--> Abstract: Sedimentation Rate as the Control on Hydrocarbon Source, Generation and Migration in the Deepwater Gulf of Mexico, by N. Piggott and A. J. Pulham; #90987 (1993).

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PIGGOTT, NEIL, and ANDREW J. PULHAM, BP Exploration, Houston, TX

ABSTRACT: Sedimentation Rate as the Control on Hydrocarbon Source, Generation and Migration in the Deepwater Gulf of Mexico

Geochemical analysis of oils from the Deepwater Gulf of Mexico defined three discrete oil families. These can be related to Mesozoic carbonate and clastic source systems, deposited within a restricted basin setting during periods of low clastic sediment supply.

Generation of hydrocarbons from these source rocks results from deep burial in a cool geothermal setting due to rapid sedimentation during Late Tertiary and Quaternary times. Detailed stratigraphic analysis reveals that this sedimentation occurs in relatively discrete pulses separated by significant hiatuses (condensed sections, but with no source potential). Each pulse of sedimentation comprises a distinct deepwater slope complex. These abandon and shift laterally in response to major avulsions in the updip deltaic/shelf system. The mature source kitchen can be traced as moving through time in response to these switches, with generation slowing during the hiatal phases.

Rapid sedimentation also provides the mechanism for extensive vertical migration of hydrocarbons. Compaction disequilibrium and poor sealing characteristics caused by development of extreme overpressure is one mechanism. In addition, extensive halokinesis due to sediment loading of shallow buried salt in the slope created high permeability conduits.

As well as being periods of diminishing hydrocarbon generation and migration, the hiatal phases allow biodegradation of shallow trapped oils, reducing oil quality and mobility. In some cases these heavy oils can be seen to have been "topped-up" by late light oil and gas generated in response to renewed rapid sedimentation as the switching depocenters re-occupied previously abandoned deepwater environments (e.g. Northern Green Canyon area).

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