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Petroleum Geology of South Carlton Field, Lower Tuscaloosa "Pilot Sand", Clarke and Baldwin Counties, Alabama
Ernest A. Mancini, J. Wayne Payton (1)
Petroleum production from South Carlton field, southern Clarke and northern Baldwin Counties, Alabama, is primarily from the lower Tuscaloosa "Pilot Sand" of Late Cretaceous age. The "Pilot" is a massive, green-brown, micaceous, fossiliferous, glauconitic, very fine to medium-grained, quartzose sandstone. Texturally, the sandstone is submature with the quartz grains being subangular to subrounded and moderately to moderately well sorted. The sand body displays a decrease in grain size, sorting, and roundness from the base to the top of the unit. The sandstone has a maximum thickness of 71 feet in the northern part of the field. Usually overlying the "Pilot" is a gray, silty, oyster-bearing packstone. The packstone is overlain by the "Marine Shale" which consists of dark-gray, silty, micaceous, fossiliferous, laminated claystone. This claystone contains a diverse fossil assemblage, indicating open marine-shelf sedimentation. The "Pilot Sand" is underlain by an interbedded sandstone and claystone interval, which is separated from the "Massive Sand" by a silty claystone.
The spontaneous-potential pattern for the "Pilot Sand" illustrates a gradational lower contact and a sharp upper contact. Constructed cross sections indicate the "Pilot" thins and becomes argillaceous immediately east and west of the field. The sedimentary rock characteristics, including an elongate northeast to southwest strike-trending morphology, and well-log properties of the sandstone suggest it accumulated as part of a marine-bar complex.
Porosity in the "Pilot" is principally intergranular and averages 27.3 percent. Average permeability is 183 md, and oil saturation can be as much as 42 percent. The "Marine Shale" above the "Pilot Sand" and claystone from the interbedded interval underlying the "Pilot" have potential as petroleum source rocks. The overlying packstone and "Marine Shale" make excellent seal rocks. The petroleum trap is a combination of stratigraphy and structure. The structural element is most critical and involves salt movement which has resulted in a domal feature. The crude oil trapped by the salt dome is a heavy oil having a chemical composition of an immature oil.
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