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Abstract: Architecture, Internal Heterogeneity, and Resulting Drainage Efficiency of Upper Oligocene Frio Formation Inner-Shelf Sandstone Reservoirs in West Fulton Beach Field, Aransas County, Texas

Paul R. Knox

The architecture, internal heterogeneity, and production history of selected reservoirs in West Fulton Beach field, Copano Bay, Aransas County, Texas, were examined as part of a project to identify additional oil and gas reserves on Texas state lands. Upper Oligocene Frio Formation reservoirs in this field have yielded more than 146 bcf of gas and 8 MMbbl of oil. A high-resolution genetic stratigraphic analysis of inner-shelf sandstone reservoirs used well log and petrophysical data to locate uncontacted or incompletely drained reservoir compartments.

Shelf sandstone reservoirs are composed of thin (2-6 ft) sandstones that are laterally isolated but commonly vertically stacked and amalgamated into units as much as 16 ft thick. Individual sandstones constitute separate reservoir compartments that are vertically isolated if surrounded by inner-shelf shales or are in partial communication if vertically stacked, separated by a low-permeability bioturbated sandstone or siltstone layer of varying thickness. Heterogeneity within individual sandstone units is low. Deposition of inner-shelf sandstones varies--from proximal settings, below fair-weather wave base but above storm wave base, where the sandstones are thicker, more commonly amalgamated, and form well-interconnected compartments--to distal settings below storm wave base, where the are thinner and more commonly isolated. Production histories indicate that completions in proximal settings can drain more than 600 ac in a gas reservoir, whereas those in distal settings drain less than 200 ac.

High-resolution stratigraphic analysis of inner-shelf sandstone reservoirs at West Fulton Beach field has identified 11 bcf of additional reserves in untapped and incompletely drained reservoir compartments. The 63 other major fields of the downdip Frio barrier-bar/strandplain play of the central Texas Gulf Coast may contain as much as 500 bcf of additional gas that could be identified through similar efforts.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994