USE OF BIOMARKER AND PYROLYSIS PROXIES TO ASSESS ORGANIC MATTER SOURCES, THERMAL MATURITY, AND PALEOREDOX CONDITIONS DURING DEPOSITION OF MARCELLUS SHALE

This study was conducted to test the efficacy of biomarker and pyrolysis proxies for reconstructing organic matter (OM) source/type, thermal maturity and paleo-redox conditions in Marcellus Shale. Samples were acquired from gas-prone (WV-6) and liquids-prone (WV-7) well in Monongalia and Wetzel Counties, West Virginia, respectively. The samples were analyzed to determine the amount of free, bound and residual hydrocarbons (HC), and distribution/concentration of aliphatic biomarkers.

The source rock analyzer (SRA) data indicates the presence of more gas prone kerogen, comparatively lower amounts of free HC (S1), and HC released from cracking of kerogen (S2) in WV-6 as compared to the WV-7 core. SRA data are unreliable for determining the type of kerogen due to low values of hydrogen and oxygen indices. However, the thermal maturity parameter (Tmax), calculated vitrinite reflectance (VRo%), fraction conversion of OM to HC, and the ratio of residual carbon/pyrolysable carbon (RC/PC) suggest that samples from WV-6 and WV-7 belong in the dry gas and wet gas window respectively, which is in agreement with production data of these wells.

Biomarker analysis indicate that samples from WV-6 had lower concentrations of n-alkanes but higher concentrations of pristane (Pr), phytane (Py), steranes and hopanes compared to WV-7. Samples from both cores show mixed sources of OM indicative of Type II kerogen. However, n-alkane distribution as well as Pr/n-17 vs Py/n-18 cross plots indicate that WV-6 core had comparatively higher amount of terrestrial OM influx as compared to WV-7. The Pr/Py, Ts/Tm hopane ratios, and Pr/n-17 vs Py/n-18 cross plots indicate alternating oxic-anoxic conditions during the deposition of shales from both cores. Our results suggest that WV-6 and WV-7 shales were deposited in different depositional environments and have different thermal histories. Hence these parameters might be responsible for the differences in the type of hydrocarbons produced from these sites.