GEOCHEMISTRY AND SULFIDE MINERAL PARAGENESIS IN MARCELLUS SUBGROUP AND UTICA FORMATION GAS SHALE INTERVALS

Organic-rich mudrocks of the Utica Formation (Upper Ordovician) and Marcellus Subgroup (Middle Devonian) are sources of natural gas and related liquids in the Appalachian Basin. Sulfide mineralization and associated enrichment of certain trace metals in the target intervals in the Utica and Marcellus are widespread. Sulfide mineral distribution controls important rock properties including fracturing behavior, porosity and permeability, and potential metal/acid leachate from well cuttings. Whole-rock major and trace element chemistry, bulk mineralogy from X-ray diffraction, and SEM-EDS analyses of outcrop, core and cutting samples of the Flat Creek Member of the Utica Formation, and Union Springs Formation of the Marcellus Subgroup reveal a variety of sulfide mineral species, and paragenetic histories.

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.