Biological mechanism of paleokarst developed in the Ordos Basin

This paper demonstrated the biological mechanism of paleokarst developed within the Ordovician formation in the Ordos Basin. An experiment in laboratory was designed for understanding the water-rock interaction with sulfate-reducing bacteria. The study is focused on the geochemical processes in the bacteria-water-rock system by tracing stable sulfur isotope (³⁴S) fractionation. The δ³⁴S_-H2S obtained in the experiments are about 6.88⁰/₀₀ to 13.11⁰/₀₀ lower than the value of sulfate rock, which provides evidence of bacterial sulfate reduction in the bacteria-water-rock system. The changes of SO₄^2- concentrations and δ³⁴S_-SO4 values indicate that SO₄^2- has been released continually from the dissolution of sulfate rocks. This continuous dissolution of sulfate rocks resulted in the development of sulfate karst in the Ordovician weathering crust. In the buried stage, the bacterial sulfate reduction enhanced the paleokarstification in carbonate rocks in the Ordos Basin, These results provide new insight into the formation of the oil-gas reservoir and helpful information for the exploration of petroleum gas.