Bedding-parallel calcite veins indicate hydrocarbon–water–rock interactions in the over-mature Longmaxi shales, Sichuan Basin

Abstract

Bedding-parallel calcite veins in shales archive hydrocarbon–water–rock interactions, but such processes in highly–over mature shales are poorly understood in terms of their geological and geochemical significance. To fill the knowledge gap, we conducted a combined petrographic, geochemical, isotopic, and fluid inclusion study of the Silurian Longmaxi Formation in the Sichuan Basin, China, to investigate these processes. Our results show that bedding-parallel calcite veins grow nearly vertically along the median plane, and exhibit antitaxial and two stages of growth. The early calcite veins have high δ13C values (0.08‰–4.75‰) and the diagenetic fluids were relatively low-temperature (120–150 °C) and high-salinity (11.2%), and formed during primary migration of hydrocarbon-bearing fluids in the oil window. The late calcite veins have relatively low δ13C values (−7.01‰ to −0.08‰) and the diagenetic fluids were high-temperature (180–210 °C) and low-salinity (8.9%), and formed in the gas window. The similar 87Sr/86Sr and δ13C values of the calcite veins and surrounding host rocks, combined with the relatively uniform fluid δ18OV-SMOW values, indicate that the calcite-precipitating fluids were derived mainly from the surrounding host-rocks and organic fluids. The formation of bedding-parallel calcite veins records the two-stage, fluid-overpressured evolution of organic matter in the oil and gas generation stages.