RELINQUISHMENT DOCUMENT
1.0. Header
Licence Number:
Licence Round:
Licence Type:
Operator/Partner %:
Work Programme Summary:
License P1022 (Block 98/11)
Awarded out of round 1999
Production, Block 98/11
Operator: BP (50.48%),
Partners: Maersk (32.19%), Premier (12.38%),
Talisman (4.95%)
1 firm well targeting Old Harry exploration prospect
2.0. Synopsis
Licence status
Licence surrendered to DECC (Licence determination of surrender document filed to DECC
November, 2009)
3.0. Exploration Activities
3.1. Seismic acquisition:
None
3.2. Well(s) drilled:
L98/06-M18 and L98/06-M18Z (Old Harry-1) – Plugged and Abandoned.
Drilled from Wytch Farm onshore M Site in Jan 2000. Drilled 60mTVT of Sherwood from
1738mSS. Encountered shaley zone in top 9m (Wytch Farm Zones 10, 20). Good 6m HC
sand at 1748 – 1754m TVD, Sw 40 – 55% Thinner sands below with net SW 40% in
shaley matrix. Overall 60m interval is N/G of 0.29 with Sw 50% cut off.
Possible OWC at 1753m TVDSS, structural spill point on post well remapping is 17501760m. Individually trapped HC’s in thin sands below this level, down to 1788mTVDSS
May be gas or oil, no oil shows in Sherwood, but OBM and disaggregated samples would
work against shows. Oil is assumed, see Comments. This well had no RFT runs despite
low Sw and good reservoir zones, and no DST’s were run.
Reason for failure – Top Sherwood 40m deeper than prognosed due to higher than
predicted Chalk velocity. Logs show oil or gas migration to this structure
3.3. Studies carried out:
•
•
Old Harry Review: Review of results from exploration well L98/06-M18 & L98/06M18Z in block 98/11. (Giles Watts, Jun 2000)
Prospectivity Review for Offshore, (D Caston, RML, 2004)
4.0. Prospectivity Analysis
Wytch Farm
a
98/11-3
Old Harry
1
3
98/11-1 Updip
Old Harry
2
98/11-2
Down Barn
Kimmeridge 1
– IOW
Purbeck
Encombe
Kimmeridge 5
Lulworth Banks
4Z
l
Reversa
Southard
Quarry
Oil Prospect
Gas Prospect
Fig. 4.: Map displaying Block 98/11 Discoveries, leads and prospects
Old Harry Prospect (Well 98/06-M18)
Pre Drill
The objective was the Sherwood Sandstone, footwall anti-clinal structure, one fault block
south of Wytch Farm.
Well and Post Drill
This prospect was drilled by BP L98/06 -18Z in Jan 2000 (Old Harry), but the structure was
40m deep to prognosis. There were good HC indications on logs, SW’s down to 40%,
indicating a valid closure, probably oil bearing, but no tests were carried out, RFT or DST.
The reservoir quality was poor due to the absence of good channel sands in Zones 30, 40,
which are present in Wytch Farm. Old Harry is off the main Sherwood sand channel axis.
There was a good quality 6m HC bearing sand at the top of Zone 30. Below this thin
individual HC bearing sands occur in a low N/G sequence, extending to TD at some 44m
below the mapped spill point. These are probably individually stratigraphically trapped, but
if they are part of a connected reservoir body, a fault linkage would be needed to seal a
structure down to at least 1800mSS, below the water level in 98/11-1 to the east.
