Microbial and thermogenic hydrogen sulfide in the Qianjiang Depression of Jianghan Basin: Insights from sulfur isotope and volatile organic sulfur compounds measurements

Abstract

Improved understanding of the hydrogen sulfide (H2S) origins is of significant importance for petroleum exploration, exploitation and refinement due to it being highly toxic and corrosive. H2S was discovered from the Qianjiang Formation (Eq) in the northern area and Xinggouzui Formation (Ex) in the southern area of the Qianjiang Depression, Jianghan Basin, China. The sulfur isotopic compositions of H2S, water-dissolved sulfates and oils and chemical compositions of volatile organic sulfur compounds (VOSCs) in natural gas as well as molecular and stable carbon isotopic compositions of oil and gas samples were measured to decipher the specific generation pathways of H2S. The conventional wisdom relays on bacteria sulfate reduction (BSR) for producing H2S because of the current low thermal regime (<80 °C). Crude oils, H2S and water-dissolved sulfates are progressively enriched in 34S. The δ34S values of H2S are around 12.0–32.0‰ lower relative to that of the dissolved sulfates and 0.8–24.0‰ greater relative to that of crude oils. This indicates that the current major contribution of H2S is actually related to BSR according to the S isotopic fractionation levels in BSR. The secondary contribution of H2S is attributed to be thermogenic mainly by thermal cracking alteration of organic matters (TCA). This clue is provided by the distribution patterns of VOSCs in natural gas rather than S isotope of crude oils, H2S and water-dissolved sulfates due to the limited input of TCA-associated H2S. The Eq TCA-associated gases are significantly enriched in thiols, sulfides, and alklyated thiophenes, the Ex BSR-associated gases contain mainly thiols and a little bit sulfides and alkylated thiophenes. The relevant early-matured Eq oils are also rich in labile organosulfur compounds, favoring the generation of VOSCs by TCA. VOSCs detected on the artificially pyrolyzed gaseous products of Eq rock and oil both show the identical distribution patterns as those of Eq natural gas samples, supporting the occurrence of TCA-associated H2S in the Eq reservoirs. The positive correlation between i-C4/n-C4 and H2S concentration for Eq gases indicates the catalytic effects of clays, promoting the generation by non-biological processes within the low thermal regimes. This study presents a novel method to clarify the origins of H2S by VOSCs in natural gas and improves our understanding of H2S generation pathways within low-temperature regime in nature.