McLAUGHLIN, O. M., Department of Geological Sciences, University College of London, England; R. S. HASZELDINE, Department of Geology and Applied Geology, Glasgow University, Scotland; and A. E. FALLICK, and G. ROGERS, Isotope Geology Unit, SURRC, East Kilbride, Scotland

ABSTRACT: Isotopically Layered Basinal And Meteoric Porewaters; South Brae Oilfield Diagenesis, North Sea, UK

Reservoirs of the South Brae Oil field compromise a 1540 m sequence of Upper Jurassic submarine fan sandstones and conglomerates inter-digitating with Kimmeridge Clay oil source rocks. The earliest diagenetic phase was calcite cementation which formed concretionary bodies 0.8-4 m diameter during shallow burial. These concretions were precipitated from a meteoric fluid (isotope){18}O = -7 o/oo), which flushed through the reservoir displacing the depositional marine pore fluid and causing aggressive dissolution of detrital shell debris, plagioclase and K-feldspar. Oxygen isotope values record precipitation at temperatures between 15 degrees and 70 degrees C at depths of 0.3 to 2.3 km (isotope){13}C and {87}Sr/{86}Sr ratios record increasing thermal decarboxylation and silicate dissoluti n.

This was followed by quartz cementation. Oxygen isotope and fluid inclusion data suggest that the greatest volume of quartz was precipitated at temperatures between 70 degrees and 110 degrees C (2.3 to 3.7 km), from a layered porewater. Warm, isotopically-modified, compaction driven, basinal waters ((isotope){18}O = +4 o/oo) overlay cooler meteoric derived waters ((isotope){18}O = 4 o/oo). Cementation occurred from 55 to 20 Ma and may be related to periods of overpressure release. Locally quartz was diffusively supplied by pressure solution within siltstones and possibly from mudrocks.

A late dissolution event has enhanced porosity, due to dissolution of diagenetic calcite and detrital K-feldspar, particularly in the upper reservoir zones. This secondary porosity formation was due to the movement of basinal fluids, compacted from the interdigitating Kimmeridge Clay and drained out of the basin laterally through the reservoir. The oil-water contact has no poroperm effect. From 2 Ma, the reservoir sealed and became overpressured by 900 psi halting fluid movement through the oilfield, and permitting final hydrocarbon accumulation to occur.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.