Diagenesis of Cotton Valley Sandstones, Catahoula Creek Field, Southern Mississippi: ABSTRACT

Abstract

Three conventional cores from the deep Cotton Valley clastics in south Mississippi were analyzed by thin-section petrography, scanning electron microscopy, x-ray diffraction, and cathodoluminescence. The ^dgr13C and ^dgr18O values were determined on carbonate cements in sandstones. The sandstones are predominantly subarkose, arkose, and quartzarenites, with minor amounts of quartz and feldspathic wackes. K-feldspar, muscovite, and plagioclase are relatively fresh. Authigenic clay in sandstones is almost exclusively chlorite that occurs as a pore-lining and pore-filling phase, most commonly associated with porous zones. Feldspar diagenesis includes overgrowths of both potassic feldspar and albite. Typically, fresh detrital K-feldspar grains have overg owths of K-feldspar that are partly dissolved, whereas plagioclase grains (with partly dissolved interiors) have unaltered albite overgrowths. Quartz overgrowth development is pervasive where not hindered by chlorite. Carbonates include intergranular calcite, dolomite, and iron-rich dolomite. The earliest phase is a pore-lining dolomite cement, followed by a later ferroan calcite. Ferroan dolomite (or ankerite) is associated with chlorite in relatively porous intervals. The ^dgr13C values of mixed carbonates in sandstones range from -0.8 to -4.4^pmil (PDB), which is typical of normal marine carbonates. The ^dgr18O values range from -5.0 to -12.3^pmil (PDB). Formation temperatures of approximately 115° -160°C are suggested by the oxygen isotope data from al ost pure late-stage ferroan dolomite. Petrographic evidence suggests that coarser grained sands were cemented early in the burial history by dolomite and calcite. These cements were later leached by formation waters, possibly related to the generation of organic acids and/or carbon dioxide released during kerogen maturation. Resultant secondary porosity was preserved by formation of bladed chlorite. The source of the iron, manganese, and magnesium necessary for chlorite formation was presumably the release of ions from smectite to illite conversion in shales, because ferromagnesian rock fragments in the sandstones are rare. The feldspar volume and stability and the ^dgr13C values of carbonate cements suggest the pore water was probably initially seawater and later brine that migrated upward from the Louann Salt Present brines contain 7,300 ppm of potassium and 51,113 ppm of sodium. Meteoric diagenesis of these rocks was likely minimal. These data suggest that original depositional porosities and permeabilities and carbonate cementation are significant controls in secondary porosity formation in the Cotton Valley sandstones. End_of_Article - Last_Page 1424------------