5.0. Reserves Summary
P50 RESERVES > IMMSTB
98/11-1 Updip Sherwood * (P1022)
TOTAL
STOIIP
mmstb
Recovery
(nominal
)
Reserves
mmstb
CoS
Risked
Reserves
mmstb
10.5
0.2
2.1
0.64
1.3
10.5
2.1
* ~ 50% of this prospect lies on open block
98/12
DISCOVERIES
98/11-3 Sherwood Oil (P1022)
98/11-2, 4Z (Gas BCF) mmboe (P1022)
98/11-2, 4Z mmboe (P1022)
Old Harry Sherwood (P1022)
TOTAL
6.0. Maps and Figures
6.1. Location plate;
Figure 6.1: Proposed Relinquishment outline
4.9
22.0
4.0
4.8
0.2
0.7
0.1
0.2
1.0
2.6
0.4
1.0
35.7
1.2
4.9
1.3
6.2. Structure Map / Illustrative seismic section(s) / Illustrative geoseismic cross
section(s)
Wytch Farm Offshore
Old Harry Updip
98/11-,3 Oil Disc.
98/11-1 Oil Prospect
‘‘98/12-2’’
Not drilled
98/12
Gas Prospect
98/11-2,4Z Gas Disc
Figure 6.2.1: Top Sherwood Structure Map (Sept 1999)
S
1M-18ZSP
Cornbrash
White Lias
Sherwood
1M-18ZSP
IL6470
Figure 6.2.2: Seismic section through 1M18ZSP
All seismic images are courtesy of CGGVeritas.
7.0 Clearance
The Operator, BP (50.48%), and its partners, Maersk (32.19%), Premier (12.38%),
Talisman (4.95%) confirm that DECC is free to publish information documented in this
document and that all 3rd party ownership rights (on any contained data and/or
interpretations) have been considered and appropriately cleared for publication purposes.
Extracted from D Caston, 2004
TECHNICAL DATA
Regional Geological Setting
A good summary of the Petroleum Geology of the Onshore UK and the Wessex Basin has
been produced in 2003 by the British Geological Survey for the UK DTI. This can be found
at the DDTI website:http://www.og.dti.gov.uk/upstream/licensing/onshore_10th/Basin_HC_prosp.htm This
sets the regional scene, as does an earlier paper from the BGS Principal features of the
hydrocarbon prospectively of the Wessex Channel Basin UK. Penn et al 1987
The greater structural unit in which the AOI is contained is the Wessex - Channel Basin,
spanning an area of 40,000 km . Figure 2 shows the basin geometry with isobaths of the
Perm- Triassic and the Jurassic-Cretaceous. The economic basement is comprised of
highly deformed pre-Variscan Devonian and Carboniferous strata, deformed in the Variscan
orogeny. Extensional graben epicentres were initiated in Perm-Triassic times as a network
over North West Europe, and the Wessex - Channel Basin inherited the pre-existing eastwest trend of the Varisacan thrusts.
2
A sequence of up to 6km of syn-rift Perm-Triassic and Jurassic sediments were laid down
in this extensional basin, characterised by rift style, rotational, down to the basin faulting,
often synsedimentary, and typical of the extensional rift basins UK North Sea.
The most marked basin (growth) fault runs along the Dorset coast, with an accumulated
throw of up to 2000m to the south towards the basin axis. The structural elements map on
Figure 3 shows the resulting syn – rift fault pattern.
Like the North Sea grabens, but much less vigorously expressed in the AOI, a phase of
post rift uplift and erosion, the Late Cimmerian unconformity, occurred in the earliest
Cretaceous. The subsequent post rift Cretaceous and Tertiary sequences, including the
Lower Cretaceous Wealden and the thick Upper Cretaceous chalk sequence, were laid
down in the now structurally calmer and thermally subsiding basin.
During the mid Tertiary, the Alpine phase of compression caused inversion of the basin,
with the east-west inversion axis lying some 10km south of the Dorset coast. The centre
of the basin was uplifted in a broad anticline along this axis, and the post Variscan basin fill
was pushed from the south into tighter anticlines and monoclines against the major east
west normal fault at the Dorset coast. This line of normal faulting is now linked with the
tight anticlines, monoclines and reverse faulting.of the inversion, and is known as the Isle
of Wight – Purbeck Disturbance.
(The Alpine inversion is very marked in the UK Southern Gas Basin and stretches to the
west as far as the Celtic Sea, where the Kinsale Gas Field and the Seven Heads Oilfield are
formed by Lower Cretaceous reservoirs in a basin inversion structure.)
