--> ABSTRACT: Depositional and Diagenetic Controls on Reservoir Quality and Gas Composition in a Mixed Siliciclastic/Carbonate Setting, Thomasville Field, Mississippi, by Roger Dean Shew; #91003 (1990).
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ABSTRACT: Depositional and Diagenetic Controls on Reservoir Previous HitQualityNext Hit and Gas Composition in a Mixed Siliciclastic/Carbonate Setting, Thomasville Field, Mississippi

Roger Dean Shew

The Smackover Formation in central Mississippi is sour gas productive from siliciclastics interbedded with tight carbonates. The sandstones have low reservoir Previous HitqualityNext Hit (porosity and permeability of 7.0% and 0.35 md, respectively), but wells are capable of producing at high rates and with large volumes because of the extremely large geopressures and locally thick continuous sandstone packages. Production is from a hostile subsurface environment which includes high temperatures (>365°F), high pressures (>0.88 psi/ft), great depths (>20,000 ft), and variable and corrosive sour gas mixtures (CH4 = 55%, H2S = 36%, CO2 = 9%). Five fields are productive in this trend; Thomasville, the largest field, is used to illustrate the depositi nal and diagenetic controls on productivity.

Deposition occurred as an upward-shoaling sequence of outer ramp to nearshore and sabkha environments. The lower sandstones are bioturbated, low-Previous HitqualityNext Hit reservoirs interbedded with carbonate mudstones and packstones; the middle interval consists of the thickest and most continuous sandstones that are interpreted to be amalgamated inner-ramp shoal and ridge deposits; the upper interval consists of the common but tight Smackover ooid grainstone shoals that are interbedded with continuous but thin shoal and shoreface sandstone. The middle interval has the best reservoir Previous HitqualityNext Hit and contains 75% of the hydrocarbons. Seismic may be used to map the sandstone packages but not the individual sandstones. In addition to the larger net/gross and more continuous sandstones in the middle zone, two postdepositional controls also govern the reservoir Previous HitqualityNext Hit. These include small-scale faulting but more importantly diagenetic events which strongly overprint but do not obscure the original depositional Previous HitcontrolNext Hit.

At least 15 different diagenetic events have occurred in the sour gas fields that have generally decreased reservoir Previous HitqualityNext Hit. The most important include the early formation of dolomite and calcite cements, migration and cracking of the oil, and thermochemical sulfate reduction (TSR). Early formed dolomite has helped to prop the framework grains to lessen compaction. An optimum amount of carbonate cement (6-20%) occurs in the best reservoir Previous HitqualityNext Hit zones. Sandstones with too little cement are often highly compacted, and those with too much, such as those adjacent to carbonate interbeds, have low porosity. The increased carbonate cement adjacent to these interbeds is the reason the thinner sandstones have poorer reservoir potential. Thermal cracking has led to the formation of large amo nts of bitumen in the pore space (5-15% by bulk volume) that reduce porosity and permeability. However, the presence of bitumen also indicates the sandstones have reservoir Previous HitqualityNext Hit. One of the latest and most significant events to the economics of the prospects is TSR (reduction of sulfate by hydrocarbons to form H2S and other by-products). The high H2S content gas requires special drilling and completion equipment, separation from the methane, and finally Previous HitprocessingTop into sulfur.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990