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1 Conoco Exploration Research and Services, 1000 South Pine, Ponca City Oklahoma 74603, USA
2 Conoco United Kingdom Limited, currently at Conoco Norway Inc., Finnestadveien 28, P. O. Box 488, 4001 Stavanger, Norway
3 Corrigan Associates, 2 The Drove, Ditchling, Sussex BN6 8TR, UK
The Rotliegendes Group in the Vanguard Field, UKCS, consists of a series of aeolian, fluvial, and reworked aeolian sediments, organized into four broadly correlatable lithostratigraphical units. Within these units, reservoir properties are strongly linked to lithofacies, with aeolian dune sandstones possessing the best porosity and permeability.
A comparison of production log spinner profiles (PLT) with detailed sedimenttological logs from three development wells shows that gas production is intimately associated with specific lithofacies and their spatial organization. Although dune sandstones in the upper section of the Rotliegendes have good petrophysical characteristics, they do not contribute significantly to production because they are contained within laterally extensive and relatively impermeable interdune sandstones. Most production comes from 15–30 feet thick and highly permeable (100–1000 mD) dune foreset intervals deposited near the bases of large aeolian dunes.
Significant interwell variation in production rates and in porosity exists for all lithofacies due to the variable extent of diagensis. Petrographical studies, combined with whole-rock geochemical analysis, have outlined a diagenetic sequence for the field. Two end-member mineralogical assemblages are identified which are closely linked to reservoir quality. High porosity wells contain a chlorite-K-feldspar-quartz-dolomite assemblage while low porosity wells contain an illite-quartz-ankerite assemblage. Empirical evidence shows that porosity loss is associated with conversion of K-feldspar, chlorite and dolomite to illite and ankerite, and is due to compaction rather than cementation. Post-depositional faults acted as conduits for fluid which altered mineralogy, removed soluble components and, in one case, introduced anhydrite cement.
