The four Triassic fields in the ETAP cluster (Marnock, Heron, Egret and Skua) have all underperformed. McKie et al. (2005, 2010) have shown that this is a combination of Skagerrak reservoir connectivity being less than expected, and fault compartmentalisation.
The reservoir connectivity issues are both lateral, with fluvial channels being poorly connected and vertical, with palaeosols being more extensive than expected. The fields start up strongly but rapidly decline as the perforated, high permeability sands are not recharged quickly enough to sustain production.
J block
The Jasmine paper in the UK Oil & Gas fields 50th anniversary commemorative volume (Geol Soc Mem 52) summarises J-block performance. Two of the J-block Triassic fields, Judy and Jasmine, have under-performed but Jade has been a success.
Jasmine’s initial production rates were as expected but then pressure and production fell rapidly, necessitating a complete rebuild of the reservoir model to understand field performance.
Appraisal drilling established that Judy contained sealing faults dividing the field into 3 volatile oil blocks and a gas condensate block. It also had reservoir connectivity issues.
Jade is a very layered reservoir but better reservoir properties gave good lateral connectivity.
Suitable for onshore development
For an offshore development, a ball-park recovery of 20 mmbbl or 120 Bcf per well is required for a commercial development. The heterogeneities in the Skagerrak sands reduce the drainage area per well so these recoveries are rarely achieved, as presented here.
If these Triassic fields would be found onshore, an eight acre well spacing would be appropriate to produce the gas economically. However, applying onshore well densities to an offshore development is still a long way away.
MALCOLM PYE
References
M. Lawrence, L. E. Armstrong, K. Ashton, A. D. Jones and I. E. Mearns, 2020, The Jasmine Field, Blocks 30/06 and 30/07a, UK North Sea Geological Society, London, Memoirs, 52, 498-510.
McKie and P. Audretsch, 2005, Depositional and structural controls on Triassic reservoir performance in the Heron Cluster, ETAP, Central North Sea Geological Society, London, Petroleum Geology Conference series, 6, 285-297.
McKie, S. J. Jolley and M. B. Kristensen, 2010, Stratigraphic and structural compartmentalization of dryland fluvial reservoirs: Triassic Heron Cluster, Central North Sea Geological Society, London, Special Publications, 347, 165-198.
