Geochemical and Isotopic Signatures of Thermochemical Sulfate Reduction in Hydrous Pyrolysis vs. Long-Term Hydrous Heating Experiments

Abstract

Understanding Thermal Sulfate Reduction (TSR) has important implications for exploration and development of deep-gas reservoirs. Prior laboratory simulations of TSR were conducted under unnatural chemical conditions (i.e. short reaction time or high temperatures). Here, a series of hydrous pyrolysis and long-term hydrous heating experiments were conducted using stainless steel and glass reactors loaded with immature Paleozoic siliciclastic and carbonate source rocks from Illinois Basin cores. Carbonate experiments utilized samples of St. Louis Limestone and siliciclastic experiments utilized New Albany Shale and Turner Mine Mudstone directly overlying Springfield Coal. Parallel comparative experiments were performed (i) for 3 and 6 days at up to 320^oC and (ii) for up to 18 months at 130^oC. Elemental sulfur (ES) with ³⁴S-enrichment was added to some siliciclastic source rock samples. Sequential sulfur extractions, geochemical analyses, and stable isotope measurements of sulfur and carbon were applied in this study to evaluate (i) effects of TSR alteration on chemical and isotopic compositions of gas produced by different source rocks, (ii) the influence of sulfur species (e.g., elemental sulfur and H₂S) on TSR, and (iii) variation in kinetics between long- and short-term experiments performed on shale vs. limestone.

Preliminary analyses of the produced gases show δ¹³C_CH4 values ranging from -28 to -58 ‰ and δ¹³C_CO2 values ranging from -7 to -31 ‰. The majority of the δ¹³C_CH4 values fall within the range of thermogenic gases. Samples from Turner Mine Mudstone are characterized by lower δ¹³C_CH4 than New Albany Shale and likely indicate a contribution from preexisting biogenic gases trapped in pore space. High-temperature/short-term and low-temperature/long-term experiments show differences in Δ¹³C_CH4 ranging from 1 to 11 ‰ with the more positive carbon isotopic composition coming from low-temperature/long-term runs plausibly related to changing kerogen reaction sites with extended reaction time.

Values of δ¹³C_CO2 appear to be influenced by both disintegration of carbonate minerals and oxidation of organic matter. Values of δ¹³C_CH4 for siliciclastic experiments with added ES are isotopically more negative by up to 14 ‰ compared to values of δ¹³C_CH4 for experiments without supplemented ES. Additionally, values of δ¹³C_CO2 for experiments with added ES are isotopically less negative by up to 4 ‰ compared to values of δ¹³C_CO2 for experiments without supplemented ES.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018