Through a cooperative research program among Schlumberger-Doll Research (Ridgefield, Connecticut), Schlumberger Well Services (Shreveport, Louisiana, and Houston, Texas), and Delta Drilling Co. (Tyler, Texas), a whole core and a conventional (DIT (FOOTNOTE *)/BHC, GR/CNL (FOOTNOTE *)/FDC (FOOTNOTE *), HDT/FIL (FOOTNOTE *)) and experimental-prototype (Digital Sonic, CNT-G, EPT (FOOTNOTE *), GST-A (FOOTNOTE *), NGT (FOOTNOTE *), NML (FOOTNOTE *)) suite of logs were examined from the lower and middle Cotton Valley sands (Upper Jurassic) of a 10,200-ft (3,100 m) well (Alice Snider 1). The well is located in the Carthage tight gas sand field, located in Panola County, about 60 mi east of Tyler, Texas. Particular attention was directed to one of the two gas-producing zones (9,3 4 to 9,638 ft, 2,851 to 2,938 m); a deeper gas-producing zone (9,700 to 10,027 ft, 2,957 to 3,056 m) was not cored.

A synthesis of the rock core and log data leads to the following conclusions.

1. The formation is dominantly a well-laminated and horizontally bedded unit with low-angle, planar cross-bedding, shallow-water ripple marks, and convoluted bedding. Bioturbation and diagenesis have dramatically altered these features in some parts of the core.

2. The lithology is an alternating sequence of lithic sandstones (sublitharenites) and silty shales. Minor amounts of conglomerates, coal laminae, and fossil-bearing zones are also noted.

3. The tight gas sands have low porosities (< 10%) and low permeabilities (< 0.1 md). Intergranular porosity (2%) is reduced by authigenic quartz and/or calcite overgrowths and/or pore-lining, filling, and bridging clay minerals--illite, chlorite, and illite-chlorite mixed layers. Relatively abundant intergranular and secondary (4%) porosity is also noted.

4. The depositional environment of the gas-producing zone is interpreted as an alternating and transgressive sequence of shallow marine water dominated by barrier bars. Interbedded with the bioturbated shoreface sediments are minor tidal-deltaic sands deposited near the northeast-southwest-trending edge of a sedimentary basin.

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