Exploration History
The Wessex-Channel Basin is a classic area of British geology. All the classic surface oil
exploration elements were in place, oil seeps, oil shales, surface anticlines, and
outcropping reservoirs, and the first drilling was carried out in 1936-1937 by d’Arcy
Exploration with Broadbench 1 in Kimmeridge Bay, found traces of live oil in the Upper
Jurassic Corallian. In 1959 three follow up wells were drilled, .and Kimmeridge 1
encountered oil in the Middle Jurassic Cornbrash Limestone. The well was completed as a
producer in the Cornbrash and since 1961 this well has remarkably produced 3.5mmstb of
light oil, and is still producing.
The BP Bere Regis well, on the western edge of the PL089 Licence, was drilled by BP in
1959 on a surface anticline and proved the Bridport reservoir in the area, though water
bearing in the well. A full Lias sequence of 530m, together with oil shows in the Lias at
Ashdown, showed the oil potential of the area due to the presence of a thick and deep
Lower Jurassic basin.
A gravity high had been identified just NW of Wareham, and reflection seismic was
acquired by SSL and BP between 1958 and 1962, to comfirm the structure. Wareham
1was drilled in 1964 and discovered oil in the Bridport, and water bearing Sherwood sands.
Wareham 2 followed in 1965, which confirmed the Bridport oil and tested oil from the
Cornbrash.
In 1973, the Bridport reservoir of the Wytch Farm Field was discovered, and the deeper
and much larger Sherwood reservoir was discovered and put on production in 1984. The
dry well at Wareham and other Sherwood exploration failures had led to the belief that the
Sherwood could not be sourced from the overlying Lias or Kimmeridge. The key was that
the down to the basin Isle of Wight – Purbeck fault had a large synsedimentary growth
element, burying the Lias to maturity on the downthrown south side of the fault during and
after the creation of the Wytch Farm structure.
Migration paths through the Bridport, the Sherwood and through faults had charged the
Wytch Farm Field, where source rocks are immature for oil.
Between 1980 and 2000 five exploration wells were drilled in 98/11a and have been
successful in discovering oil and gas. They have proved the extension of good reservoir
rocks over the area, and a number of wells have good oil and gas shows.
Two tested the early fault structures:98/11-1
98/11-3 (Uneconomic oil discovery)
The remainder have tested the younger inversion structures
Old Harry, (Minor HC column, unknown oil or gas)
98/11-2 (Uneconomic gas discovery)
98/11-4,4Z (Gas appraisal)
These wells are discussed in more detail individually below. Some have been shown by
later seismic to be off structure, while those with good oil shows have indicated a
migration path, or later failure of the structure. It is clear that a late structure can still be
charged, but may be gas prone.
Seismic Data Base
2D and 3D seismic covering the area is stored in UKOGL/CDA.
Reservoirs
The regional stratigraphy and the reservoir and source rock horizons are shown on Figure 6
Sherwood Sandstone (Early Triassic – Scythian)
The Sherwood Sandstone Group is a 100 to 300m thick sequence of terrestrial red-beds
composed of arkosic sandstones and mudrocks resting on the Zechstein Aylesbeare
Mudstone Group. It shows a complex and interdigitating assemblage of floodplain,
aeolian, lacustrine, channel fill and sheetflood facies. The channel, sheetflood and
floodplain facies are the most common, with the first two providing the dominant
reservoirs.
The dominant upward trend is from perennial braidplain to sheetflood to ephemeral
lacustrine conditions, which finally give way to the persistent playa lake conditions
resulting in the Mercia Mudstone caprock. This represents a proximal to distal
progression, showing an overall regressive sequence.
The Sherwood becomes thin and silty to the north around 51° and to the east across
the Isle of Wight. The provenance area was the Armorican Massif to the south, but the
regional northward transport direction was modified by the east-west synsedimentary
faulting. Exploration wells have shown the Sherwood to have good reservoir potential
over the AOI.
