ABSTRACT: Early Diagenesis, Oil Migration and Carbonate Cementation of Tertiary Reservoir Sands: U.K. Sector, North Sea

WATSON, ROSELEEN S., and NIGEL H. TREWIN, Kings College, University of Aberdeen, Scotland, and ANTHONY E. FALLICK, Scottish Universities Research & Reactor Centre, East Kilbride, Glasgow, Scotland

Heavy oil and gas in the Forth field are reservoired in well-sorted medium- to fine-grained turbidite sands at maximum burial depths of 2.3 km. Sandstones range from totally unconsolidated to well cemented. The timing of hydrocarbon migration and the extent of carbonate cementation controlled reservoir quality, preventing overgrowths and pore-filling kaolinite from further development.

In well 2 (Early Eocene), Carbon 13 PDB values of -25 to -31o/oo are present in tight, calcite-cemented sands. Pre-cement porosities of 40% and the presence of hydrocarbon inclusions within the cement suggest oil emplacement and carbonate cementation occurred simultaneously within a few meters of the depositional surface. Biodegradation of the migrated oil under oxidative conditions possibly formed isotopically light carbonates with Carbon 13 PDB values more negative than -25o/oo.

Late Paleocene carbonates with Carbon 13 PDB isotopic signatures of -29.5 to +13o/oo were probably formed at greater depths, by a wide range of microbial and possibly abiotic reactions dependent upon the oxidation potential of the precipitating environment.

Similar Oxygen 18SMOW values for both oil- and gas-bearing units indicate a common pore fluid regime. Calculations relating temperatures of precipitation to isotopic composition, assuming oxygen 18SMOW of Eocene sea water, indicate precipitation temperatures in excess of 50 degrees C--higher than expected for shallow burial cementation. Alternatively, modified pore fluids carrying hydrocarbons from greater depths in the basin during the early Eocene may have displaced the original marine pore fluid prior to precipitation, resulting in carbonates with an analogous Oxygen 18 isotopic signature.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)