ABSTRACT

The rocks of the Tuscaloosa group, basal Upper Cretaceous unit in the eastern Gulf Coastal Province, record a complete depositional cycle. The Lower Tuscaloosa comprises the transgressive stage, the Middle Tuscaloosa the inundated phase, and the Upper Tuscaloosa the regressive. Many stratigraphically controlled hydrocarbon traps are present in the basal portion of the transgressive Lower Tuscaloosa unit in east central Louisiana. Southeastern Tensas Parish was chosen as a type area in which to analyze the Lower Tuscaloosa in detail.

The study of the type area revealed that although numerous Lower Tuscaloosa sands produce hydrocarbons, only one, herein termed Buckhorn sand, contains significant reserves. It is concluded that this sand was deposited in depressions on the pre-Tuscaloosa erosion surface. Stratigraphic cross sections, a Lower Tuscaloosa isopach map, a structure map of the pre-Tuscaloosa unconformity, and a pre-Tuscaloosa paleogeologic map can be collectively used to form interpretations of pre-Tuscalocsa topography and related Buckhorn sand distribution patterns. The interpreted sand patterns can then be presented in the form of a Buckhorn sand isopach map. If this map is considered in conjunction with a top of the Lower Tuscaloosa structure map, potential hydrocarbon traps can be found. Since basal Tuscaloosa depositional patterns are similar throughout east central Louisiana, the methods used in the type area are applicable to the entire area.

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