The sandstones overall are good quality. The Lower Sherwood has a mean porosity of
18% and permeability often >1.5D. The Upper Sherwood ranges from 10 to 15%
porosity and permeability around 150mD. The lacustrine deposits at the top of the
Sherwood can reduce gross reservoir quality, particularly with a vertically restricted oil
column. At Wytch Farm, this zone is about 10 to 20m thick
Bridport Sands (Lower Jurassic - Aalenian)
The Bridport Sandstones, between 25 and 100 m thick are massive, mica-rich, very fine grained
sandstones and siltstones, highly bioturbated and deposited in a shallow shelf environment.
Strongly calcite-cemented sandstone hardbands are prominent in section, representing storm
events which brought shell debris from shallower waters. These show a high lateral continuity.
The sand provenance was the South Wales area. The sands form part of a unit extending north
as far as the Cotswolds, becoming progressively younger as the early Jurassic coastline
prograded southwards.
Sedimentologicaly, the Bridport Sandstones are very uniform, typical of shallow shelf
bioturbated sand bodies. The major control on porosity and permeability is the extent of the hard
bands. These have very poor reservoir quality and are non-pay, and form extensive production
barriers. Average net porosities in the Bridport Sandstones range from 20-27%, and layer
average permeabilities range from 5mD at the base of the sands to 64mD at the top.
Bridport oil is produced from the Wytch Farm and Wareham Fields. Conventional sub vertical
wells in the Bridport on Wytch farm and Wareham sustained production of 500bopd with
pressure support.
The operators of the recently confirmed Waddocks Cross Bridport discovery on PL090 quote
studies which support an average rate in excess of 200 barrels of oil from a planned horizontal
well, with reinjection of produced water.
Great Oolite Group
The limestones of the Bathonian Great Oolite Group form the main reservoir in the
Humbly Grove Oilfield and in a number of other discoveries in the Weald Basin oil.
Sandhills well, on the Isle of Wight had oil indications in the Great Oolite, and a follow
up well has been planned. They are proximal oolitic carbonates deposited in shallow
water, and traced southwest into the Wessex-Channel Basin. The carbonates grade
laterally into a deeper water shalier more distal facies known as the Frome Clay, with a
reduction in reservoir quality.
These high resistivity limestones are difficult to evaluate from log and conventional core
plug analysis. Reservoir stimulation, long term testing and care to avoid formation
damage are needed for full evaluation.
Frome Clay (Bathonian)
At Wytch Farm the Frome Clay is a calcareous mudstone. At its base it contains a patchily
distributed limestone, the Frome Clay Limestone, representing shelly debris, principally oyster
(ostrea) shells redistributed by storm episodes. The Frome Clay Limestone is at 850 m depth,
mapped by 3D AI seismic. Vertical Wytch Farm wells1K-09FP and 1L-11FP produce 400 and 190
b/d respectively, and the dedicated horizontal Frome well 1M-04FP can produce approximately
400 b/d.(historical data).
The Frome Clay Limestone varies dramatically in thickness. To the north and east of the Wytch
Farm area, the limestone thickens rapidly from about 2 m to over 40 m. Moderate oil shows
(especially where the limestone is thickest) and poor gas shows are common.
As an exploration target the Frome Clay should prove more prospective to the north east of the
AOI, and requires high quality seismic for detection of the good reservoir facies.
Vertical Wytch Farm wells1K-09FP and 1L-11FP produce 400 and 190 b/d respectively, and the
dedicated horizontal Frome well 1M-04FP can produce approximately 400 b/d.(historical data).
Forest Marble ( Late Bathonian)
The Forest Marble Formation underlies the Cornbrash and is approximately 50m thick,
thickening east to west. It is a similar sequence of calcareous, ooilitic pelletal
mudstones, interbedded with limestone units composed of oolitic shelly lime sand bars
or redeposited storm beds of up to 2m thick. These limestone beds make up around 510% of the sequence, and their reservoir characteristics are the same as the Cornbrash
given above. In practice the Cornbrash and Forest Marble form a single reservoir target.
