COMPARISON OF PHOSPHOLIPID AND DIGLYCERIDE FATTY ACID BIOMARKER PROFILES IN MARCELLUS SHALE CORES OF DIFFERENT MATURITIES

The Middle Devonian Marcellus Shale which occurs at depths of 1.5 to 2.5 km, with formation temperatures of 49˚- 60˚C, is one of the largest gas shale plays in North America. Microbial activity has been detected in subsurface shale environments as deep as 3 km with temperatures as high as 70^˚C, suggesting that microbial life may exist in the Marcellus prior to energy development. Core samples were collected from a less mature, liquids-prone well in Wetzel County and a more mature, gas-prone well in Monongalia County, West Virginia. A prior comprehensive study of these cores suggests that the sediments in these cores were deposited in different geological settings. This study was conducted to determine whether (1) the viable biomass was higher in the samples of lower maturity from Wetzel County, and (2) the microbial profiles were different due to differences in depositional environment and thermal history. Ester-linked phospholipid fatty acids (PLFA) and diglyceride fatty acids (DGFA) were extracted from samples collected from selected intervals and the total yield and variety of PLFA (proxy for recent, viable microbiota) and DGFA (proxy for non-viable or relic biomass) profiles were examined. The lipid profiles consisted of normal saturated, monounsaturated, polyunsaturated, branched, epoxy, and hydroxyl fatty acids. As hypothesized the variety and yield of both lipid fatty acid profiles (PLFA and DGFA) were higher in the less mature liquids-prone well compared to the more mature gas-prone well. Our analyses suggest that PLFA and DGFAs profiles may be useful for assessing microbial life within the shale prior to energy development, along with biologic derivatives of relic communities within the formation. However, it is important to note that the samples described here do not account for the introduction of microbial cells during drilling, collection, and sample storage as well as present and previous engineering practices used in the well field that could alter lipid profiles and biomass. Currently, a study is underway to obtain pristine shale core samples from a test well at the Marcellus Shale Energy and Environment Laboratory (MSEEL) at West Virginia University to better understand the biomarker signatures of indigenous microbial communities in shales.