The Upper Jurassic Smackover Formation throughout most of the U.S. Gulf Coast is composed of a regressive sequence of carbonate ramp deposits. From central Mississippi to eastern Louisiana, however, carbonate deposition was influenced or interrupted by a large influx of siliciclastics derived from an ancestral Mississippi River source. Longshore currents and storm processes, combined with sea-level fluctuations, led to interfingering of the sandstones and carbonates in nearshore to basinal depositional environments.
In central Mississippi the sour gas trend (five fields at depths greater than 19,300 ft (5,880 m)) is characterized by mixed siliciclastic/carbonate deposits that occur in a general upward-shoaling sequence. The upward-shoaling sequence is evident both within fields as well as from downdip to more updip locations. In Thomasville, the most updip field, the upward-shoaling sequence is composed of (1) outer ramp bioturbated storm deposits interbedded with mudstones and wackestones, (2) lower shoreface and shelf ridge sand bodies that are gradational with pelletal packstones and minor ooid grainstones, and (3) shoreface and shoal sandstones with gradational contacts into high-energy ooid grainstone deposits with local skeletal buildups. Tidal-flat carbonate mudstones and Buckner Formation sabkha evaporites are the uppermost units of the upward-shoaling sequence and are the top seals to the hydrocarbon accumulation. Harrisville, the most downdip field, contains a basal sandstone that is interpreted to be a basinal turbidite deposit interbedded with microlaminated carbonates. Only the lower half of the upward-shoaling sequence is present in Harrisville, with the uppermost sandstones interbedded with middle to outer ramp skeletal and pelletal packstones and wackestones.
Thomasville and Harrisville are sour gas (H2S concentrations of 35 to 41 percent), productive from low porosity and permeability sandstones that are interbedded primarily with nonreservoir carbonates. Even though a complex diagenetic history has strongly overprinted the original depositional fabric and reduced the reservoir quality, high geopressures, large drainage areas, and enough altered primary and secondary porosity remain for both high production rates and economic prospects in these hostile subsurface environments. Important diagenetic events include early compaction, cementation by both calcite and more importantly dolomite, quartz and feldspar overgrowths, hydrocarbon migration and calcite dissolution, bitumen formation, and thermochemical sulfate reduction to form the high H2S concentrations. The original depositional setting and proximity of the carbonate interbeds to the sandstones have played major roles in defining the trap, reservoir quality, hydrocarbon accumulation, and final hydrocarbon mix in these reservoirs.
