Depositional Influences in Sandstone Diagenesis of Lower Cretaceous Hosston Formation, Marion and Walthall Counties, Mississippi: ABSTRACT

Abstract

ABSTRACT Poor reservoir quality is a significant impediment to exploration success in the deep Hosston gas/condensate trend of south-central Mississippi. Regionally, depositional lithology of the Hosston exerts significant influence on cementation and porosity reduction. The primary variables are depositional carbonate content, quartz sand-grain size, and the presence of matrix clay. Feldspar content is a variable of lesser significance. In Hosston fluvial and deltaic facies, carbonate content is low, and coarser sediments are present in channel sandstones. These coarser sandstones preserve commercial porosities, whereas finer-grained and shaley sandstones offer marginal porosities. Quartz overgrowths, pressure solution, and authigenic kaolinite are responsible for most diagenetic porosity reduction in this facies. In Hosston marine, prodeltaic sandstones, depositional carbonate is more common. Carbonate cements, dominantly ankerite and dolomite, are recrystallized from shell fragments and carbonate grains to occlude porosity. Quartz overgrowths, pressure solution, and stylolitization are also common in these finer sandstones. In distal, marine shelf sandstones, depositional carbonate is abundant, and quartz sand is very fine-grained. Porosities are very low in this lithofacies because of pervasive early carbonate cementation and later quartz overgrowths in intervals not cemented by carbonate. The first stage of Hosston sandstone diagenesis was early calcite or quartz cementation. Calcite was later replaced by ankerite or dolomite. Deeper burial initiated plagioclase feldspar dissolution and the development of pore-filling kaolinite. Hydrocarbons subsequently accumulated in structural and stratigraphic closures and retarded further diagenetic porosity reduction in these sandstones. Deeper burial caused thermal degradation of oil reservoirs to dry gas and pyrobitumen. Minor galena, sphalerite, pyrite, and barite are present in open, tensional fractures, associated with stylolites and as intergranular cements. These sulfides and sulfates were probably precipitated from metalliferous brines common in the Hosston.