DOMINANT CONTROLS ON DEPOSITION OF ORGANIC-RICH INTERVALS IN MARCELLUS SHALE: GEOCHEMICAL, ISOTOPIC AND PETROGRAPHIC EVIDENCES

To understand dominant controls on organic matter deposition geochemical, isotopic and petrographic analyses were performed on a 100 feet core of Marcellus Shale obtained from Greene County, Pennsylvania. Sampling intervals were determined by lithologic and well log characteristics. The black shale intervals with higher gamma ray values, higher total organic content (TOC) values, and more positive δ¹³C in carbonate are interpreted as periods of high primary productivity. The high photosynthetic demand is likely to preferentially remove lighter ¹²C leaving the dissolved inorganic carbon pool enriched in ¹³C resulting in higher δ¹³C of precipitating carbonate. Peloidal texture in these shale intervals suggests terrestrial input which may have supplied nutrients for enhanced productivity. The lowermost part of Marcellus displays extremely high TOC values along with high concentration of redox sensitive metals (e.g. U, Mo, V). In addition, the δ¹⁵N values of organic matter is more negative (δ15N_org is close to 0) in those intervals, indicating anoxic conditions. The petrographic analysis of the samples from these intervals shows abundance of pyrite grains suggesting anoxic and sulfidic (euxinic) conditions during the deposition. In contrast, the low gamma-ray limestone intervals show an abundance of carbonate mud and skeletal grains in thin sections, depleted δ¹³C in carbonate and low concentrations of redox sensitive elements (Mo and U). The multiple lines of evidence suggest that these periods reflect short term shifts in an oxic/anoxic boundary near the sediment-water interface and low paleo-productivity. Our preliminary study indicates how well log data can be integrated with isotopic, geochemical and petrographic observations to better understand dominant controls on deposition of organic-rich intervals in Marcellus.