ABSTRACT

Reef limestones within the Lower Cretaceous Sligo Formation form a trend which has 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 reefs separate into two trends in central Louisiana. The main bioherm extends roughly east-west through Vernon and Rapides Parishes, thence in a southeasterly direction through Avoyelles, southern Catahoula, and southern Concordia Parishes. It has a maximum known width of about 40 miles and a maximum known thickness of about 500 feet. The few deep tests that have been drilled within this reef indicate a fossil assemblage consisting mostly of caprinids (sessile pelecypods) and algae in addition to miliolids and other small fossils in a sparry cement or micrite matrix. Up to 50 percent of the Sligo in this trend is dolomite. Porosity has generally been less than 9 percent in tests drilled to date.

North of the main reef is another bioherm up to 250 feet thick which forms an arcuate trend through eastern Natchitoches, Winn, southern Jackson, and western Caldwell Parishes. It is possible that it extends southeastward into Franklin and Catahoula Parishes. The term patch reef has been applied to some local thickenings within this reef trend. The lithology of the northern reef is similar to the main reef. 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 few, if any, dolomite zones as in the main reef trend. In some localities within the northern reef, porosity and permeability are very high.

Commercial oil and gas production has yet to be found in the main reef trend in Central Louisiana, but the possibilities by no means have been exhausted. In contrast, the Black Lake field in Natchitoches Parish was discovered in the northern biohermal trend in 1964. This is a major gas-distillate-oil field within a stratigraphic-structural trap containing ultimate recoverable reserves of approximately 150 million barrels of oil equivalent. This discovery caused an active wildcat play in search of additional traps similar to Black Lake but so far, without success.