Reservoir Characterization of Tight Gas Sand: Taylor Sandstone (Upper Cotton Valley Group, Upper Jurassic), Rusk County, Texas

C. L. Vavra, M. H. Scheihing, J. D. Klein

An integrated petrographic, sedimentologic, and log analysis study of the Taylor sandstone in Rusk County, Texas, was conducted to understand the geologic controls on reservoir performance and to identify pay zones for reserves calculations.

The Taylor sandstone interval consists of tightly cemented, fine-grained quartzose sandstones interbedded with mudstones, siltstones, and carbonates that occur in upward-coarsening sequences. Helium permeability rarely exceeds 0.1 md, and porosity is rarely greater than 10%. Relationships between porosity and permeability are diffuse because of a strong diagenetic overprint.

Six major rock types or petrofacies are distinguished on the basis of pore type and dominant cement mineralogy. Three sandstone petrofacies--primary macroporous quartz cemented, moldic macroporous quartz cemented, and microporous clay cemented--have reservoir potential. Although these petrofacies have similar porosities and permeabilities, fluid saturations differ considerably due to differences in pore geometry as indicated by petrographic and capillary pressure analyses. These three reservoir-quality petrofacies can each be identified directly on wireline logs by applying cutoffs to the porosity and normalized gamma-ray logs.

Reservoir-quality petrofacies can be tied to larger scale depositional facies. The Taylor interval consists of stacked barrier island sandstone bodies with associated shoreface and inner-shelf sandstones. The primary macroporous petrofacies is associated with upper shoreface/beach and tidal-channel facies. Moldic macroporous petrofacies is associated with shoreface and foreshore facies. Microporous petrofacies is associated with clay-rich, more poorly sorted and bioturbated sandstones of the lower shoreface, inner shelf, and some back-barrier sandstones.

This study demonstrated the value of an integrated approach to reservoir characterization by combining pore level, interwell, and fieldwide level description and identifying reservoir-quality rock types on wireline logs.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.