GEOCHEMISTRY AND SULFIDE MINERAL PARAGENESIS IN MARCELLUS SUBGROUP AND UTICA FORMATION GAS SHALE INTERVALS
In the Flat Creek Member (Utica), early diagenetic framboidal pyrite is widespread. Burial recrystallization produced coarser, blocky pyrite in the mudrock matrix. Other sulfide minerals, including sphalerite and minor galena, are rarely observed in mudrock, but present and associated with coarse pyrite in carbonate-sulfide veins. Flat Creek Member sulfide-carbonate veins often contain aromatic hydrocarbons, and are related in time to fluid hydrocarbon generation. Later, apparently higher-temperature carbonate veins lack sulfides, but contain methane fluid inclusions.
In the Union Springs Formation (Marcellus), early framboidal and coarser, blocky pyrite are also common, and are accompanied by abundant sphalerite in some mm-scale laminae and flattened concretionary masses. Sphalerite occurs as equant to irregular-blocky aggregates of the same size as detrital clasts. Sphalerite infills uncompacted algal cysts (‘Tasmanites’), suggesting a synsedimentary or early diagenetic origin. Chalcopyrite and minor galena are also relatively common, and associated with sphalerite. The common occurrence of this sulfide mineral suite suggests that Union Springs bottom waters were metal-enriched. Later carbonate veins associated with thrust fault systems in the Marcellus are generally sulfide mineral-poor, and sulfide mobility was relatively limited during fluid hydrocarbon migration and subsequent thermal over-maturation.