Shallow Clues for Deep Exploration
Seismic interpreters study images of present-day sedimentary deposits to recognize similar features in
the subsurface. Applying this technique to surveys offshore West Africa reveals features that could be
high-quality sands containing hydrocarbons.
Douglas Evans
Gatwick, England
The best place to look for oil is near existing
discoveries—especially when it comes to exploring offshore. This simple axiom is encouraging
governments to offer leases on acreage blocks
adjacent to known discoveries, attracting a number of oil and gas companies to explore in
ever-increasing water depths. In these environments, a deepwater well can cost $25 million or
more, with no guarantee of success.
To improve the chance of success, seismic
interpreters evaluating unexplored regions rely
on high-quality three-dimensional (3D) seismic
data that provide a continuous image of the subsurface. This gives a clear picture of promising
structures, sedimentary sequences and other key
elements of potential drilling targets. Every
prospect needs the requisite source rock, reservoir, trap and seal—all occurring in the proper
geologic timing and successions—to become a
viable target. However, because of the high risk
associated with wildcat exploration, many
For help in preparation of this article, thanks to John
English, Gatwick, England; and George Jamieson, Houston,
Texas, USA.
1. www.angola.org/fastfacts/economic.html
2. Kwanza is sometimes spelled Kwanzaa or Cuanza.
2
exploration and production (E&P) companies
delay acquiring proprietary 3D surveys until they
have licensed the acreage. In the absence of 3D
surveys, companies turn to widely spaced twodimensional (2D) seismic lines, between which
the interpreter may have to make unsupported
guesses about geologic structures and stratigraphic changes.
To satisfy a need for early high-quality 3D
seismic data on unlicensed acreage at lower risk
and lower cost to E&P companies, seismic
contractors offer surveys for use by multiple
clients. At their own risk, seismic contractors
acquire and process 3D surveys over blocks of
acreage not yet licensed for exploration, then sell
access to, or license, the seismic data to companies interested in evaluating the block before
entering a bid. In the case of WesternGeco, multiclient 3D marine surveys have been acquired
over an estimated 500,000 km2 [193,000 sq miles]
of the Gulf of Mexico, offshore West Africa, the
North Sea, Indonesia and Australia.
An example of where these surveys have
been used extensively is offshore Angola. Here,
the Congo River has been depositing sediments
for more than 90 million years. The conditions for
a productive petroleum system have come
together to create some of the largest discoveries of the last decade (next page). In
Block 17, at 1350-m [4429-ft] water depth,
TotalFinaElf (TFE) is operating the giant
Girassol field, which contains 700 million barrels
[111 million m3] of oil reserves. Industry experts
say Blocks 14 (operated by ChevronTexaco),
15 (ExxonMobil), 17 (TFE) and 18 (BP) may contain
up to 10 billion barrels [1.5 billion m3] of recoverable oil.1 Most of these deepwater discoveries
are in channelized sands deposited by the Congo
River system during the Tertiary period. The
Angolan government has agreements with companies to explore Blocks 31, 32 and 33. Sonangol
and Norsk Hydro are exploring the recently
licensed Block 34.
The deepwater blocks to the south of the
giant fields have not yet been licensed. These lie
in the deepwater Kwanza basin, where conditions for source rock, hydrocarbon generation,
structure and seal have yet to be proved.2 Given
the size of the finds to the north, the undesignated and unlicensed areas, also known as
“open” areas, are of significant interest to exploration companies.
Oilfield Review
Boma
14
Plutao
Marimba
Kissanje
Congo R
Essungo
iver
1
31
15
2
16
3
32
Girassol 17
Rosa
Tulipa
4
Dalia
A N G O L A
Cromio
Platina
33
5
18
34
6
Luanda
19
Kwanza River
20
7
21
8
22
0
km
100
0
miles
62
23
9
> Large discoveries offshore Angola. In Block 17, at 1350-m [4429-ft] water depth, TotalFinaElf, with partners ExxonMobil, BP, Statoil and Norsk Hydro, is
operating the giant Girassol field, containing 700 million barrels [111 million m3] of oil reserves. Blocks 14 , 15, 17 and 18 may contain up to 10 billion barrels
[1.5 billion m3] of recoverable oil. The deepest discovery so far is the BP Plutao well in Block 31.