Stoborough 1 Recovered 31 bbls oil from the Forest Marble sands on DST, followed by
a 2 month Forest Marble production test in 1978 at 140-90 bpd, pumped
Cornbrash (Bathonian)
The Cornbrash in the AOI is a laterally persistent 5 – 15m unit of thinly bedded bioclastic
limestones, interbedded with silty mudstone. The Upper Cornbrash (Callovian) is
composed of thinly bedded limestones with interbedded mudstones, and has a rich
assemblage of brachiopods.and bivalves. The Lower Cornbrash (Bathonian) is a grey
brown, rubbly argillaceous fossiliferous limestone.
Log and core analysis has shown that the Cornbrash has porosities of 5 to 19%,
averaging 11%., with conventional core plug permeabilities very variable as most
permeability is fracture origin.
Wells from the Wytch Farm, Wareham and Kimmeridge Fields and a Stoborough
exploration well have produced oil from the Cornbrash
Wytch Farm
X1 BP Cornbrash flowed 18bbls 37.2API oil in 8 hors after acidising
F5 SP Cornbrash flowed 53 bbls fluid, 10% oil in 4 hours after acidising
Wareham-2
Cornbrash DST 14.5bbl/d oil, 33 API, after frac and acidising
Kimmeridge 1 Completed in Cornbrash, has produced 3.8mmstb of light oil since 1961,
and is currently producing 73bopd. Mapped pore volume in the Cornbrash is inadequate for
this oil volume. The light oil character at this shallow depth +-600m suggests charging
from a deeper reservoir via the fault and an associated fracture system. The underlying
Forest Marble in this well is dry.
Structures and Trapping mechanisms
Two major trap types, shown on Figure 7, result from the structural history described
above. The first are the early, syn-rift, generally elongate, east west combination fault and
dip structures which hold the Wytch Farm and Wareham oilfields and the small
Stoborough discovery.
In the AOI the Late Cimmerian erosion has not cut down as far as the reservoirs, and the
cap rocks are the original conformable overlying shales of the Liassic for the Sherwood
reservoir and the Oxford Clays for the Bridport. These same shales form the downfaulted
seal across the faults.
Experience from Wytch farm shows that although intra-reservoir faults can support
production pressure differentials, the field initially had a single original OWC and pressure
system. Consequently sand to sand contact faults must be considered a high trapping risk.
The second trap type is the suite of late anticlines resulting from the Alpine inversion,
superimposed on the pre-existing extensional fault structures. They are elongate anticlines,
sometimes four way dip closed and sometimes fault limited along one or both long sides.
The Isle of Wight-Purbeck Disturbance is the line of most intense Tertiary structure. The
Kimmeridge Field lies on this line as a late anticline trap type, and indeed the associated
fracturing has enhanced the Kimmeridge Field production. The Old Harry prospect, and the
98/11-2 gas discovery are also of this trap type. Waddocks Cross structure is an early fault
structure modified by late inversion, though possibly created by strike slip movement
rather than by direct compression.
Source rocks and Migration
Both the Kimmeridge Clay and the Lower Lias source rocks are immature for oil north of
the Isle of Wight – Purbeck disturbance, ie immature at structure. It is well established that
the source of most of the oil in the Wessex basin is the Lower Lias south of the
disturbance, where it reached maturity. Figure 8 shows a generalised migration pathway
map. The Lower Lias of the Winterbourne Kingston Trough (presumably the Pewsey Basin)
north of the AOI could have been the source for Wareham (Selley 1987) and hence
possibly Waddocks Cross.
The migration path for Wytch Farm is believed to have been from the Lias vertically up into
the Bridport Sands, from the Bridport sands by direct contact across the main fault to the
Sherwood, and across the intermediate fault terraces to the Wytch Farm structure. The
downthrown Lias may also be in direct contact with the Lias across the main fault. Wytch
Farm is believed to be directly adjacent to the main volume of mature Lias
Migration to the Bridport on the north side of the fault has most likely involved migration
from both the downfaulted Bridport, and the upfaulted Sherwood, through faults and
fractures.
