Relinquishment Report
for
Licence P.272
Block 20/7a
Licence P.272Relinquishment Report –
February 2007
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CONTENT
1 - Introduction
2 - Exploration Activities
2.1 - Purchase 130km2 of Aker 1999 survey
2.2 – Reprocessing of 3D Aker data
2.3 - Biostratigraphic Study
2.4 - Well 20/7a-4 Results, Squirrel Prospect
2.5 - Well 20/7a-6 – tight hole
3 - Prospectivity Analysis
LIST OF FIGURES AND TABLES
Figure 1: Location Map
Figure 2: Middle Volgian – J73B MFS Depth Map
Figure 3 : 3D Seismic Data - Aker 1999
Figure 4 : Aker 1999 3D Seismic Reprocessing Results
Figure 5: Upper Jurassic Well Correlation
Figure 6: 20/7a-4 CPI
Figure 7: Attic Volume up-dip of Well 20/7a-4
Figure 8: Top Triassic Depth Map
Figure 9: Pre-Jurassic Lithology
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1 - Introduction
Licence Number:
P.272
Licence Round:
5th Round – Awarded 14th February 1978
Licence Type:
Traditional
Block Number:
20/07a
Equity Holdings:
Nexen Petroleum U.K. Limited (Operator)
45.3920%
Nexen Ettrick U.K. Limited
19.8160%
Hunt Petroleum UK Ltd
22.3900%
Bow Valley Petroleum (UK) Limited
12.4020%
Initial and Second Term Working Obligations:
•
The Licensee shall before the expiry of the initial term of this Licence carry out
not less than 200 kilometres of seismic survey work in the licensed area.
•
The Licensee shall before the expiry of the second term of this Licence carry out
not less than 200 kilometres of seismic survey work in the licensed area and drill
therein one exploration well.
Fallow Status at time of relinquishment: N3 – Not Fallow – Well drilled
Licence P.272 was fully relinquished on 1st November 2006 and no outstanding
obligations remained on the Licence. Three wells were drilled on the Licence and a total
of 6 wells were drilled in block 20/7 (Figure 1), which were all Plugged and Abandoned
Dry. The Licence obligations were fulfilled by the original group of Licensees by the
drilling of well 20/7-1 in 1978. Wells 20/7a-4 and 20/7a-6 were drilled by the current
operator in 2003 and 2006 respectively.
Block 20/7a is divided into two main structural elements by a major north throwing, NESW oriented, normal fault, which separates the Ettrick Graben to the north and the
Peterhead Graben to the south. Wells 20/7-1, 20/7-2, 20/7a-3 and 20/7a-4 were drilled
on the footwall of the major fault (Figure 2).
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2 - Exploration Activities:
2.1 - Purchase 130km2 of Aker 1999 survey
130 square kilometers of full fold 3D Aker 1999 seismic data were purchased in 2002
over block 20/7a (Figure 3), in order to define the Squirrel and Black Cat leads. The data
in the Upper Jurassic section appeared to be very noisy and the reflectors critical for the
mapping of the leads showed a lot of discontinuity. Reprocessing of the Aker 1999 data
started in 2003.
2.2 – Reprocessing of 3D Aker data
The Aker 1999 3D seismic reprocessing was completed by CGG in 2004. It was part of a
bigger project including the adjacent acreages operated by Nexen. Only the NW corner
of 20/7a was reprocessed, over the Black Cat prospect. Pre-processing paid careful
attention to editing of noisy traces; correcting for variations in shot strength an/or
receiver sensitivity and maximising resolution of data (zero phase/Q phase only
correction). A lot of effort was directed (pre- and possibly post-stack) towards the
attenuation of multiples. As well as conventional xt and tau-p methods, typical UKCS
processing sequence were used, including full Kirchhoff 3D PSTM. A true amplitude
processing sequence was required to allow for any subsequent AVO investigation. A line
comparing the original and the reprocessed version shows that there is significant
improvement in the resolution and continuity of the Upper Jurassic section as well as far
better noise attenuation (Figure 4).
2.3 - Biostratigraphic Study
A revised Humber Group stratigraphy of wells 20/7-1, 20/7-2, 20/7a-3, 20/7a-4, 20/1-2,
20/2-4 was carried out in 2003 by BVR International Ltd (Les Riley). In 2006,
biostratigraphic analysis was carried out on 20/7a-6.