Winter 2002/2003
3
Several recent wells have been drilled in the
shallower water of the Kwanza basin to test play
concepts different from those found in the
deeper water farther offshore. The wells have
come in with mixed results; only the ExxonMobil
Semba-1 well flowed, at about 3000 B/D
[477 m3/d] during testing.3 This has provided
encouragement that there is an active petroleum
system in this basin.
Farther offshore, WesternGeco has acquired multi-
of the Kwanza basin, including a 7000-km2
[2700-sq mile] subset of the 3D dataset that we
examine in more detail (below). The dominant
feature visible in the high-resolution seafloor
image is the Kwanza Canyon, an active submarine
canyon that serves as a “surface” example, or
analog—albeit on the present-day seafloor—for
understanding subsurface structures interpreted
in the 3D seismic images. The canyon is of particular interest because it is presumed to act as the
conduit that carries sands from the African continent to the deep waters offshore Angola.
This article describes how seismic interpreters use images of present-day deposits in
this offshore province to detect similar features
in the subsurface, features that could be highquality sands charged with hydrocarbons. We
begin with the shallow, seafloor expression of
the Kwanza Canyon, and track its manifestation
in deeper, more mature sediments, before examining the resultant abyssal-plain deepwater fan
client 3D data over 19,000 km2 [6840 sq miles]
Boma
14
Congo R
15
31
iver
1
2
A N G O L A
16
3
32
17
33
4
5
18
34
6
Luanda
19
Kwanza River
20
7
21
8
22
0
km
100
0
miles
62
23
9
> Multiclient 3D seismic surveys (pink) acquired by WesternGeco. Light pink areas show surveys
acquired over unlicensed acreage, and darker pink areas show surveys acquired over areas that
were licensed at the time of seismic survey acquisition. The trapezoidal area outlined in magenta is
the area covered by the bathymetric data shown on the next page.
4
Oilfield Review
by analogy to the Congo Fan. Finally, we show
some possible exploration targets that have been
identified using the knowledge built on these
Kwanza and Congo examples.
Starting at the Surface
The Kwanza Canyon was first identified on
bathymetry data acquired, in association with a
gravity survey by ARK Geophysics Ltd., at the
time of the 3D seismic data acquisition (below).
The underlying salt diapirism is currently active
and causes the noticeable north-south grain, or
topography, of the seabed. The canyon starts
near Luanda, Angola, in 50-m [164-ft] water
depth and extends east to west—cutting across
the regional grain but always following the continental slope—for about 300 km [188 miles],
eventually reaching the abyssal plain in water
depths of more than 4000 m [13,100 ft].
Abandoned meanders and “oxbow lakes” cut off
by chute channels correspond to earlier channel
Kwanza Canyon
cuts of the submarine “river” before it became as
deeply incised as it is today.4 The canyon is of
variable width, but generally about 1 to 2 km
[0.6 to 1.2 miles] wide and 400 m [1300 ft] deep
in the area covered by the 3D survey. The water
depth varies from 1200 m [4725 ft] in the east to
2500 m [8203 ft] in the west.
3. http://www2.exxonmobil.com/Corporate/Newsroom/
Newsreleases/corp_xom_nr_140601_1.asp
4. An oxbow is a U-shaped bend in a river. The gradual
meandering of rivers commonly produces cutoff
bends, visible as oxbow lakes, that mark the river’s
former course.
Feeder canyon
0
km
10
0
miles
6.2
Incipient channel
Pockmark
> Seabed topography of the Kwanza Canyon, first identified on bathymetry data acquired in association with a gravity survey. Color indicates depth below
sea level, with yellow as shallow and purple as deep. Underlying salt diapirism causes the north-south trends visible in the seabed. The canyon crosscuts
these north-south features. It starts in the shallower water in the east and extends westward for about 300 km [188 miles]. This image covers about 7000 km2
[2700 sq miles] of seabed.