Since the Lias is not oil mature in the AOI north of the Isle of Wight – Purbeck disturbance,
oil charge has to be by lateral migration along carrier beds, which may also involve a fill and
spill process through intervening closures.
In the south of the AOI migration routes can be by:Spill from Wytch Farm (eg to Wareham and Stoborough)
East to west migration along the east west fault terraces south of Wytch Farm (eg to
Old Harry, Harmans Cross, Kimmerdge Deep, Church Knowle)
Direct fault migration from the Lias to the shallow reservoir at Kimmeridge Field (which
may include secondary migration from deeper reservoirs, through fracture zones).
In the north of the AOI by
Spill from Wytch Farm (eg to Carnford Cliffs)
From the north from the Winterbourne Kingston Trough (Selsey Basin), (eg Waddocks
Cross, Wareham, Cold Harbour Morden)
The mid Tertiary inversion structures are thought to have post-dated the main phase of oil
generation and may be gas prone, eg – 98/11-2 gas discovery. The mid Tertiary inversion
may also have added a northerly tilt to the existing structures, with some consequent
remigration. Late gas generation and secondary oil migration due to structural tilting allow
oil and gas charge to the late structures.
98/11-1 Updip Prospect
DESCRIPTION
This is narrow elongate footwall closure at Sherwood level with high relief. It is the next
fault corridor north of the 98/11-2 gas discovery. This late generated gas has not reached
through to 98/11-3 (oil discovery) and the 98/11-1 structure is well placed for oil charge via
the 98/11-2 fault block.
SEISMIC DATA/MAPPING
Good coverage of offshore 2D data, Gas Council 82 - 85
WELL DATA
98/11-1
Closure was believed to exist at Bridport and Sherwood Sandstone horizons and this
deviated well was proposed to test both reservoirs.
Sherwood - Cores 1958.9-1986.4m oil stained. Oil staining decreases below 1984.2m.
Total gas max 500ppm with C1-C4 to 1986.1m and trace of C5 at 1956.8m. Below
1986.1m total gas drops to zero. DST 1 74.1bbls of formation water
Bridport - Patchy oil bleeds (1227.7-1229.9m). Tiny oil bleeds (1235.2-1236.0m). Total gas
max 440ppm (1226.8m) C1=250ppm, C2=30ppm, C3=25ppm, C4=17ppm, average 50300ppm
98/11-3
1986 Sherwood – No resistivity log over the Sherwood, so Sw unknown. No visible oil or
significant fluorescence in core. Total gas max 10000ppm C1-C5. Gas levels drop on
coring and remain below 100ppm to TD. Sidewall samples from uncored section showed
good fluorescence 1901.3-1916.6m. DST 1 - 109.4bbls production with 65% water cut,
initial rate 4000 bpd, final rate 45 bpd after 3.5 hrs, Oil gravity 38.78 API, H2S/CO2 zero.
No gas rate measured. 12m Oil column,OWC at 1740m TVDSS
Bridport - oil stain in top 30m of formation. Total gas max 2000ppm C1-C5, Variable shows
corresponding to occurrence of C5 in ditch gas. Patchy oil bleeds throughout Core 1.
Visible oil on broken surfaces and strong hydrocarbon odour in core 2. Bridport DST 2 No
trace of oil, no production to surface. 46.7bbls into test string, initial rate 240 bbd, final
rate (est) 50 bbd after 6hrs. Losses into Bridport while casing may have caused
inconclusive results. Clearly water bearing on logs.
