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Reservoir architecture and diagenesis in downthrown fault block plays: the Lowlander Prospect of Block 14/20b, Witch Ground Graben, Outer Moray Firth, UK North Sea

Abstract The Lowlander Prospect is an Upper Jurassic, downthrown hydrocarbon accumulation, with three-way dip closure and fault seal developed within the Sgiath and Piper formations. The prospect was discovered in 1987 and has since been appraised by more than 900 ft of core from five wells. Detailed core description, conventional core analysis, thin section petrography, fluid inclusion microthermometry, stable isotope analysis and over 2000 ft of wireline petrophysical log interpretation were utilized in constructing an integrated reservoir and diagenetic model for the reservoir units in the prospect. Piper Formation sandstones form the main reservoir in the Lowlander Prospect. Deposition of Piper Formation sandstones was controlled by three major structural elements which served as separate hinterlands; the Fladen Ground Spur and Piper Ridge to the northeast, Halibut Horst and Platform to the southwest, and regional Toarcian-Aalenian domal uplift to the southeast. Two major coarsening up sequences are present in the Lowlander Piper Formation that represent progradational strandline deposits. Transgressive abandonment sandstones, locally preserved beneath a top Piper Formation unconformity, represent a Volgian transgression and associated reworking of Piper Formation sediments. Mechanical compaction and diagenetic cementation, the latter including early non-ferroan calcite, kaolin, quartz overgrowths, ankerite, late calcite and barite-sulphide mineralization, have resulted in generally low intergranular porosities in Piper Formation sandstones. However, total porosities are enhanced by the development of late, post-quartz overgrowth feldspar grain dissolution. Microthermometric study of fluid inclusions in quartz overgrowths indicates that precipitation of authigenic quartz took place at temperatures of between 88 and 122°C, concomitant with hydrocarbon migration. Best reservoir quality is developed adjacent to basin centre faults where early carbonate cements and detrital feldspars have been extensively dissolved. Higher depositional energy facies adjacent to the faults were the initial focal point for aggressive fluid ingress into the reservoir. Fluid migration away from the faults resulted in reprecipitation of carbonate cements up-structure towards the graben edges where reservoir quality is now the lowest. Structural highs thus need not be the prime exploration target in downthrown, hanging-wall basin centre plays.