The Olmos Sandstone is part of the Upper Cretaceous Taylor Group of south Texas. It is overlain by shales and sands of the Escondido Formation, and underlain by shales of the lower Taylor Group. In the subsurface of Webb County, the Olmos has produced over 142 bcf of gas from 11 fields.

The composition, texture, and sedimentary structures of the Olmos were examined from more than 300 ft (91 m) of full-diameter, diamond bit cores and 50 thin sections. The morphology of the sandstones was determined by correlation of over 300 electric logs.

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Lithologic and petrologic analysis indicates that the Olmos was formed in two major sedimentary environments. Deltaic distributary channel, levee, marine bay, marsh, and crevasse splay sequences are recognizable in cores from updip wells. However, cores from downdip wells show open marine shelf sequences, occasionally interrupted by ordered, graded, and thin-bedded sandstones deposited by density flows.

Net sandstone isopach maps of the Olmos show that the Olmos was deposited updip as a series of overlapping, lobate sand bodies. Downdip sands have a sheetlike morphology and are much thinner. Structure maps of the top of the Cretaceous show gentle southeast dip in updip areas, indicating stratigraphic trapping of gas in those areas. However, downdip, gas is trapped against a series of down-to-the-coast normal faults.

Gas production trends closely parallel depositional trends. Updip wells produce an average of 52 mmcf/year/well, whereas downdip wells average only 33 mmcf/year/well. Depositional environment is the controlling factor on Olmos sand thickness and morphology, and thus, gas production.

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