LICENCE RELINQUISHMENT REPORT
1.
LICENCE INFORMATION
Licence
P1969
Licence Round
27th Round
Block
3/2b
Licence Type
Traditional
Licence Operator
TAQA Bratani Limited
Operator/Partners
TAQA Bratani (op): 50% / Fairfield Cedrus: 50%
2.
LICENCE SYNOPSIS
Licence P1969 was awarded as a two year drill-or-drop licence to a group comprising TAQA Bratani
th
Limited and Fairfield Cedrus Limited in the 27 Licensing Round. Following completion of the Work
Programme the licence is being relinquished at the end of the Initial Term.
Licence P1969 consisting of Block 3/2b is located in the Northern North Sea, within the East
Shetland Basin, approximately 90 miles northeast of the Shetland Islands. The acreage lies between
the Hutton and Ninian Fields southeast of the abandoned Northwest Hutton Field (Figure 1) acreage
jointly held by TAQA Bratani and Fairfield Cedrus (the Partnership).
Block 3/2b contains the 3/2-7 Prospect, a three-way dip closed structure with an eastern boundary
defined by the main Hutton/Ninian boundary fault. At the time of application the seismic interpretation
and depth conversion provided a top Brent depth structure map which suggested the potential for a
sizeable dip closed prospect. The mapped spill point coincided with the ODT observed in the
petrophysical analysis of the only well drilled on the licence (3/2-7) which was drilled on the southwestern margin of the prospect (Figure 2).
TAQA Bratani & Fairfield Cedrus confirm that the Department of Energy and Climate Change is free
to publish the contents of this report and that all third party ownership rights (on any contained data
and/or interpretations) have been considered and appropriately cleared for publication purposes.
Figure 1 – Licence P1969/Block 3/2b Location Map
Figure 2 – Application Document Prospect Outline
3.
WORK PROGRAMME SUMMARY
•
Re-processing of 28.8 km² of existing 2009 seismic data to create a pre-stack depth migrated
(PSDM) volume across the entire prospect and the northern part of Block 3/2b. Inclusion of
additional seismic data, from the Ninian 3D survey, may also be completed in order to give a
sufficient migration aperture for the PSDM processing
•
Detailed fault seal studies
•
One drill or drop well with two year decision point. The well would be drilled to 3250 m or
120 m (400 feet) below the Top Brent Group, whichever is the shallower, to evaluate the
prospect and define the net to gross, level of oil saturations and reservoir pressure
The PSDM seismic processing (through ION GXT) was completed in March 2014. Although not
offered in the original work programme a pre stack inversion was undertaken (through CGG) in order
to ensure maximum resolution across the 3/2b Prospect defined in the application document specifically its eastern flank where the key uncertainty resided.
Indications from the interpretation of the new seismic data were that fault seal was less of a risk to the
prospect than had been thought previously. The main challenges were the extent of erosion across
the eastern flank and the apparent impact on dip closure. Whilst the most recent top Brent structure
map does suggest the potential for a slightly larger STOIIP in the 3/2b Prospect, associated with
various combinations of fault seal than dip closure alone, pressure depletion in well 3/2b-7 (from
Ninian production) suggests limited potential for the high degree of fault seal required to reach
economic STOIIP levels.
With the revised mapping and updated volumetric assessment, the dip closed 3/2b Prospect is
looking less attractive than at the time of application. There is insufficient potential for the Partnership
to commit to drilling as required to extend the licence at the end of the initial two-year drill-or-drop
term.
4.
DATABASE
Seismic:
Block 3/2b is partially covered by several seismic surveys but the two main ones of interest, shown in
Figure 3, are the multi-client ESB09 (PGS/TGS 2009) 16 streamer seismic survey, partly funded by
both TAQA and Fairfield, and the released Ninian 3D survey (CGG Quad 3 North - 1995). Both
surveys were processed to post-stack time migration.
The southernmost part of the 3/2-7 Prospect lies on the edge of the higher quality ESB09 survey in an
area of low fold and with limited migration aperture. In this area the interpretation was reliant on the
Ninian 3D. Amplitude balancing and a 16 ms shift were applied to this survey to match the prime
dataset.
