ABSTRACT

Three conventional cores from deep Cotton Valley clastics (approximately 19,000 to 20,000 ft; 5,791 to 6,096 m) in southern Mississippi were analyzed by thin-section petrography, scanning electron microscopy, X-ray diffraction, and cathodoluminescence. ¹³C and ¹⁸O values were determined on carbonate cements in sandstones. The sandstones are predominantly subarkoses, arkoses, and quartz arenites, with minor amounts of quartz and feldspathic graywackes. Potassium 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 potassium feldspar and albite. Typically, fresh detrital potassium feldspar grains have overgrowths of potassium feldspar that are partially dissolved, whereas plagioclase grains (with partially dissolved interiors) have unaltered albite overgrowths. Quartz overgrowth development is pervasive where not hindered by chlorite. Carbonates include intergranular calcite, dolomite and Fe-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. ¹³C values of mixed carbonates in sandstones range from -0.8 to -4.4 (PDB), which are values typical of normal marine carbonates. ¹⁸O values range from -5.0 to -12.3 (PDB). Temperatures of formation of approximately 115°C to 160°C (239°F to 328°F) are suggested by the oxygen isotope data from almost pure late-stage ferroan dolomite.

Petrographic evidence suggests that the coarser-grained sands were cemented early in the burial history by dolomite and calcite. These cements were later leached by acidic brines, or formation waters possibly related to the generation of organic acids and/or CO₂ released during kerogen maturation. Resultant secondary porosity was preserved by formation of bladed chlorite. The source of the Fe, Mn, and Mg 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 ¹³C values of carbonate cements suggest the pore water was probably initially seawater and later brine that migrated upward from the Louann Salt. Meteoric diagenesis of these rocks was likely minimal. Present brines contain 7,300 ppm K and 51,113 ppm Na. This study suggests that original depositional porosities, permeabilities and carbonate cementation are significant controls in secondary porosity formation in the Cotton Valley sandstones.