--> Abstract: Shallow Calcite Cementation in a Submarine Fan: Biodegradation of Vertically Migrating Oil?, by R. Stewart, S. Haszeldine, T. Fallick, R. Anderton, and R. Dixon; #90987 (1993).

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STEWART, ROBBIE, and STUART HASZELDINE, Glasgow University, Glasgow, Scotland; TONY FALLICK, SURRRC, East Kilbride, Scotland; and ROGER ANDERTON and RICHARD DIXON, BP Exploration, Glasgow, Scotland

ABSTRACT: Shallow Calcite Cementation in a Submarine Fan: Biodegradation of Vertically Migrating Oil?

Early Palaeocene sand-rich submarine fans of the Andrew Formation were deposited as regionally extensive sand bodies in the Witch Ground and Central Grabens of the North Sea Basin, UK. Diagenetic Fe-calcites form as concretions up to several metres thick and as dispersed cements (3%) within the fan sandstones. High minus-cement porosities (35%) within the concretions suggest growth at burial depths less than 500 m. Oxygen isotope analyses from the center to the rim of the concretions (isotope{18}O = -10.6 to -8.5 o/oo PDB) indicate that meteoric water displaced depositional marine water. The radial increase in the isotope{18}O values reflects the mixing of this meteoric water with compactional marine-derived waters sourced from the enclosing mudrocks. Carbon isotope trends in the conc etions (isotope{13}C= -23.1 to -29.0 o/oo PDB) are unusual in that they show an increasingly negative trend towards concretion rims. This suggests an increasing input from the oxidative biodegradation of oil (isotope{18}O = -29.4 o/oo PDB).

Preliminary analyses of a large well log database from the North Sea basin indicates that calcite cementation is more prevelant in the vicinity hydrocarbon discoveries rather than dry holes. Occasionally calcite-cemented zones are sufficiently thick (up to 50 m) to be resolvable as high amplitude events on seismic data. These events are not flat but are tilted by 2-3 degrees and possibly represent palaeo-hydrocarbon contacts.

We infer that vertical hydrocarbon migration occurred from deeply buried (3 km+) Jurassic source rocks into shallow reservoirs at the same time as meteoric flushing. Carbonate cements were produced in the mixing zone.

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