Paper No. 135-7
Presentation Time: 9:50 AM
TRACING THE ROLE OF WATER IN CH4 GENERATION VIA LONG-TERM, LOW TEMPERATURE EXPERIMENTAL MATURATION OF IMMATURE SHALES
The conventional paradigm attributes the process for generation of most economically viable petroleum reserves to thermal cleavage of C-C bonds within sedimentary organic matter. There is increasing evidence, however, that alternative chemical pathways yield non-microbial CH4 generation at low temperatures. Samples of two immature shales (Cretaceous Second White Specks Formation, SWSF; Ro = 0.42 % and Devonian New Albany Shale, NAS; Ro = 0.51 %) were subjected to hydrous heating experiments in sealed Pyrex® ampoules at low temperatures (80 or 100 °C) over long-term periods (14 to 36 months). Utilizing 2H-enriched water at three levels of 2H-enrichment (-137 ‰ to 2H2O) in these experiments provided an opportunity to evaluate the role of water in CH4 generation at temperatures insufficient to achieve thermal cleavage of C-C bonds. Free CH4 was produced from both source rocks heated at 80 °C and 100 °C, with the latter temperature producing slightly higher yields of CH4 (relative to organic C contents) for NAS, and greater CH4 generation from the SWSF. Moreover, CH4 from the hydrous maturation experiments conducted at 100 °C exhibited higher δ13C values for both samples compared with those at 80 °C, matching the expectation for δ13CH4 values to increase with increasing maturity. δ2H values for CH4 generated from samples treated with 2H-enriched water confirm incorporation of water. In addition, the samples treated with 2H-enriched water at 100 °C maturation experiments yielded significantly higher δ2H values for generated CH4 than those conducted at 80 °C, indicating greater hydrogen incorporation from 2H-enriched water. The evidence for incorporation of 2H derived from 2H-enriched water in hydrous maturation experiments demonstrates that water incorporation is a key process in CH4 production at low temperatures (80-100 °C) that precludes cleavage of C-C bonds, and further supports a role for catalytic processes in petroleum generation in natural systems.