Abstract: A Geological Exploration Model for Offstructure Gas Accumulations in Geopressured Lower Miocene Sandstones, Offshore Texas

BREWSTER, CHARLES
Texaco, New Orleans, LA.;
CHARLES SHARPE
Amoco, Houston, TX,
SHARON LANDECHE
Amoco, New Orleans, LA;
LORI HATHON
Shell E&P, Houston, TX;
BOB KLEIN
Amoco, Houston, TX

Diagenetic traps formed in geopressured sandstone beds that lap onto a shale ridge are observed at Matagorda Island 519 field. The compartmentalized reservoir at Matagorda 519 is thought to form as a result of complex diagenetic processes, including the generation of secondary porosity, related to fluid flow and pressure cell formation. Study of this field has given rise to an exploration model that predicts exploration targets in similar offstructure positions of neighboring ridges. The prediction of such fields can be enhanced through geochemical, geopressure and seismic interval velocity analyses.

The proposed geologic model is dynamic; it involves several periods of geopressuring, and it includes diagenetic mineral assemblages consistent with hydrothermal fluid flow alteration. The updip portion of the sandstone bed is located proximal to a regional growth fault that may have acted as a conduit for basinal fluids. A noneconomic sandstone cemented by pyrite and polycrystalline quartz is found updip of the hydrocarbon accumulation. The non-economic sandstones underlie two, banded, geopressured shale compartments. Downdip of the non-economic zone, the sandstone grades laterally into a quality reservoir characterized in thin section by patchy Feankerite and Fe-calcite residual cements. Interval velocities within the sandstone gradually slow downdip where secondary porosity is saturated with hydrocarbons.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas