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Reef limestones within the Lower Cretaceous Sligo Formation form a trend within the Gulf Coastal province and have been traced in the subsurface from Mexico to Mississippi. This trend probably continues beneath the Gulf of Mexico in the general vicinity of the West Florida shelf.
The Sligo forms 2 reef trends in central Louisiana. The main biohermal trend extends roughly east-west through Vernon and Rapides Parishes, then southeast through Avoyelles, southern Catahoula, and southern Concordia Parishes. It has a maximum known width of about 40 mi and a maximum known thickness of about 500 ft. The few deep tests that have been drilled within this reef indicate a fossil assemblage consisting mostly of caprinids (sessile pelecypods) and algae plus miliolids and other small forms in a sparry or micrite matrix. Up to 50% of the Sligo in this trend is dolomite, and porosity is generally less than 9% in tests drilled to date.
North of the main reef is another biohermal trend up to 250 ft thick which forms an arcuate pattern through eastern Natchitoches, Winn, southern Jackson, and western Caldwell Parishes. The term "patch reef" has been applied to some local thickenings within this
trend. It is possible that this reef extends southeastward into Franklin and Catahoula Parishes, but subsurface data is lacking to substantiate this. The lithology of the northern reef is similar to the main reef in that it contains about the same fossil assemblage in sparry, granular, or micrite matrix. Some zones within and on the north edge of the reef contain an abundance of oolites and algal pisolites. In part the limestone is slightly dolomitic, but there are no dolomite zones as in the main reef. In some localities within the northern reef, porosity and permeability are very high.
Commercial oil and gas production has not been found in the main biohermal trend in central Louisiana, but the possibilities have by no means been exhausted. In contrast, the Black Lake field in Natchitoches Parish was discovered in the northern bioherm in 1964. This is a major gas-distillate-oil field within a stratigraphic-structural trap, containing ultimate reserves of approximately 150 million bbl of oil equivalent. This discovery set off an active wildcat play in search of additional traps of the Black Lake type, so far without success.
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