ABSTRACT

The Jurassic (Oxfordian) Smackover Formation in Thomasville Field, Rankin County, Mississippi is sour gas productive from deep (19,000'+), geopressured (>.88 psi/ft.) siliciclastics that are interbedded with nonreservoir carbonates. The siliciclastics are moderately to moderately-well sorted, fine- to medium-grained subarkosic sandstones. They are characterized by low average porosities (7.0 percent) and permeabilities (0.35 millidarcies (md)) resulting from both diagenetic and primary textural controls. Secondary and altered primary porosity are present. Permeability is reduced by several diagenetic events including the formation of anthraxolite and authigenic fibrous illite.

Smackover carbonate ramp deposition was interrupted in Thomasville by the periodic influx of siliciclastics. Siliciclastics were delivered to the basin of deposition by ancestral drainage systems and distributed on the ramp by current and storm processes. They are interpreted to be shallow marine (beach and bar), inner shelf (sand waves and storm deposits), and outer shelf to upper basinal (storm and turbidite channels) deposits based on sedimentary structures, trace fossils, and lateral and vertical facies association with the interbedded carbonates. The carbonates are indicative of a shoaling-upward sequence with outer shelf and upper basinal mudstones and wackestones at the base grading upward through shelf packstones, high energy ooid grainstone shoals, and intertidal mudstones to grainstones.

Rock properties and interpretations of the depositional environments were based on core and drill-cutting analyses. These analyses supplemented the use of computer-enhanced wireline logs to establish rock-log calibrations, correlation markers, structural growth, and reservoir continuity.