 1982. Petroleum in Basement Rocks. Bull. AAPG,
66, 1597-1543.
POWERS, S., 1932. Symposium on occurrence of petroleum
in igneous and metamorphic rocks. Bull. AAPG, 16, 717859.
QUE, P. H., 1994, History of geological development of the
Cuu Long Basin. PetroVietnam Review, 2-1994, 5-16.
QUY, H. V., I. I. DEMUSKIN and P. D. HAI, 1997. Geological
characteristics in the model of the Bach Ho Basement.
PetroVietnam Review, 2-1997, 22-25.
REID, I. S. A., 1997. Petroleum exploration in Vietnam. PetroMin,
July, 32-34.
RIDD, M. F., 1981. Petroleum geology west of the Shetlands. In:
L. V. Illing and G. D. Hobson (Eds), Petroleum Geology of
the Continental Shelf of North-West Europe. London,
Heyden and Son.
ROZANOVA, E. P., I.A. BORZENKOV, and A. L.TARASOV, 2001.
Microbial processes in a high-temperature oil field (in
Russian). Microbiology (Mikrobiologiya), 66, 718-725.
SALAH, M. G. and A. S. ALSHARHN, 1998. The Precambrian
basement: a major reservoir in the rifted basin, Gulf of
Suez. Journal of Petroleum Science and Engineering, 19, 201222.
SCHLUMBERGER, 1992. Well Service Manual: Fractures
Trinh Xuan Cuong and J. K. Warren
analysis. 5.1.
SCHLUMBERGER, 1997a. Well Service Manual: Fractures. 4.14.28.
SCHLUMBERGER, 1997b. Log Interpretation Charts.
SCHUTTER, S. R., 2003a. Hydrocarbon occurrence and
exploration in and around igneous rocks. Geological Society,
London, Special Publications, 214, 7-33.
SCHUTTER, S. R., 2003b. Occurrences of hydrocarbons in
and around igneous rocks. Geological Society, London, Special
Publications, 214, 35-68.
SEARLE, M. P., 2006. Role of the Red River Shear zone,Yunnan
and Vietnam, in the continental extrusion of SE Asia. Journal
of the Geological Society, 163, 1025-1036.
SHI, X., X. QIU, K. XIA, and D. ZHOU, 2003. Characteristics of
surface heat flow in the South China Sea. Journal of Asian
Earth Sciences, 22, 265-277.
SIBBIT, A. M., 1996. Quantifying porosity and estimating
permeability from well logs in fractured basement
reservoirs. SPE 30157.
SMITH, J. E., 1956. Basement Reservoir of La Paz-Mara Oil
Fields, Western Venezuela. Bull. AAPG 40, 280-385.
SNEIDER, R. M., F. H. RICHARDSON, D. D. PAYNTER, R. E.
EDDY, and I. A. WYANT, 1977. Predicting reservoir rock
geometry and continuity in Pennsylvanian reservoirs, Elk
City field, Oklahoma. J. Petrol. Tech., 29, 851-866.
SORENSEN, R. P., 2005. A dynamic model for the Permian
Panhandle and Hugoton fields, western Anadarko basin.
AAPG Bull. 89, 921-938.
STANDEN, E., 1991. Tips for analyzing fractures on wellbore
images. World Oil, 212, 99-118.
STEARNS, D. W., and M. FRIEDMAN, 1972. Reservoirs in
fractured rock. AAPG Memoir, 16, 161-164.
TANDOM, T. M., T. X. NHUAN, H. D. TJIIA, and S. A.
SPAGNUOLO, 1997. Fractured basement reservoir
characterization, offshore Southern Vietnam: Third
International Well Logging Symposium of Japan, Chiba,
Japan, September 1997.
TAPPONNIER, P., M. PELTZER, and R. ARMIJOR, 1986. On the
mechanisms of the collision between India and Asia. In:
Collision Tectonics. Geological Society, London, Special
Publication,19, 115-157.
TAPPONNIER, P., G. PELTZER, A.Y. LE DAIN, R.ARMIJO, and P.
COBBOLD, 1982. Propagating extrusion tectonics in Asia:
new insights from simple experiments with plasticine.
155
Geology, 10, 611-616.
TAYLOR, B., and D. E. HAYES, 1983. Origin and history of the
South China Sea. In: D. E. Hayes (Ed.), The tectonic and
geological evolution of South East Asian seas and islands.
Washington DC. Geophys. Monogr. Am. Geophys. Union, 27,
23-56.
TEN HAVEN, H. L., 1996. Applications and limitations of
Mango’s light hydrocarbon parameters in petroleum
correlation studies. Organic Geochemistry, 24, 957-976.
TJIIA, H. D., P.TANDOM, M.,A. MOHAMAD, and T. X. NHUAN,
1998. Fracture basement study on the blocks 01 & 02 the
Cuu Long Basin, Southern Vietnam. Conference on Vietnam
Petroleum Institute; 20 year development and prospects.
Hanoi, May 1998, p. 68-90.
TRAN, C., V. H. DO, H. CARSTENS, and S. BERSTAD, 1994.
Vietnam — attractive plays in a new geological province.
Oil & Gas Journal, 93, 78-83.
TUAN, P. A., O. F. MARTYNSIV, and T. L. DONG, 1996.
Heterogeneity of fractured granite: effects on petrophysical
properties and water saturation of preserved cores: 1995
SCA Symposium, San Francisco, 12-14 Sept. 1995.
VINH, N. X., 1999. Main alteration processes of granitoid
basement rocks of the Cuu Long Basin and the relationship
to their reservoir properties. PetroVietnam Review, 4-1999,
18-30.
WALTERS, R. F., 1991. Gorham Oil field, Russell County, Kansas.
Geological Survey Bulletin Kansas, 228, 112 p.
WILLIAMS, J. J., 1972. Augila field, Libya: Depositional
environment and diagenesis of sediment reservoir and
description of igneous reservoir. In: Stratigraphic oil and
gas fields. AAPG Mem., 16, 623-632.
WILSON, J. T., G. F. MAINZER, F. ESCOBAR, G. AGUIRRE, and
J. S. DEAN, 1993, Preliminary Assessment of the Lago
Mercedes Discovery, Magallanes Basin, Chile (abstract).
AAPG Bull., 77, 313.
Xiaoguang, T., and H. Zuan, 1991, Buried-Hill Discoveries of
the Damintun Depression in North China. AAPG Bull., 75,
780-794.
YOUNES, A. I., T. ENGELDER, and W. BOSWORTH, 1998.
Fracture distribution in faulted basement block: Gulf of
Suez, Egypt. In: M. P. Coward,T. S. Daltaban, and H. Johnson,
(Eds), Structural Geology in Reservoir Characterization.
Geological Society, London, Special Publication 127, 167-190.