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Abstract: Enhanced Porosity from Diagenesis of Deepwater Sandstones: Brae Oilfields, North Sea

HASZELDINE R S, University of Glasgow, Scotland; C MACAULAY, A E FALLICK, Scottish Universities Research and Reactor Centre, Scotland; A Marchand, R Swennen, University of Leuven, Belgium.

Summary

The faulted edge of the North Sea Viking Graben contains a complex of oilfields in Jurassic syn-rift submarine fan deposits of the Brae sandstone petroleum system. These are overlain by the West Brae oilfield in Palaeocene submarine channels deposited during thermal subsidence. These fields are well-drilled and form useful deepwater analogues. Jurassic reservoirs show extensive calcite cementation from early ingress of meteoric water. Carbon isotopes record a distinctive organic supply (d13C-12^pmil) from the intimately enclosing Kimmeridge Clay source rock. Quartz forms a major porosity filling cement. The North Brae field has lost oil updip to West Brae oilfield. Exceptionally high poroperm occurs within the Jurassic sandstones:- 25%, 2,000mD at 4.0km, compared with 15%, 200mD in cemented sandstones stratigraphically adjacent. These zones are explained:- EITHER by fluid inclusions showing an early oil charge preventing cementation and so preserving primary porosity, such zones are probably discontinuous; OR by stratigraphically layered oxygen isotopes (d180 = 25.3-17.1^pmil) showing expulsion of basinal porewaters and oil overlying meteoric water, dissolving carbonate and feldspar, thus producing secondary porosity and resurrecting primary porosity, such zones are laterally continuous along the top of sandstone aquifers. Clay has been exported from these sandstones, further improving quality. Updip, the West Brae field has distinctive excess calcite (d13C -27^pmil) from oxidation of fill-spill oil, and has localised excess clay cement, from partial transport of deeper-derived clay. Diagenetic fluid migration has improved the reservoir in predictable locations, and these can be inferred from effects shallower in the basin.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah