Formation of hoodoo-upland on Ordovician karst slope and its significance in petroleum geology in Tahe area, Tarim Basin, NW China

Abstract

Based on a large number of geological and geophysical data, the formation, fracture-caves types and hydrocarbon distribution of hoodoo-upland on the Ordovician karst slope in the Tahe area, Tarim Basin, are discussed by analyzing faults and strata thickness. The hoodoo-upland was made of high peaks and narrow valleys in the Ordovician karst slope during the Early Hercynian karst period, which were distributed along the NNE positive flower structure and had inherited evolution. The fault-fractures and fracture-vugs complex were extremely developed, with a thickness of 100 m. The cumulative oil production of 60% oil wells was more than 20×104 t per well in the hoodoo-upland, where the residual thickness of the Ordovician Yingshan Formation was greater than karst depressions. Caves formed by the shelter of collapsed breccias were developed in the valleys. They were 1.6 to 13.5 m high, with a filling rate of 51.6%. The positive flower structure under the settings of strike-slip compression controlled the early formation of the hoodoo-upland on the karst slope, resulting in the differences of drainage distribution and karstification. Compared with the water-rich karst valley, the hoodoo-upland with lean water suffered weaker karstification, had thicker residual stratum, and was higher in terrain. In rainy season, the meteoric water flew and corrode along the cracks, forming a complex network of fractures and caves. Combined with inherited uplift and the effective match of the NNE deep faults, oil and gas continuously charged into the reservoir space in the upland, forming the hoodoo fracture-cave reservoir with vertically quasi continuous distribution, high hydrocarbon abundance and high production.