Differential structure of Ordovician karst zone and hydrocarbon enrichment in paleogeomorphic units in Tahe area, Tarim Basin, NW China

Based on a large number of drilling, logging, seismic and production data, the differential structures of karst zone and hydrocarbon distribution in different paleogeomorphic units of the Tahe area, Tarim Basin, are discussed by analyzing the karst drainages and flowing channels. The karst paleogeomorphy of Ordovician in Tahe area is composed of watershed, karst valley and karst basin. The watershed has epikarst zone of 57.8 m thick on average and vadose karst zone of 115.2 m thick on average with dense faults, fractures and medium—small fracture-caves, and 76.5% of wells in this area have cumulative production of more than 5×104 t per well. The karst valleys have epikarst zone, vadose karst zone and runoff karst zone, with an average thickness of 14.6, 26.4 and 132.6 m respectively. In the runoff karst zone, the caves of subsurface river are mostly filled by fine sediment, with a filling rate up to 86.8%, and 84.9% of wells in this area have cumulative production of less than 2×104 t per well. The karst basin has no karst zone, but only fault—karst reservoirs in local fault zones, which are up to 600 m thick and closely developed within 1 km around faults. Different karst landforms have different water flowing pattern, forming different karst zone structures and resulting in differential distribution of oil and gas. The watershed has been on the direction of oil and gas migration, so medium—small sized connected fracture-caves in this area have high filling degree of oil and gas, and most wells in this area have high production. Most caves in subsurface river are filled due to strong sedimentation and transportation of the river, so the subsurface river sediment has low hydrocarbon abundance and more low production oil wells. The faults linking source rock are not only the water channels but also the oil-gas migration pathways, where the karst fractures and caves provide huge reservoir space for oil and gas accumulation