The fate of CO2 derived from thermochemical sulfate reduction (TSR) and effect of TSR on carbonate porosity and permeability, Sichuan Basin, China

This article discusses the role of methane in thermochemical sulfate reduction (TSR), the fate of TSR-derived CO2 and the effect of TSR on reservoir porosity and permeability, and the causes of the anomalously high porosity and permeability in the Lower Triassic soured carbonate gas reservoirs in the northeast Sichuan Basin, southwest China. The Lower Triassic carbonate reservoirs were buried to a depth of about 7000m and experienced maximum temperatures up to 220°C before having been uplifted to the present-day depths of 4800 to 5500m, but they still possess porosities up to 28.9% and permeabilities up to 3360md. The present-day dry gas reservoirs evolved from a paleo-oil accumulation and experienced varying degrees of TSR alteration as evidenced from the abundant sulfur-rich solid bitumens and varying H2S and CO2 concentrations. TSR occurred mainly within the oil and condensate/wet gas windows, with liquid hydrocarbons and wet hydrocarbon gases acting as the dominant reducing agents responsible for sulfate reduction, sulfur-rich solid bitumen and H2S generation, and calcite precipitation. Methane-dominated TSR was a rather late event and had played a less significant role in altering the reservoirs. Intensive H2S and CO2 generation during TSR resulted in calcite cementation rather than carbonate dissolution, which implies that the amount of water generated during TSR was volumetrically insignificant. ¹³C-depleted CO2 derived from hydrocarbon oxidation preferentially reacted with Ca²⁺ to form isotopically light calcite cements, and the remaining CO2 re-equilibrated with the ¹³C-enriched water–rock systems with its δ¹³C rapidly approaching the values for the host rocks, which accounted for the observed heavy and relatively constant CO2 δ¹³C values. The carbonate reservoirs suffered from differential porosity loss by TSR-involved solid bitumen generation and TSR-induced calcite and pyrite precipitation. Intensive TSR significantly reduced the porosity and permeability of the intervals expected to have relatively high sulfate contents (the evaporative-platform dolostones and the platform-margin shoal dolostones immediately underlying the evaporative facies). Early oil charge and limited intensity of TSR alteration, together with very low phyllosilicate content and early dolomitization, accounted for the preservation of anomalously high porosities in the reservoirs above the paleo-oil/water contact. A closed system seems to have played a special role in preserving the high porosity in the gas zone reservoirs below the paleo-oil/water contact. The closed system, which is unfavorable for deep burial carbonate dissolution and secondary porosity generation, was favorable for the preservation of early-formed porosity in deeply buried carbonates. Especially sucrosic and vuggy dolostones have a high potential to preserve such porosity.