Geopressuring Mechanism of Smackover Gas Reservoirs, Jackson Dome Area, Mississippi (includes associated papers 6558 and 6559 )

Abstract

To help understand the problems associated with geopressures in theMississippi salt basin, Smackover sour gas reservoirs were studied anda geopressuring mechanism was concerned. The concept presented is that gaswas sealed in a confined container, and then geopressured by thetemperature and pressure of the earth's crust with subsequent burial. Introduction Geopressures of the Smackover sour gas reservoirs in theJackson Dome area of the Mississippi salt basin appear tobe created by the temperatures and overlying weight ofthe earth's crust. Calculations indicate that the geopressure cell was sealed and geopressuring started ata depth of about 12,500 ft. With burial that occurredthereafter, the pore-gas pressure increased in accordancewith the equation for real gases that combines Boyle's and Charles' laws.The Smackover exists at 12,000 ft on the north flank and plunges to about 27,000 ft at the synclinal axis of thebasin. The geopressure gradients of the reservoirsincrease with depth from hydropressures in the 12,500-ftLoring reservoir to 19.0 lb/gal, or 0.99 psi/ft, in the 22,100-ft Southwest Piney Woods reservoir. The measuredreservoir pressure is 21,835 psi. This is the highestpressure and pressure gradient of a potential reservoirever recorded in the Gulf of Mexico basin.Calculations indicate that this pressuring mechanismcould create a geopressure gradient up to 24.4 lb/gal(1.28 psi/ft) at the synclinal axis of this subregionalprovince;however, it is not known whether a rock canhold such a pressure gradient. Porosity reduction bycompaction appears to decelerate with geopressuring andessentially stops when the pore-gas pressure approachesthe geostatic pressure. Geopressures Geopressures are found in increasing numbers in theMississippi salt basin as more deep wells are drilled. Tohelp understand the problems associated with thesepressures, a geopressuring mechanism for the Smackoversour gas reservoirs was conceived and is presented here.The concept is simply that gas was sealed in a confinedcontainer, and then was geopressured by the temperatureand pressure of the earth's crust with subsequent burial.The concept conforms to the "seal and pressurize"method of geopressuring. The pressuring source of heatand weight of the earth's crust is essentially the same asthat proposed for the generation of geopressures for theentire Gulf of Mexico salt basin and other basins.The seal of the geopressures in the review area isdifferent from that for the Texas-Louisiana Cenozoic saltbasin. The seal, however, is not the subject of thisdiscussion.Several pressuring sources for creating geopressureshave been proposed by other investigators; some haveproposed that the Mississippi salt basin has a differentpressuring source than the Cenozoic salt basin. Other pressuring sources have been proposed by Parkers for theorigin of geopressures in Mississippi.The area discussed is a corridor traversing the Loring, Pelahatchie, Thomasville, Piney Woods, and SouthwestPiney Woods fields, adjacent to the Jackson Dome in theMississippi salt basin, as illustrated in Fig. 1. Smackoversour gas has been found in the five fields, ranging indepth from 12,450 to 22,100 ft. The Smackover area ofthe Jurassic age in this basin is 1,200 ft thick, exists at12,000 ft on the north rim, and plunges to 27,000 ft in thegeosynclinal axis of the basin. JPT P. 579^