Silicon isotopic constraints on the genesis of cherts in the Ordovician sedimentary succession in Tarim Basin, Western China

The Tarim Craton is the largest petroliferous basin in China, and thick chert have been found in the Ordovician carbonate reservoir during drilling. Owing to the tight chert, the storage performance may decrease and lead to drilling failure. In this study, silicon, oxygen, and strontium isotope compositions of chert were tested to constrain the source and migration of Si-rich fluids, which is helpful for understanding the reservoir and avoiding drilling failure. The Ordovician silicon dioxide in cherts have delta 30Si values from +1.29%o to +3.50%o, delta 18O values from 20.21%o to 27.50%o, and 87Sr/86Sr values from 0.709 to 0.714. The correlation between delta 30Si- and delta 18O reveals that the formation of cherts is related to the activity of low-temperature hydrothermal fluids, and the value of 87Sr/86Sr indicates the contribution of the deep crust source to the silicon-rich fluid. Comprehensive analyses showed that the deep thermal fluid was active during the formation of cherts. The late Hercynian tectonic movement led to the development and activity of deep and large faults, and the deep-source silica hydrothermal fluid entered the Ordovician reservoir along the fault from bottom to top, filling the pores, densifying the rocks, and causing deterioration of the storage performance. Therefore, through multigeochemistry (Si-O-Sr isotopes), the spatial distribution characteristics of silica can be determined to avoid the distribution interval and member of silica during drilling. Moreover, the identification and distribution prediction of the genesis of cherts is of great significance for oil and gas exploration.

Automated summary

The study found that the compositions of silicon, oxygen, and strontium isotopes in chert from the Ordovician reservoir can help constrain the source and migration of Si-rich fluids, preventing storage performance decline and drilling failure.