ABSTRACT: Geologic Controls on Tight-Gas Production from a Storm-Dominated Shelf: Upper Cretaceous Olmos Formation, Texas

BARROW, KENNETH T., Bureau of Economic Geology, University of Texas at Austin, Austin, TX, and GEORGE B. ASQUITH, Texas Tech University, Lubbock, TX

Tight-gas sandstones of the Olmos Formation in the downdip Las Tiendas trend were deposited on a storm-dominated shelf proximal to a major deltaic complex. Detailed correlations of logs from more than 500 wells show that production is independent of structure and that the shelf sands can be tied directly back to deltaic deposits. Nine parasequences in an aggradational stacking pattern contain one to six strike-elongate sand bodies, each 20 to 50 ft thick and separated by 5 to 100 ft of mudstone. Sand bodies extend up to 10 mi downdip and 40 mi along strike.

Sand bodies are composed of sharply based, thin-bedded sandstones that are massive, horizontally laminated, or, more rarely, low-angle crossbedded. Sandstones grade upward into thoroughly bioturbated silty sandstones and sandy mudstones. These beds are interpreted to be storm deposits derived from the nearby deltaic headland. Detrital and authigenic clays, primarily illite/smectite and Fe-rich chlorite, comprise up to 30% of some sandstones. Secondary calcite further reduces porosity and helps form the updip trapping mechanism.

Strike-oriented belts of high production parallel sand thickness trends. However, highest production occurs in the longshore-distal part of the trend, where sand thicknesses are generally lower than in the area proximal to the delta. This may be due to increased concentrations of calcite in proximal areas. Hydrocarbon pore volume corrected for volume of clay has been calculated from available log suites and compared with cumulative gas production. Gas recovery efficiency varies across the trend; significant reserves remain in areas where average drainage radius is much less than regulated well spacing. Field designation and well spacing in the downdip trend should be reexamined in light of the geologic controls on reservoir properties and gas production.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)