Genesis and geological significance of pyrite in the Ordovician carbonate-evaporative strata of central Ordos Basin, China

Abstract

Deep burial carbonate-evaporative strata (4000–10000m) have significant exploration potential for gas-rich petroliferous basins. However, high temperature and thermal evolution destroy most hydrocarbon related organic geochemical parameters used to analyze the alteration of oil and gas reservoirs. This study is the first systematic attempt to assess hydrocarbon alteration and cracking based on generational evolution and geochemistry of pyrite. Pyrites in the subsalt carbonate of Ordovician Majiagou Formation of central Ordos Basin are formed in four stages (Py1, Py2, Py3 and Py4). The mineral morphology and geochemical characteristics indicate that Py1 was formed from microbial sulfate reduction. Py2, Py3 and Py4 are derived from thermochemical sulfate reduction (TSR). This is supported by coexistence between pyrite and other TSR products (calcite, bitumen and elemental sulfur), reaction condition and rich TSR reactants (hydrocarbon and sulfate). TSR occurs in a closed system, proved by increasing δ³⁴S values from Py2 to Py4 (6.2 ‰–73.9 ‰) and unusually high δ³⁴S values of residual vein anhydrite (36.2 ‰–44.8 ‰) in the reservoir. TSR pyrites (Py2, Py3 and Py4) have relatively high concentrations of transition metal elements, which is consistent with worldwide oil. Significantly decreasing trace element concentrations from Py2 to Py4 and from core to rim are attributed to TSR reactant transformation from oil to wet gas. Py2, Py3 and Py4 probably correspond to oil-dominated TSR, TSR-induced oil cracking and wet gas-dominated TSR respectively. Therefore, TSR pyrite is a potentially effective mineral to the evolution of deep burial gas reservoirs.