Winter 2002/2003
5
1
3
2
4
Kwanza Canyon
Feeder
canyons
Seafloor “island”
4
3
2
1
> Bathymetry of the Kwanza Canyon (bottom) and cross sections extracted from 3D seismic data (top) in the eastern portion of the canyon. Cross sections
are numbered starting at the shallower end of the canyon. Panel 1: Two feeder canyons south of the main canyon are seen close to their point of origin,
which is not seen in the 3D volume. Panel 2: The feeders form a subsea ‘island.’ The bases of the channels show high-amplitude reflections, possible indicators of sand. Panel 3: The main Kwanza Canyon comes in from the east, and at this point is at a depth of 500 msec, or 400 m [1312 ft]. The canyon exploits
the crest of a salt diapir in the subsurface (outlined in red), probably a zone of weakness that is more easily eroded. Panel 4: The Kwanza Canyon is less
deeply incised and is on the flank of the salt diapir.
Round depressions, or pits—”pockmarks,”
as they are known in the industry, are visible on
the seafloor image throughout this area. These
are caused by escaping gas or fluids bubbling up
from the subsurface, and can provide direct evidence of present-day hydrocarbon migration
from deeper hydrocarbon source rocks. The
pockmarks exploit zones of weakness and fracturing, eventually coalescing into continuous
linear features, forming the troughs that are
6
commonly seen in the seafloor along the flanks
of salt diapirs. These troughs may also have
some influence on the routes taken by features
such as the Kwanza Canyon.
Cross sections through the shallow section of
the 3D volume show the canyon at different
points along its course (above). In the first two
panels, smaller feeder channels, or side canyons,
can be seen cutting through shallow sediments
before joining the main Kwanza Canyon. In the
third and fourth panels, the canyon cuts near a
salt subcrop. Interpretation of the seismic lines
determined that, at the time they were acquired,
there was no sediment in the Kwanza Canyon;
there are no rugose or mounded reflections seen
at the base of the channel cut, either of which
would be indicators of unconsolidated sediments
progressing down the canyon. Interpreters speculate that sediment movement along the canyon
could be intermittent but rapid, taking the form of
a debris- or mass-flow deposit, similar to those
seen occasionally onshore.
Oilfield Review
6
5
Filled earlier canyon
8
7
High-amplitude
canyon bottom
Oxbow
Main canyon
8
7
6
5
> Bathymetry of the Kwanza Canyon (bottom) and cross sections extracted from 3D seismic data (top) in the western portion of the canyon. Panel 5: The
canyon follows the predominant north-south grain imposed by underlying salt diapirism. A filled earlier canyon can be seen to the left of the present-day
canyon. Panel 6: Earlier traces of the canyon are seen in sediments beneath the current canyon. Salt diapirism is no longer evident. Panel 7: The main
canyon cuts across an oxbow, abandoning it and leaving the oxbow higher. Older cuts that have been filled are preserved below the current channel.
Panel 8: The main channel widens and deepens in the deeper water near the boundary of the 3D survey.
As the canyon moves into deeper water, its
course becomes independent of salt-related
structures (above). In the first two panels, labeled
5 and 6, an earlier cut of the canyon can be seen
to the left of its present-day position. By the second panel (6), salt diapirism no longer seems to
control the canyon’s location or direction. Panel 7
shows the canyon and a nearby oxbow.
Immediately under the canyon cut, high-amplitude reflections point to preserved older channel
Winter 2002/2003
fill. In the last panel, the canyon widens and
deepens just before it disappears across the
boundaries of the 3D survey. The meandering
form of the canyon is believed to be a consequence of the interaction between salt diapirs
and the angle of slope of the continental shelf.
Submarine canyons seen on the Nigerian continental shelf beyond the Niger delta tend to be
more linear, because there is no salt tectonism to
alter the seabed topography and interact with
the canyons.
What happens to the Kwanza Canyon beyond
the boundaries of the 3D survey? Despite a tight
grid of 2D data in the nearshore area of Block 6,
the canyon cannot be traced back to the continent. This could indicate that it is not related to a
present-day river system onshore. However, the
canyon may connect to the mouth of the Kwanza
River, some 50 km [30 miles] to the south via the
strong northerly current known as the Benguela
Current, which passes along the West African
7
coast here (below). The Kwanza Canyon starts
where there is a break in the coastline, near a
spit, 50 km north of the mouth of the Kwanza
River. The Kwanza Canyon may also relate to an
earlier and different drainage system than the
one that exists today.
