From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau Seminar, October 26, 2007

A geological exploration model for offstructure, geopressured, gas accumulations in Lower Miocene Sandstones, offshore Texas

Charles Brewster
Gulf Energy Exploration, Austin, Texas


Studies of the Matagorda Island 519 field offshore Texas led to an exploration model to predict the existence of similar exploration targets in geopressured sandstone in flanking structural positions. The extensional tectonic setting includes coast-parallel, Lower Miocene expansion faults. The deltaic depocenters, formed on the downthrown sides of these faults, deformed older, underlying shales into ridges. These ridges are oriented perpendicular to the strike of the expansion faults, and structures over the ridges are preferred areas for hydrocarbon accumulations. An exception to this simplified setting is the Matagorda Island 519 field. This field is on the structural flank of a coast-perpendicular ridge, in geopressured sandstone, showing pressure and diagenetic compartmentalization. These compartmentalized reservoirs are thought to form as a result of a complicated secondary porosity diagenetic process related to fluid flow and pressure cell formation. Prediction of similar exploration targets can be enhanced through geochemical, pressure, and seismic interval velocity analysis.

Exploration Model.
The proposed geologic model is dynamic, involving several periods of pressure formation and includes diagenetic mineral assemblages consistent with hydrothermal fluid flow alterations.