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Recent oil discoveries in the St. Louis and Salem Limestones of Middle Mississippian age, on the eastern flank of the Illinois basin in Indiana, have been significant enough to stimulate exploration. Most previous oil exploration in the area was confined to depths shallower than the St. Louis and the Salem.
Along with the oil discoveries came problems of stratigraphic identity in Gibson County and adjoining counties. The problems were resolved by a study in which the Salem Limestone and associated rocks were traced from the outcrop area into the subsurface, where geophysical logs and drill cuttings were used for correlation. Within the St. Louis a geophysical log marker--the X marker--and the Sisson Member were introduced as new names.
Two adjoining fields in Gibson County, Owensville North Consolidated and Mt. Carmel Consolidated, were studied in detail in an attempt to determine the conditions for oil entrapment in the St. Louis and Salem reservoirs. Production in both fields abut one another, and therefore the reservoirs are treated as one unit. Production is from the St. Louis (Sisson Member) and the Salem. The Salem reservoir is about 180 ft (55 m) below the St. Louis reservoir, and both produce from porous zones in a calcarenite facies composed of microfossils, fossil fragments, and oolites. Porosity studies of the reservoirs show that the lower limit of producible porosity is 6%, and most oil wells have at least 5 ft (1.5 m) of net porosity greater than 6%. Maximum porosity recorded in the St. Louis was 15%, nd in the Salem 21%. Density logs were used for porosity determinations. Salem wells had higher initial production than St. Louis wells. Entrapment is influenced by both stratigraphic and structural conditions.
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