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

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 delta C-13 values (0.08 parts per thousand-4.75 parts per thousand) and the diagenetic fluids were relatively low-temperature (120-150 degrees 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 delta C-13 values (-7.01 parts per thousand to -0.08 parts per thousand) and the diagenetic fluids were high-temperature (180-210 degrees C) and low-salinity (8.9%), and formed in the gas window. The similar Sr-87/Sr-86 and delta C-13 values of the calcite veins and surrounding host rocks, combined with the relatively uniform fluid delta O-18(V-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 re-cords the two-stage, fluid-overpressured evolution of organic matter in the oil and gas generation stages.

Automated summary

The study shows that bedding-parallel calcite veins in shale have two stages of growth, with early veins formed in low-temperature, high-salinity diagenetic fluids and late veins formed in high-temperature, low-salinity diagenetic fluids, reflecting the evolution of organic matter in the oil and gas generation process.