At top reservoir level (Top Brent Group) the ESB09 data is of good quality. Fault terminations are well
imaged and coherency products give a good picture of the reservoir structure and faulting.
However in detail the Top Brent reflector and its tie to the existing well database can be variable
across Northwest Hutton/Darwin area. Detailed study suggested that the variability in the Top Brent
reflector definition is due not to variation in the Brent sand section but to variation in the overlying
Heather Shale Formation. The Heather Formation has numerous hard layers and calcareous bands
that onlap the Brent within the area covered by the survey. These are sub-seismic in scale but affect
the reflectivity coefficient at the Heather-Brent interface. This in turn leads to variable reflection types
when convolved with the seismic wavelet.
By adding the low frequency component (from the wells and a velocity model) and by effectively
adding the higher frequency component (by deconvolving the seismic trace to remove the wavelet)
the resulting data has a very broad (theoretically infinite) bandwidth. The seismic dataset was
subsequently inverted in an attempt to improve the resolution.
Figure 3 – Seismic coverage across Block 3/2b used for the evaluation
(The ESB09 is highlighted in cyan, the Ninian 3D in magenta)
Wells
The 3/2-7 Prospect is penetrated on the western flank by well 3/2-7 which was drilled in November
1984 by Conoco (UK) Limited (with partners Britoil PLC and Gulf Oil Corporation) using the semisubmersible rig Dundee Kingsnorth.
Conoco had mapped the Prospect as a down-stepped fault block oriented northeast-southwest with
fault closure on three sides and dip closure only to the southwest. The nearest well control was 3/3-6
(4.5 km to the southeast on the northern part of Ninian).
The Brent Formation was the primary objective with the Statfjord Formation being a secondary target.
Top Brent was encountered 89 feet deep to prognosis. Only poor oil shows were recorded over the
upper part of the Brent section with electric logs suggesting residual oil saturations over the
uppermost 20 feet (water saturation in the zone was never less than 60%).
Conoco inferred the presence of residual oil which in turn suggested that the south eastern boundary
fault was not an effective seal but permitted communication between the prospect and the main
Ninian field. This was supported by the RFT results which supported the depletion in the upper
135 feet of the reservoir.
st
The well reached total depth in the Cormorant Formation on 21 November 1984 and the well was
plugged and abandoned.
5.
PROSPECTIVITY UPDATE
5.1
PROSPECT DESCRIPTION
Reviewing the final PSDM and inversion seismic volumes has resulted in greater definition across the
prospect compared to the original application. Two key changes have been made to the
interpretation from the time of application.
1) The depth migration confirms that the prospect is not dip closed to the south and is reliant on
fault seal to provide a robust closure (Figure 4). This sealing mechanism is not supported by
the pressure data which confirms connection between the Prospect and Ninian.
Figure 4 – 2014 Final PSDM Top Brent Structure in Depth
Note: that the maximum potential closure (analogous to the prospect outline in the original application document)
requires fault seal to the south
2) The revised eastern erosion edge of the fault block has an impact further west than had
originally been interpreted - this has resulted in greater erosion of the primary reservoir over
the Prospect and a reduced gross rock volume (Figures 5 and 6).
Figure 5 – Quality of seismic data at time of Application (courtesy of PGS and TGS)
Figure 6 – PSDM Final Stack (in time) showing extent of erosion across the 3/2-7 prospect (Figure
courtesy of PGS and TGS)
Recent experience of wells drilled across analogous Brent crestal erosion sequences on
Cormorant Block 1 confirm the absence of slumped Brent as had been proposed for the 3/2-7
Prospect in the application document.
Reservoir
The Middle Jurassic Brent Sandstones in well 3/2-7 are at a burial depth of ~10,500 feet TVDSS and
the Ness distributary channel sands, which compose the reservoir interval, display very high porosity
(20%) and permeability in excess of 2 D as a result of mineralogical maturity and the clean arenites
that make up the channel sands. The well is at the deepest point in the 3/2-7 Prospect and so
deterioration in reservoir quality across the structure is unlikely.
