Petrology and Depositional Environments of Lower Tuscaloosa Formation (Upper Cretaceous) Sandstones in North Hustler and Thompson Field Areas, Southwestern Mississippi: ABSTRACT

Abstract

ABSTRACT The lithology, diagenesis, and depositional environments of the Dykes, Denkman, and McComb sands of the Stringer Sand Member of the Lower Tuscaloosa Formation in the North Hustler and Thompson field areas of Amite County, Mississippi, have been determined by detailed description of cores from nine wells and examination of 61 thin-sections. The cores were recovered from driller depths of about 11,100 ft in the updip Tuscaloosa trend, which is being actively explored for stratigraphically trapped oil. Most of the sandstone samples (65%) are quartz arenites, and the remainder are sublitharenites: both types consist dominantly of fine- to very fine-grained sand. The average sandstone consists of 67.3% framework grains, 14.8% matrix, 9.2% cement, and 8.7% pore space. Monocrystalline quartz is by far the most abundant framework grain (88.7%), with polycrystalline quartz (6.6%), potassium feldspar (0.3%), plagioclase (0.1%), chert (1.9%), igneous (mostly volcanic) rock fragments (0.5%), metamorphic rock fragments (0.2%), and sedimentary rock fragments (1.5%) also present. The matrix for the most part consists of pore-filling clays, and includes organic matter and secondary pyrite. Quartz and ferroan dolomite (ankerite) are the most common cements; high concentrations of siderite and calcite occur as cement within some thin layers, and siderite is also found commonly as small concretions. Porosity is mostly secondary, resulting from partial dissolution of framework grains and cement. Quartz overgrowths probably formed during an early stage of diagenesis, followed by deposition of calcite cement which corroded and embayed quartz and other framework grains. Apparently the calcite was later replaced by ankerite. Late-stage pore-filling clays include vermicular kaolinite and chlorite, the latter occurring as rims and in dissolution-enlarged pore space. The depositional environment of the Dykes and Denkman sands in all wells is interpreted to be fluvial; stacked point bars are obvious in one well. The McComb sand, best developed in a wildcat well between Thompson and McComb fields, is marine, based on the occurrence of glauconite and shell fragments. The presence of a basal scour surface on shale indicates that this sandstone may be the product of marine reworking of a transgressed distributary mouth bar.