RISK./ VOLUMES
98/11-1 Updip
Risking
Trap integrity
Reservoir
Source
Migration
0.8
1
1
1
Retention
CoS
0.8
0.64
P90
P50
P90
STOIIP
mmstb
Recovery
(nominal)
Reserves
Risked
Reserves
3.7
10.5
28.2
20%
20%
20%
0.74
2.1
5.64
0.5
1.3
3.6
50% on block 98/11 remainder on
98/12
COMMENTS
A very slim and elongate structure along the crest of a fault block which has a high chance
of oil, which would need a horizontal well for a reasonable recovery rate.
98/11-3 Oil Discovery
Shown on map for 98/11-1 on previous page
DESCRIPTION
Sherwood - Minor oil discovery in Sherwood sands, with a vertical column in 98/11-3
estimated at 12m, OWC 1740. This is assumed to be the closing contour with a consistent
OWC across the fault block. OWC matches closing contour of 1740m from mapping, with
spill north directly up to Wytch Farm. Bridport reservoir present in 98/11-3, good
fluorescence and oil bleed from cores, but water bearing on logs.
SEISMIC DATA/MAPPING
Gas Council 82.
WELL DATA
98/11-1, 98/11-3 see above, under 98/1-1 Updip Prospect
RISK / VOLUMES
98/11-3 Sherwood
STOIIP mmstb
P90
P50
P10
2.8
4.9
8
Recovery
(nominal)
20%
20%
20%
Reserves
0.6
1.0
1.6
COMMENTS
A Sherwood oil discovery with a very flat structure, making a good recovery factor difficult
to achieve. There is some upside, depending on the north west extension of the closure
up to the Wytch Farm bounding fault, if this is sealing. Approximately the same distance
from Wytch Farm M drill site as the proposed 98/7-2 ultra long reach well. Bridport oil
bleed from core supports oil migration through the Bridport to Wytch Farm
98/11-2
Gas Discovery
DESCRIPTION
Sherwood - Gas discovery in Sherwood with 8m oil rim. Discovered by British Gas in
1984. Lies 4Km south of Wytch Farm Offshore). Drilled on the first fault block north of the
IoW Purbeck disturbance, this anticlinal structure results from effect of Tertiary
compression superimposed on the earlier extensional fault block. The gas is probably from
the late stage, more evolved maturation of the Lias, and the gas has displaced earlier oil
from the reservoir leaving an oil rim. There is another very similar structure to the east in
Block 98/12 for which one can assume the same volumes range. Elf planned at one time
to drill it.
Bridport – Reservoir present, but dry with weak oil and gas shows.
SEISMIC DATA/MAPPING
Good coverage of offshore 2D data, Gas Council 82 – 85. Remapped by BG after the
drilling of the discovery and appraisal wells, maps held by BP in ZMAP
WELL DATA
98/11-2 BG 1984 Gas discovery. Sherwood tested at 9.6mmscfd 48/64” choke 70’
interval
Sherwood oil rim of 8m, oil on DST. Top Sherwood 1819m SS GOC 1851mSS
OWC1859mSS OWC. N/G 0.55, Net Ф 14%, Net K ± 50mD Sw 30-40 FVF 180.
Cores were taken in the Inferior Oolite and Bridport, Bridport oil shows, total gas max
50ppm
Bridport water bearing on logs, not tested.
98/11-4 Dry well. Top Sherwood at 1876mSS, downfaulted below GOC and OWC.
Bridport faulted out.
98/11-4Z
Sidetracked north back on to structure, Top Sherwood 1819m. Sherwood recovered
120bbls oil, only 25-50,000scfd gas
Reservoir damaged by fault proximity? Sherwood core available
Bridport faulted out.
RISK / VOLUMES
98/11-2,4Z Gas Structure
Volumes
Deterministic
98/11-2,4Z Oil Rim
Volumes
Deterministic
GIIPP
mmstb
22
Recovery (nominal)
STOIIP
mmstb
4
Recovery (nominal)
Reserves
10%
0.4
70%
Reserves BCF
15
There is an undrilled prospect on the neighbouring open block 98/12 – Assume same volumes
here as for 98/11-2, 4Z. (Elf calculated 12 BCF Reserves for 98/11-2 in a document for the DTI in
March 1998 prior to releasing the block)
COMMENTS
The presence of the undrilled eastern prospect in neighbouring Block 98/12 is positive. It
could make the combined recoverable gas up to 30 BCF. Is it possible that this could make a
viable project supplying gas to a local energy supplier, if an environmentally acceptable
wellhead and pipeline solution could be found?
