RELINQUISHMENT REPORT FOR LICENCE P. 766 Block 44/21d
Licence:
P.766
Block: 44/21d
12th Round Traditional Licence
Operator:
Ithaca Energy UK(Ltd)
65.85%
Partner:
Centrica North Sea Gas Ltd
34.15%
Work Programme Summary:
Following the acquisition of an interest in Block 44/21d from GDF SUEZ E&P UK, an agreement
was made with the regulatory authorities (DECC) to extend the licence term to allow Ithaca Energy,
as incoming operator, to make its own assessment of the production potential of the Opal discovery.
An extensive review of all the subsurface data was performed in Q1 2011 in order to fully assess the
development potential of the discovery. A “drop” decision was made in July 2011.
SYNOPSIS
Block 44/21d was issued under licence P.449 in May 1983 as part of the 12th licensing round
awards and has been through various partnerships since. Ithaca Energy UK (Ltd) acquired an
interest in Block 44/21d as well as the operatorship from GDF SUEZ E&P UK in December 2010
as part of a broader deal with GDF. Centrica North Sea Gas Ltd retained their interests in the block
at this time.
Block 44/21d is a small block located in the Carboniferous Play Fairway of the Southern Gas Basin,
some 12km west north west of the Boulton Field (Fig 1) and approximately 150 km to the northeast
of the Theddlethorpe Terminal.
The only remaining feature of interest in Block 44/21d is a small part of the Opal gas discovery.
The Opal gas accumulation was discovered in 2005 by well 43/25a-2W, a sidetrack of well 43/25a2X which encountered shows in a downflank location. The Opal Field straddles Block 43/25a,
P.449, a licence that has also been relinquished and for which a separate relinquishment report has
been produced.
Subsequent evaluation of this discovery has encountered difficulties in establishing a reliable GRV
due to a) poor seismic imaging, b) uncertainties in the depth conversion and c) net to gross of the
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P.766, Block 44/21d August 2011
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Ketch reservoir. The volumes for the Opal Field are mapped as being too small and risky to do
further work.
The partnership decided to relinquish the block.
Fig 1. Location of Block 44/21d
EXPLORATION ACTIVITIES
The Opal Field and Block 44/21d is covered by a set of merged 3-D seismic data sets which was
latterly reprocessed by GXT in 2003/2004 with both PreSDM and PreSTM volumes now available.
As well as the discovery well(s) a regional set of wells from adjacent areas and covered by the
seismic were available for Ithaca Energy’s evaluation.
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PROSPECTIVITY ANALYSIS IN BLOCK 44/21d
Block 44/21d is located in the Silverpit Basin in the Southern North Sea. In the Silverpit area,
principal sections of interest are the fluvial distributary channel deposits of Westphalian C/D age,
comprising of the Lower Ketch 1, Lower Ketch 2 and Upper Ketch Formations. The Lower Ketch
formations consist of continental, alluvial-fluvial channel sandstones and shales that thicken to the
southeast. The deeper Westphalian B, Westoe Coal Formation is principally composed of nonreservoir shales and coals, some of which act as a source rock in the area.
The Opal Field that straddles Blocks 43/25a and 44/21d was discovered in 2005 by GDF SUEZ
E&P UK. Well 43/25a-2x was the first well to enter the reservoir and encountered gas shows in the
Lower Ketch 2 Formation of the Westphalian C/D sequence in a 140.5 m thick water-wet section
of Lower Ketch 2 sandstone. Geological sidetrack well 43/25a-2w was drilled up-dip and
intersected a 37 m gas column in the Lower Ketch 2. MDT pressure data was acquired which
proved a gas water contact at -3,933.4 m TVDSS. The well was not tested.
The Opal Field comprises westerly dipping reservoir sequences preserved beneath the Base Permian
Unconformity on the crest of a tilted fault block (Fig 2). Top seal is provided on the flanks by the
Upper Westphalian C/D Upper Ketch Formation and the Boulton Formation and by early Permian
shales and evaporates over the crest. The Opal Field is fault closed to the north and south, dip
closed to the west with up-dip erosional truncation in the east. Base seal is provided by Westphalian
B shales.
The Opal Field reservoir is mostly composed of sands within the Lower Ketch 2 Formation. The
Upper and Lower Ketch Formations consist of fluvial, low-sinuosity channelized sandstones,
believed to flow broadly north-south to northeast-southwest across the Southern Gas Basin.
Reservoir properties are generally consistent across the Westphalian C/D reservoirs of the Southern
Gas Basin, with the greatest variety in the net to gross ratio, or channel density. Porosities are
broadly in the range of 9% - 11% with Sw of 0% - 40% and good permeabilities. Net to gross ratio
was 40% in the 43/25a-2w well for the Lower Ketch 2 but is likely to be highly variable.
On a regional basis the Ketch net to gross ratio distribution in Opal is likely to have occupied a
peripheral area to the main distributary fluvial pattern, characterised by less extensive channel
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threads, channels with a low aspect ratio (i.e. low channel width/thickness relationship) and
development of thicker overbank (coal, soils, alluvial fines), levee/crevasse splay deposits and
short-lived, shallow streams.
No further prospectivity has been identified in the block.
RESERVES SUMMARY
GIIP volumes are provided below for the Opal Field within Block 44/21d.
There is significant gross rock volume uncertainty associated with depth conversion. The
overburden is comparatively simple, however, the low relief nature of Opal and magnitude of depth
residuals suggest some lateral velocity uncertainty which results in the degree of structural
uncertainty observed. The seismic imaging of the Ketch reservoir sands is also very poor and a
robust pick is not possible (Fig 3). Artefacts such as multiples and velocity pull-ups associated with
the Intra-Zechstein Plattendolomite further hinder the interpretation. 43/25a-2x was the only well
that encountered a full Ketch and Upper Cleaver section; the sidetrack (43/25a-2w) penetrated only
the Ketch, 83m deep to prognosis.
The other key uncertainty is the net to gross ratio of the Ketch reservoir units. The distribution and
connectivity of the reservoir sands within the gas bearing area of the accumulation has significant
bearing on the uncertainties affecting resource estimates.
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Opal Discovery ‘on block’
Type: Tilted Fault Block with Westphalian C/D Ketch sandstone reservoir.
Risks:
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Structure: The Opal reservoir structure consists in a westerly dipping
reservoir sequence below the Base Permian Unconformity (BPU) on the crest
of a tilted fault block, fault closed to the north and to the south and dip closed
to the west with up dip erosional truncation to the east (Fig 2 & 3). Seismic
imaging and depth conversion is very poor and there is therefore a moderate
risk on GRV estimates (0.7)
Source/Migration: Proven to work. (1.0)
Seal: Proven to work. (1.0)
Reservoir: Westphalian C/D Lower Ketch channel sands. Net to gross ratio
sensitivity is related to the channel distribution, and proximity to source,
there is a risk the Opal reservoirs are located in a peripheral area to source.
Reservoir risk is moderate. (0.7)
Overall Risk = 0.5
Volumes in Block 44/21d
Gas in Place: P90 3 bcf
P50 5 bcf
P10 18 bcf
Fig 2. Opal Discovery Top Reservoir.
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Figure 3. Depth Line across the Opal Discovery orientated along 43/25-2w (LK = Lower Ketch ).
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