The Lower Tuscaloosa Formation (Upper Cretaceous) in the Greensburg and Joseph Branch Field Areas, St. Helena Parish, Louisiana

Abstract

Detailed examination of five conventional cores and geophysical well logs in the Greensburg and Joseph Branch field areas of St. Helena Parish, Louisiana, were used to determine the geology of the First Tuscaloosa sand (an informal term for the objective sandstone in this part of the Lower Tuscaloosa production trend). Greensburg and Joseph Branch fields occur in the intermediate-depth part of the productive trend at an average depth of about 13,000 ft in an area of limited low-relief structural nosing. Neither anticlinal closures greater than 20 ft nor faults were recognized by structure contour mapping. Based on the updip and lateral shaling-out of the First Tuscaloosa Sand in the field areas, and the absence of structure, these fields are deemed stratigraphic traps. The First Tuscaloosa Sand probably was deposited as a distributary channel that flowed in the subaerial lowland part of a delta. Intense bioturbation of sand tops, unidirectional trough cross-bedding, and flaser structures were recognized in the core, indicating that this part of the overall deltaic environment was influenced or reworked by primarily sublitharenite, with scattered quartzarenite intervals. The reservoir facies was extensively modified by diagenesis. Initial (primary) porosity was reduced by compaction and later by formation of quartzmore » overgrowths. Secondary porosity, developed as a result of the dissolution of unstable grains and late-stage carbonate cements, can range up to 25%.« less