Three major sequences were identified in the Upper Jurassic (Figure 5): the
Kimmeridgian-Early Volgian sequence (J56-J66B Maximum Flooding Surfaces of
Partington et al, 1993) during which the Buzzard Sands were deposited, the Middle
Volgian sequence (J66B-J73B MFSs) including the Ettrick Sands and the Late VolgianLicence P.272Relinquishment Report –
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Early Ryazanian sequence (J73B-J76 MFSs) corresponding to the Burns Sand unit. The
Burns Sand is not present in the block.
2.4 - Well 20/7a-4 Results, Squirrel Prospect
20/7a-4 was drilled by the Sedco 714 semi-submersible and spudded on 9th March 2003.
TD was reached on 27th March 2003 at a depth of 8909ft MDBRT in the Kimmeridge
Clay Formation. 20/7a-4 was plugged and abandoned as a dry hole.
The Squirrel structure was a stratigraphic pinchout in Upper Jurassic Ettrick and Buzzard
sands towards the crest of a major east-west fault (Figure 2). The prospect contained up
to 1300 ft of vertical relief updip and 2 km to the west of well 20/7-2. The latter
encountered thick, good quality Jurassic sands with hydrocarbon shows, but was
interpreted as being too far down dip. Two other wells, 20/7-1 and 20/7a-3 were drilled
too far west of the pinchout and failed to encounter any sand (Figure 5).
The top Ettrick/Buzzard sandstones was penetrated at 8495 ft MDBRT (-8407 ft
TVDSS), with a 196 ft gross vertical thickness of reservoir section. The reservoir section
consisted of interbedded claystones and good quality sandstones. No hydrocarbon
shows were observed (Figure 6). Because of problems with borehole stability in the 12
¼” section wireline logs were not run. After petrophysical analysis of the LWD logs the
reservoir was confirmed as water bearing .
The well reached TD at a depth of 8909 ft MDBRT (-8820 ft TVDSS) having penetrated
214ft MD of the Pre-Buzzard Unit Kimmeridge Clay Formation. After evaluation of the
logs the well was plugged and abandoned.
2.5 - Well 20/7a-6 Black Cat Prospect
20/7a-6 was drilled by the John Shaw rig and spudded on 9th January 2006 as a tight
hole.
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3 - Prospectivity Analysis
•
P.272 Group drilled Squirrel well 20/7a-4 in 2003, operated by EnCana. The well
objectives were the pinch-out of the Volgian sands to the west, between 20/7-2
water-wet Volgian sands and 20/7-1 condensed Upper Jurassic shale section.
The well penetrated the reservoir target but was P&A dry. The potential
remaining attic volume on the Squirrel structure ranges from 4 mmbo and 9
mmbo of STOIIP (Figure 7).
•
No prospectivity in the pre-Jurassic section was identified because of a lack of
structural closure (Figure 8) and a lack of potential reservoir in the Triassic and
Permian sections (Figure 9)
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Figure 1: Location Map
Block 20/7a
Nexen
Operated
Blocks
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Figure 2 : Top Middle Volgian – J73B MFS Depth Map
Ettrick
Field
Ettrick Basin
Peterhead Graben
0
2km
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Top Ettrick Silt – J73B MFS
Depth Map
Contour interval 50ft
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Figure 3 : 3D Seismic Data - Aker 1999
Block 20/7a
3D Aker 1999
Nexen
Operated
Blocks
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Figure 4 : Aker 1999 3D Seismic Reprocessing Results
Far better noise attenuation, apparent at BCU
BCU
Top
Buzzard
BUJ
Aker 1999
Far more resolution & continuity
Reprocessed version
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Figure 5: Upper Jurassic Well Correlation
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Figure 6: 20/7a-4 CPI
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Figure 7: Attic Volume up-dip of Well 20/7a-4
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Figure 8: Top Triassic Depth Map
0
2km
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Base Upper
Jurassic
Depth Map
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Figure 9: Pre-Jurassic Lithology
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REFERENCES
Partington, M.A., Mitchener, B.C., Milton, N.J. and Fraser, A.J. 1993. Genetic sequence
stratigraphy for the North Sea late Jurassic and Early Cretaceous: distribution and
prediction of Kimmeridgian-Late Ryazanian reservoirs in the North Sea and adjacent
areas. In: Parker, J.R. (ed) Petroleum Geology of Northwest Europe: Proceedings of the
4th Conference, 347-370. © 1993 Petroleum Geology ’86 Ltd. Published by the
Geological Society, London.
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