Abstract: Interpretation of Cotton Valley Depositional Environment from Core Study, Frierson Field, Louisiana

Stephen A. Sonnenberg

Upper Cotton Valley sandstones are gas productive in northeast Texas and northern Louisiana. The Frierson field is a recently developed area that has a potential for significant gas reserves in the Davis, "B," and other sandstone members. The sandstones are interpreted as having been deposited as barrier bars that can be divided into three distinct facies based on vertical changes in texture, composition, and bedding. The facies are termed upper, middle, and lower shoreface and are similar to the modern Galveston Island barrier sequence.

The detrital composition of the Davis sandstone has an average of 86 percent quartz, and minor amounts of feldspars (1 percent), rock fragments (7 percent), and clay matrix (6 percent). These mineralogic components constitute 88 percent of the rock by bulk volume. The remaining 12 percent is primary porosity which has been occluded by 75 percent of silica quartz overgrowth and 25 percent of sparry calcite, each representing 9 and 3 percent, respectively, by bulk rock volume. Detrital composition ranges from 93 percent quartz in the upper shoreface, to 89 percent quartz in the middle shoreface, and to 76 percent quartz in the lower shoreface. "B" sandstones also are highly quartzose but have an average of 29 percent calcite matrix.

Grain size decreases downward; the lower shoreface has a mean of 0.22 mm, the middle shoreface has a mean of 0.14 mm, and the lower shoreface has a mean of 0.10 mm.

Because the role of cementation is important with regard to the migration, accumulation, and storage of petroleum in a sandstone reservoir, the order of deposition of cementing materials in the sandstones was established as authigenic quartz overgrowths followed by authigenic calcite. The results of this study may be applicable to other areas of Cotton Valley deposition in northern Louisiana and northeast Texas.

AAPG Search and Discovery Article #90975©1976 GCAGS- GC Section SEPM Annual Meeting Shreveport, Louisiana