ABSTRACT

Through a cooperative research program among Schlumberger-Doll Research (Ridgefield, CT), Schlumberger Well Services (Shreveport, LA and Houston, TX), and Delta Drilling Company (Tyler, TX), 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*, GST-A*, NGT*, NML*) suite of logs were examined from the Lower-Middle Cotton Valley sands (Upper Jurassic) in a 10,200 foot well (Alice Snider No. 1). The well is located in the Carthage tight gas sand field, Panola County, Texas. Particular attention was directed to one of the two gas-producing zones (9354-9638 ft); a deeper gas-producing zone (9,700-10,027 ft) 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 crossbedding, shallow-water ripple marks, and convoluted bedding. Bioturbation and diagenesis have dramatically altered these features in some portions 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/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 a regressive/transgressive sequence of shallow-water, bioturbated, shoreface sediments dominated by barrier bars with minor interbeds of tidal-deltaic deposits near the northeast-southwest trending edge of a sedimentary basin. Using these criteria and selected logs, an exploration model was developed.