The Early Hercynian paleo-karstification in the Block 12 of Tahe oilfield, northern Tarim Basin, China

It is indicated in this paper that the karstification occurred in Block 12 of Tahe oilfield during Early Hercynian stage on the basis of identification features obtained through the latest research on seismology, petrology, and geochemistry. Distinguishable markers of the paleo-karstification mainly include: the lost strata and unconformable contact occurring between Lower-Middle Ordovician carbonates and Lower Carboniferous Bachu Formation, the occurrence of collapse breccias and fillings in subsurface stream, the evident difference between cave-fill calcite and matrix carbonate in the Sr/Ba ratio, and the log response for cavernous formations. The paleo-geomorphology of Early Hercynian karstification can be divided into four units: karst highland, I- and II-level karst slope, and karst basin in Tahe area, and the Block 12 was basically located in II-level karst slope. Three karst cycles developed in the Block 12 according to Early Hercynian tectonism characterized by multiphase pulse-type uplift movement and three sets of cavernous formation formed. II- and III-level cavernous formations mainly developed in Block 12. The Early Hercynian karstification is distributed on the north area of the pinch-out line of Upper Ordovician Sangtamu Formation in the plane, and is vertically developed in the Lower-Middle Ordovician carbonates. The karstification is essentially controlled by the paleo-geomorphology and fault. Fissures and caves formed by the karstification are the keys to affecting reservoir quality. The evolution model of Early Hercynian karstification has been established on the basis of the synthesis of the above-mentioned research results. It can be divided into three stages: rapidly uplifting and denudation stage, three episodes of uplifting and karstification, and subsiding and reworking stage.