At its distal, deepwater end, the Kwanza
Canyon is not covered by the 3D survey, but can be
traced across 2D surveys, eventually reaching a
point at which there currently are no data to trace
it farther. Interpreters can conjecture that a deepwater fan exists at the end of the canyon system,
where sands funnelled by the Kwanza Canyon are
deposited on the abyssal plain; an image of this
system for the Kwanza Canyon remains unavailable. However, 3D datasets acquired in the
abyssal plain of the prolific Congo Fan to the north
show the likely depositional regime. These will be
discussed later in this article.
cuts stacked within and beneath the yellow surface. This system is located on a present-day
structural high. Sediment thinning into this high
suggests that this was also a high at the time of
canyon cutting. The high-amplitude events in the
bottom of the canyon indicate sand-rich channel fill.
To visualize this canyon and the sediments
that fill it, seismic amplitudes measured in an
interval including the channel bottom and
extending up to 50 to 100 msec above the yellow
event are plotted (next page, bottom). The high
amplitudes colored in yellow resemble a meandering system within the overall canyon cut. Fine-tuning
the time windows selected for amplitude extraction
probably would show more detail and complexity
within this system. The high-amplitude events
Evaluating Canyon Fill
An interpreted seismic line from the area to the
south of the Kwanza Canyon shows a preserved
canyon cut in the shallow subsurface about
500 msec below the seafloor, which itself is at
about 2.5 seconds two-way traveltime (next
page, top). This feature can be used to demonstrate the likely style of deposition and
preservation of a recent but now buried canyon.
A large channel complex delineated in yellow
can be interpreted through the volume. Detailed
examination of the data shows multiple channel
14
Boma
Congo Canyon
Congo R
15
31
iver
1
2
A N G O L A
16
3
32
17
33
4
5
18
34
Kwanza Canyon
6
Luanda
19
Kwanza River
20
7
0
km
100
0
miles
62
21
8
22
> The Kwanza Canyon, offset about 50 km [30 miles] from the mouth of the Kwanza River, and an abyssal
fan that might be inferred to exist in the deep water to the west.
8
Oilfield Review
Earlier filled canyon
A
High amplitudes in canyon bottom
A’
Two-way traveltime, sec
2.5
3.0
3.5
4.0
0
km
2.0
0
miles
1.2
> An interpreted seismic line showing a canyon (yellow) filled and preserved south of the Kwanza
Canyon. The bright reds and blacks represent high amplitudes at the base of the canyon and are signs
of sand-rich sediments filling the canyon. The seismic amplitudes at the bottom of the channel are
shown in the figure below.
A
0
km
4.0
0
miles
2.5
A’
> Amplitudes measured across a 3D seismic volume in a time interval constrained to the high-amplitude
bottom of the canyon interpreted in yellow in the figure above. High amplitudes in yellow show the
meandering nature of the preserved canyon. The location of the cross section in the above figure
is shown by the A to A’ traverse.
Winter 2002/2003
9
Meandering channel
Meandering channel in profile
Fan deposit in profile
Fan deposit
> A horizontal time-slice, or plan view, through a 3D seismic survey north of the Kwanza Canyon, revealing the high amplitudes of a large fan deposit on the
right and a meadering channel on the left.
are interpreted to be sands; the highest amplitude sands may be hydrocarbon-charged. The
gray low-amplitude areas are interpreted to be
mudstones or shales. Offshore Angola, sediments of the Tertiary period with high seismic
amplitude are generally indicators of sand
deposition, whereas low amplitudes indicate
mudstone, clay and shale.
Later movement of deeper salt has caused
uplift and erosion, and controls the limits of the
meandering system in the east-west direction;
the system cannot be traced any farther, and
10
therefore constitutes a potential hydrocarbon
trap. This is interpreted to be the mechanism of
reservoir deposition, preservation and trap formation that has occurred in the Congo and
Kwanza basins throughout the Tertiary period.
