Isotopic Biogeochemistry of Lipids and Organic Matter in Deep Subsurface Sediments of the Marcellus Shale
Isotopic composition of organic matter (OM) in sediments is dependent on a range of factors such as; sources of OM, isotopic effects associated with OM assimilation, rate of OM recycling, and physiology and nutrient status of the organisms present. As a result, isotopes have become a good proxy in understanding the biogeochemical processes that control the formation of ancient and modern OM rich deposits. Compound specific isotope analysis (CSIA) is also an emerging tool that allow for better elucidation of OM source identification, paleo-reconstruction, and petroleum geochemistry. The compound specific carbon isotope fractionation effects captured by lipids (δ13Clipids) represent both the carbon sources and metabolic pathways, and as such can also be used to understand carbon cycling relationships. We utilize both bulk δ13C of OM and CSIA of phospholipid fatty acids (PLFA) and diglyceride fatty acids (DGFAs) associated with OM in Marcellus Shale. The PLFAs and DGFAs are biomarkers for viable and non-viable microbes respectively. The samples were obtained from the Marcellus Shale Energy and Environment Laboratory (MSEEL), a scientific well site in Morgantown, West Virginia. Total organic carbon (TOC), stable carbon isotopic composition of bulk organic carbon (δ13Corg), and nitrogen (δ15Norg) were analyzed using an elemental analyzer (EA), with a Delta Advantage IRMS via a ConFlo IV, inorganic carbon (δ13Ccarb) was measured using a GasBench. CSIAs for the PLFA and DGFA biomarkers were analyzed using GC-irMS. TOC concentrations showed a general increasing trend from top to bottom of Marcellus. The δ13Corg was relatively higher in the OM poor zone compared to the OM rich zone of the lower Marcellus, suggesting more influence of terrestrial OM in OM poor zone as opposed to marine OM in the OM rich zone. Both δ13Corg and δ15Norg show higher fluctuations in the bottom organic rich zone of Marcellus an indication of nutrient recycling that releases more N and C of varying isotopic values. However, there was no difference between the average δ13Corg of bulk OM and δ13Clipids, although the δ13Clipids were consistent with CSIA values of dominantly heterotrophic communities. We identified 17 DGFA and 8 PLFA in the Mahantango, 9 DGFA and 12 PLFA in the Marcellus Top, and 10 DGFA and 11 PLFA profiles in the Upper Marcellus. The individual lipid profiles do show variations in isotopic values which can be related to the community structure and/or metabolic pathway.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017