Implications of solid bitumen association with low- to medium-temperature hydrothermal minerals in hydrocarbon reservoirs of Southwest China: Insights from organic petrology and sulfur isotopes

Solid bitumen in Southwest China often coexists with metal deposits, indicating a genetic link between organic matter and ore formation. However, this linkage remains underexplored. This study investigated the relationship between sulfur origin and organic matter maturity through sulfur isotopes of metal sulfides. Organic petrology revealed that solid bitumen associated with minerals exhibited stronger optical anisotropy (ΔBRo >3.32 %) and higher S/C atomic ratios (0.029–0.033) than non-associated bitumen (0.005–0.021). Fluid inclusion analysis showed that ore-stage calcite inclusions have higher homogenization temperatures than those in paleo-oil reservoirs, indicating that ore-forming processes accelerated hydrocarbon thermal evolution. Solid bitumen-mineral associations were classified into Mississippi Valley-type (MVT), stratiform mercury, and disseminated gold deposits. Sulfur isotope compositions identified three sulfur sources: bacterial sulfate reduction (BSR), thermochemical sulfate reduction (TSR), and thermal decomposition of sulfur-containing organic matter (TDS). In the Lanping-Simao Basin, δ³⁴S_CDT values below 0 ‰ suggest sulfur derived from BSR, with hydrocarbons contributing reduced sulfur. In the Xuefeng Mountains Uplift, δ³⁴S_CDT values exceeding 20 ‰ indicate TSR-driven sulfur, with metallogeny enhancing hydrocarbon accumulation and mineralization. In the Kangdian Axis, δ³⁴S_CDT values under 20 ‰ suggest sulfur from TSR and TDS, with mineralization causing oil cracking and reservoir destruction. The findings demonstrate that metallogeny accelerates organic matter maturation, reshapes reservoirs, and drives oil cracking. Simultaneously, organic-inorganic interactions govern metal sulfide formation and precipitation, underscoring their critical role in ore genesis.