However, the exact mechanism for the establishment and abandonment of this type of canyon is
not well understood.
The seismic expression of a deepwater fan
can be seen in some of the seismic volume
acquired to the north, over the deepwater end of
the Congo Fan system. This provides an example
of the probable style of depositional regime that
may have occurred or be occurring at the end of
the Kwanza Canyon, where no direct evidence
exists. The 8000-km2 [3090-sq mile] northern survey covered part of the abyssal plain of the
Congo Fan as well as the salt-diapir province in
shallower water. A time-slice through the 3D
volume shows a fan and a meandering channel
(above). To a first approximation, the abyssal
plain is flat and horizontal, so the time-slice is
consistent with a depositional horizon. The
image displays roughly in plan view the two
types of high-amplitude reflections commonly
seen in this region, and also shows how these
Oilfield Review
look in cross section, or along seismic lines.
Long, extensive high-amplitude reflections are
likely to be fans or sheet sands. Short, stacked
high-amplitude reflections are likely to be
channelized sands. This information can be
applied in areas of greater structural complexity where horizon-consistent time-slices are
difficult to make.
Seen in vertical section, the sand amplitudes
diminish with depth (below). This is interpreted to
be due to a change in acoustic-impedance contrast caused by compaction. Sand channels in this
seismic section have been interpreted first in
time-slice displays, and then the intersections of
the channels with the seismic section are displayed as yellow circles. The channels clearly
correspond to short high-amplitude events, indicating that channels in this basin can be picked
with confidence from seismic sections, even at
6- to 7-second two-way traveltimes.
Below the blue horizon at about 7.5 seconds
two-way traveltime, sand channels are not evident on time-slices, so the first input of sand to
this part of the basin occurs at or above this
level. The regional uplift of the African coastal
areas, which occurred in the Oligo-Miocene
about 35 million years ago, provides the source
of the earliest sand influxes to the basin. Prior to
the Oligo-Miocene uplift, little or no sand is present. This information has been used to date the
age of these sections, giving the blue horizon an
age of the top of the Eocene epoch, just prior to
the Oligo-Miocene. The seismic character
changes below the top of the Eocene, indicating
major changes of environment and deposition.
Regional information identifies the interbedded
high-amplitude, laterally continuous events
found beneath the top of the Eocene as a potential source-rock interval, equivalent to the Iabe
formation. The Iabe is a proven source in Block 2
N
offshore fields such as Essungo. If the Iabe is present as a source rock in the Congo basin, then it
is early to mid-mature, given the thickness of
Tertiary sediment burying it, and therefore
presently capable of generating hydrocarbons.
Deeper Cretaceous (Albian) source rocks, close to
the basement, would also be mature if they exist.
Clues to Hydrocarbons
Direct hydrocarbon indicators (DHIs) provide evidence that hydrocarbons can be found in this
area. These DHIs typically are anomalously highamplitude reflections resulting from the
additional acoustic-impedance contrast generated by hydrocarbon as compared with water in
sands.5 Migration of hydrocarbon fluids occurs
through the minor fractures evident on the seismic sections.
5. Acoustic impedance is the velocity multiplied by the density of a rock. Both of these quantities vary, and usually
increase, with depth for most rock types.
S
5.5
6.0
Two-way traveltime, sec
6.5
7.0
Top Eocene
7.5
Top
Cretaceous
8.0
8.5
0
km
4.0
0
miles
2.5
Break-up
unconformity
> Seismic image showing several high-amplitude (black and red) sands near the top of the section
(down to about 6.5 seconds), lower-amplitude sand-filled channels deeper (down to about 7.5 seconds), and no obvious sand below about 7.5 seconds. The decrease in amplitude of the sand-filled
channels with depth is attributed to a lower acoustic-impedance contrast with increased compaction.
Yellow circles correspond to intersections of this seismic image with channels that were interpreted
on time-slices between 6.0 and 7.0 seconds.