The 3/2-7 well clearly has the upper part of the Brent sequence eroded with the preservation of the
Broom, Rannoch, Etive and Lower Ness (Figure 7). The Ness sequence is dominated by a distinctly
sandier upper part with a thick shalier section toward the base.
The most northerly well on Ninian (3/3-6) is 300 feet shallower than 3/2-7 and encounters a Brent
sequence indicating more section has been eroded (Figure 7). The erosion cuts deep into the Lower
Ness preserving only the relatively low net to gross section at the base of the unit. Although it
experienced a greater degree of erosion, being closer to the Ninian boundary fault, the 3/3-6 well had
very consistent Broom, Rannoch, Etive and Lower Ness sections (correlatable with 3/2-7 (Figure 7)
and, despite its proximity to the eastern boundary fault, still preserved the lowermost of the high
quality Ness sands.
At the time of application it was proposed that 3/3-6 would represent the worst case for the extent of
the erosion but the revised seismic mapping suggests a lower angle to the boundary fault at the 3/2-7
prospect location. This is likely to result in greater erosion at the Prospect location for an equivalent
standoff to the boundary fault than at the 3/3-6 location. As such, it is now expected that a reduction in
the effective net to gross will occur in the 3/2-7 Prospect area than is seen at the Ninian boundary.
Source
The Upper Jurassic Kimmeridge Clay Formation is the primary source rock in the area although there
may in places be a contribution from the Heather mudstones. Coals within the Middle Jurassic Brent
Group are a secondary source. The Kimmeridge Clay Formation is mature for oil and gas over much
of the area. On the western side of block 211/27 the Kimmeridge Clay and underlying petroleum
prone intervals are deeply buried (12,500 to 16,000 feet). This deep burial and its effect on maturity
has resulted in a prolific source kitchen very close to Northwest Hutton with a viable migration route
up into Block 3/2b. Published data offers a suggestion that Hutton may have been charged from the
east rather than the west although no core or sample material is currently available to confirm this.
Figure 7 – Correlation between the 3/2-7 well and the nearest offset well 3/3-6 (north of Ninian) showing the
extent of the erosional incision into the stratigraphic sequence.
Trap
The 3/2-7 Prospect is a conventional tilted fault block trap with a major eastern fault boundary, dip
closure to the west and a combination fault and dip closure to the north. Evaluation of the reprocessed seismic data suggests the requirement for fault closure to the south to seal in order to
create a robust trap and provide a closed structure which will not have been produced at Ninian.
The revised interpretation of the Prospect, given the additional erosion and the uncertainty over the
closure of the prospect, is that there is between 1 and 5 MMstb in the most likely case whilst the high
side volume is around 21 MMstb. The high side volume relies on more significant fault seal across all
stratigraphic units to the south. This is difficult to support due to the depletion observed at 3/2-7 well
which can be directly linked to offtake at Ninian. All cases require a degree of fault seal and the
chance of success of 40% reflects the uncertainty of finding a remaining hydrocarbon accumulation
within the STOIIP range presented for the Prospect (ie it is likely Ninian will have produced some
volume and/or reduced the pressure significantly).
6.
FURTHER TECHNICAL WORK UNDERTAKEN
The PSDM data set was subsequently inverted in an attempt to improve the resolution.
7.
RESOURCE AND RISK SUMMARY
STOIIP (MMstb)
3/2b-7 Prospect
8.
P90
P50
P10
1.2
5.0
21.2
40%
CONCLUSION
The 3/2b-7 Prospect was the principal target for the application.
recognised on Block 3/2b outside of the 3/2-7 discovery.
9.
COS
No additional prospectivity is
CLEARANCE
TAQA Bratani & Fairfield Cedrus confirm that the Department of Energy and Climate Change is free
to publish the contents of this report and that all third party ownership rights (on any contained data
and/or interpretations) have been considered and appropriately cleared for publication purposes.