Old Harry Updip
DESCRIPTION
Sherwood objective, footwall wall anticlinal structure, one fault block south of Wytch farm.
This prospect was drilled by BP L98/06 -18Z in Jan 2000 (Old Harry), but the structure was
40m deep to prognosis. There were good HC indications on logs, Sw’s down to 40%,
indicating a valid closure, probably oil bearing, but no tests were carried out, RFT or DST.
The reservoir quality was poor due to the absence of good channel sands in Zones 30, 40,
which are present in Wytch Farm. Old Harry is off the main Sherwood sand channel axis.
There was a good quality 6m HC bearing sand at the top of Zone 30. Below this thin
individual HC bearing sands occur in a low N/G sequence, extending to TD at some 44m
below the mapped spill point. These are probably individually stratigraphically trapped, but
if they are part of a connected reservoir body, a fault linkage would be needed to seal a
structure down to at least 1800mSS, below the water level in 98/11-1 to the east.
SEISMIC DATA/MAPPING
Largely covered by the Merge99 set of 3D data. Any further interest in this structure
would need to include additional work on the Chalk velocities.
WELL DATA
Old Harry BP L98/06-M18Z Jan 2000, drilled from Wytch Farm onshore M Site.
Drilled 60mTVT of Sherwood from 1738mSS.
Top 9m shaley zone (Wytch Farm Zones 10, 20)
Good 6m HC sand at 1748 – 1754m TVD, Sw 40 – 55% Thinner sands below with net SW
40% in shaley matrix. Overall 60m interval is N/G of 0.29 with Sw 50% cut off.
Possible OWC at 1753m TVDSS, structural spill point on post well remapping is 17501760m. Individually trapped HC’s in thin sands below this level, down to 1788mTVDSS
May be gas or oil, no oil shows in Sherwood, but OBM and disaggregated samples would
work against shows. Oil is assumed, see Comments. This well had no RFT runs despite
low Sw and good reservoir zones, and no DST’s were run.
Reason for failure – Top Sherwood 40m deeper than prognosed due to higher than
predicted Chalk velocity. Logs show oil or gas migration to this structure.
98/11-1 Water up to 1764m, 98/11-3 Oil down to 1728m
RISK / VOLUMES
Old Harry
Risking
Trap integrity
Reservoir
Source
Migration
Retention
CoS
0.9
1
1
0.9
1
0.81
Stratigraphic?
P90
P50
P10
STOIIP
mmstb
2.0
4.8
11.9
Recovery
(nominal)
0.2
0.2
0.2
Reserves
0.4
1.0
2.4
Risked
Reserves
0.3
0.8
1.9
Gas?
COMMENTS
The reservoir evaluation of this well has not evaluated the HC indications apparent from
the logs. no RFT or DST runs were made. It was assumed that the potential STOIIP within
the reduced structure and the low N/G reservoir was uneconomic.
Chalk velocity anomalies lowered the structure 40m from predicted. Chalk velocities
increase towards the centre of the inversion anticline, southwards towards the IOWPurbeck disturbance. Old Harry is off the main sand channel axis seen to the north at
Wytch Farm, giving much reduced net reservoir.
Oil would be expected because of the oil in 98/11-3 in the same uninterrupted structural
corridor. The upside takes a contact at 1753mSS where there is a drop in log Sw.
(compatible with the mapped spill point at1750m), while the low side takes an OWC
above the Old Harry Top Sherwood penetration of 1738mSS.
Old Harry and the 98/11-3 oil structure lie in a low relief fault terrace which has been a
migration corridor for Wytch Farm. (This is also a positive indication for 98/11-1 Updip
prospect)