Winter 2002/2003
11
Sand-rich deposit
Overbank deposit
Shale-plugged channel
Levee
Earlier fan deposit
> High-resolution detail of meandering channel from the deepwater Congo Fan, showing high-amplitude sand fill (yellow and red), low-amplitude shale
(gray), and middle-amplitude channel levees (green).
Dip-closure horizon
Fan
Canyon
DHI
Fan
Channels
DHI
Channel
3.0
3.5
Two-way traveltime, sec
4.0
4.5
TopIabe
Iabe
Top
5.0
Top salt
5.5
> Channels, fans and direct hydrocarbon indicators (DHIs) interpreted on a seismic line from the salt-diapir province of the Congo Fan, north of Kwanza
Canyon. A horizon with dip closure is interpreted in orange.
12
Oilfield Review
Winter 2002/2003
salt
6. Facies variation is the variation in rock type within a unit
as a result of the depositional process.
it of
y lim
t-da
sen
Pre
To evaluate the variation in direction and
complexity of the channel systems, eight timeslices from the abyssal plain were picked at
40-msec intervals in the range 6620 to 6900 msec,
which is about 2000 msec, or 2000 m [6560 ft],
below the seafloor (right). The current limit of salt
shows the eastern edge of the tectonically active
region, although this has varied with time. It is
assumed that channels would flow westward
beyond the limit of salt. The channels show wide
variation in orientation, from north-south to eastwest. Clearly, it cannot be assumed that
channels always follow the regional dip away
from their source. Exploring for and developing
hydrocarbons in these channels require the information contained in seismic data.
The 3D seismic data contain high-resolution
detail of individual channels and other depositional features on the abyssal plain, even at
1.5 seconds, or 1500 m [4920 ft], below the
seafloor, which is 4 seconds or 3 km [2 miles],
below sea level (previous page, top). In this
example from the northern 3D survey over the
Congo Fan, high amplitudes in yellow and red
delineate sands. The time-slice shows facies
variations within a channel, where channel fill
changes from the red and yellow—indicating
sand—to the gray of shale.6 Where the channel
is plugged with shale, sometimes it has levees
marking its position. Overbank deposits, or
crevasse splays, can also be identified.7
All the information from the Kwanza Canyon
and Congo abyssal plain can be applied in areas
that currently are being evaluated for hydrocarbon exploration and future development
potential (previous page, bottom). This seismic
line comes from the salt-diapir province of the
Congo Fan. By applying the knowledge and models developed from the shallow seafloor down to
the abyssal plain, we see that two types of sand
deposition can be identified in this province:
channels appear as short, bright reflectors, and
sheet or fan sands as more extended lines of high
amplitudes. Recognition of sand-prone intervals,
combined with regional knowledge, allows
approximate geologic ages to be assigned to different parts of the section in areas where no well
information is available.
The salt provides four-way dip closures of
varying areal extent within the Cretaceous and
Tertiary overburden, of which there are examples
in this area. Sands can be interpreted by their
N
Abyssal plain
0
km
5
0
miles
3
> Orientations of channels interpreted in 3D seismic data in the interval 6620
to 6900 msec. The channels do not always follow the east-west regional dip
away from their source. Channel orientation cannot be predicted, but it can
be mapped from 3D seismic data.
characteristic amplitude, and are seen over the
crests and on the flanks of the closures, offering
attractive hydrocarbon-trapping configurations.
Post-salt source rocks, such as the Iabe formation and those of Albian age, are mature in the
deep synclines between the salt diapirs and on
the adjacent abyssal plain. The DHIs that are
apparent in these data strongly suggest that
hydrocarbons are present, have migrated into
structures and have been trapped—making the
area highly attractive for future exploration.
7. An overbank deposit, or crevasse splay, is made of sediments deposited when the channel breaks through or
runs over its banks.
In this example, the present is clearly a clue
to the past. By using information from the present-day seabed and from deeper analogs, we
have built a model of processes that have probably been occurring offshore Angola for the last
30 million years. Many of Angola’s recent discoveries are in these older channels, but the
existence and economic value of surrounding
accumulations have yet to be proved. The proof
may lie in further examination of the 3D seismic
data that form the foundation for the exploration
process in this deepwater region.
—LS
13