Significant gas reserves at depths ranging from 19,800 to 20,400 ft (6,035 to 6,218 m) have been established in Tuscaloosa sandstones at False River and Judge Digby fields (Chevron U.S.A. Inc. discoveries) located within the deep Tuscaloosa gas trend. The sandstones are within an Upper Cretaceous clastic wedge that thickens in front of, and to the south of, a Lower Cretaceous carbonate shelf.

Reservoirs at Judge Digby field are part of a thick clastic section comprised of gray shales and medium to coarse-grained sandstones, in part conglomeratic. Paleontology, textural grading, bed contacts, and sequential arrangement of sedimentary structures suggest fluvial and deltaic environments. Reservoirs at False River field are stratigraphically younger and are interpreted as part of a barrier bar sequence. These sandstones are fine to medium-grained and occur at the top of upward-coarsening gradational sequences. In both fields, the reservoirs commonly have maximum porosities above 25% and permeabilities in excess of 100 md. Although these reservoirs are presently geopressured, petrographic evidence and density of the associated shales indicate that the sandstones are not underco pacted. A study of the sandstone framework suggests that burial diagenesis dramatically reduced the primary pore system, eliminating significant permeability. Leaching of the grain framework created an important contribution to effective secondary porosity and reestablished lost permeability. The secondary pores are easily identified by relic rims of authigenic, grain-coating chlorite cement which persisted after selective grain removal by dissolution. Commercial

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Figure

production from these deeply buried Tuscaloosa sandstones is dependent upon secondary enhancement of a reduced primary pore system and its subsequent maintenance by geopressure.

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