Isotopic evidence for the role of microbes in paleokarstification in the Ordos Basin, China

Abstract

The paleokarst developed in the middle Ordovician Majiagou formation is the main oil-gas reservoir in the Ordos Basin. The mechanisms of the Ordovician karstification in the epidiagenetic stages and burial diagenesis stages have not been well understood because of the complexity of genetic types and ambient environment. The stable sulfur isotope data provide a unique insight into the importance of microbes in karst development. The isotope data from laboratory experiments on water-sulfate-rock-microbe system and field samples of pyrite fillings in the fracture of paleokarst in the Ordos Basin, suggest that the bacterial sulfate reduction (BSR) occurred and had a significant isotope fractionation for stable sulfur isotope under closed conditions in the Ordos Basin. The pyrite was a secondary mineral formed by BSR, which confirms that the formation of pyrite and dissolution of sulfate rocks were driven by BSR in Ordovician oil-gas reservoir. The continuous dissolution of sulfate rocks results in the development of sulfate karst. The results from this study provide evidence for microbe activity in karstification, and give a good interpretation for the development of paleokarst in the Ordovician formation.