Sulfur and carbon isotopic compositions of the Permian to Triassic TSR and non-TSR altered solid bitumen and its parent source rock in NE Sichuan Basin

Abstract

Reservoir solid bitumens from the Permian to Triassic and potential source rocks in the northeastern Sichuan Basin were measured for elemental and stable carbon (δ13C) and sulfur (δ34S) isotopic compositions to determine the chemical and isotopic composition of non-TSR and TSR-altered solid bitumen and to develop proxies to reflect the extent of bitumen alteration by TSR. Non-TSR altered reservoir bitumen samples showed S/C atomic ratios ⩽0.03 and had δ13C values closer to Longtan Formation (P3l) kerogen than to the Lower Silurian and Cambrian kerogens, and δ34S values slightly more enriched than P3l kerogens. The TSR-altered solid bitumens have incorporated significant amounts of TSR-derived H2S resulting in S/C atomic ratios (0.047–0.074) and δ34S values (13.4–24.0‰) being significantly higher than those of the solid bitumen not altered by TSR (S/C: 0.013–0.030; δ34S: 5.8–9.6‰) and the potential source rock kerogen. The incorporation has produced a decrease in N/C ratio and negative shift of −2.7‰ in δ13C values that resulted from increasing TSR-H2S back reactions with 12C-rich n-alkanes from the gas phase, soluble solid bitumen or paleo-oil to generate organic sulfur compounds such as ethanethiol that were incorporated into the solid bitumens, which is followed by further aromatization and polymerization. The solid bitumens show positive relationships of S/C ratios and δ34S values to the associated gas H2S/(H2S+ΣC1–6) or TSR extents, and thus the S/C ratios and δ34S values can be used as proxies to reflect TSR extents.