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

Abstract

• Hydrothermal fluids play an important role in the formation of cherts . • Si-O-Sr isotopes of the cherts to constrain the source and migration of the fluids. • Fault activity links the channels of hydrothermal migration during the late Hercynian. 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 δ 30 Si values from +1.29‰ to +3.50‰, δ 18 O values from 20.21‰ to 27.50‰, and 87 Sr/ 86 Sr values from 0.709 to 0.714. The correlation between δ 30 Si- and δ 18 O reveals that the formation of cherts is related to the activity of low-temperature hydrothermal fluids, and the value of 87 Sr/ 86 Sr 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 multi-geochemistry (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.