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Previous HitFaultNext Hit-Zone Seals in Offshore Trinidad Oil Fields

GIBSON, RICHARD G., Amoco Research Center, Tulsa, OK

Hydrocarbon columns in Pliocene sands of offshore SE Trinidad occur in 3-way closure, primarily in the footwall of normal faults. Multiple reservoir sands and numerous Previous HitfaultNext Hit blocks result in a large number of individual hydrocarbon accumulations. Previous HitFaultNext Hit-plane sections demonstrate that Previous HitfaultNext Hit sealing is unrelated to juxtaposition of intercalated shales. These Previous HitfaultNext Hit-zone capillary seals were studied by (1) inferring their fluid-flow properties from the pattern of trapped hydrocarbons and (2) direct examination and measurement of cored faults.

Buoyancy pressures for hydrocarbon columns were calculated from fluid property data for each reservoir and Previous HitfaultNext Hit block. Buoyancy pressures range widely, increasing nonlinearly with Previous HitfaultNext Hit displacement and percent shale in the faulted section, but do not vary systematically with stratigraphic position or depth. Small-displacement faults observed in core are narrow zones of cataclasis within porous sandstone. Mercury injection tests indicate Previous HitfaultNext Hit-zone displacement pressures that coincide with buoyancy pressures calculated for hydrocarbon columns sealed by large-displacement faults.

The agreement between measured displacement pressures and calculated buoyancy pressures indicates that (1) the reservoirs are filled to their capacity, dictated by the displacement pressure of the Previous HitfaultNext Hit zones, and (2) the Previous HitfaultNext Hit-zone seals are primarily the product of deformation of the sands, with some enhancement by incorporation of argillaceous material into the Previous HitfaultNext Hit zones. The observed relationship between Previous HitfaultNext Hit displacement and calculated buoyancy pressure of the hydrocarbon columns implies that Previous HitfaultNext Hit-zone continuity is a factor that needs to be assessed in Previous HitfaultTop-zone seal analysis.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)