Depositional Environments and Gas Production Trends, Olmos Sandstone, Upper Cretaceous, Webb County, Texas: ABSTRACT

Abstract

ABSTRACT 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 BCFG from 11 gas fields. The composition, texture and sedimentary structures of the Olmos were examined from more than 300 feet of full-diameter, diamond bit cores and 50 thin sections. The morphology of the sandstones was determined by correlation of over 300 electric logs. Lithologic/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 sheet-like morphology and are much thinner. Structure maps on top 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. Production trends closely parallel depositional trends. Updip wells produce an average of 52 MMCFG/year/well, while downdip wells average only 33 MMCFG/year/well. Depositional environment is the controlling factor on Olmos sand thickness and morphology, and thus, gas production.