Discovery of the ancient Ordovician oil-bearing karst cave in Liuhuanggou, North Tarim Basin, and its significance

Abstract

A typical ancient karst cave is found in the Yijianfang Formation of Ordovician at Liuhuanggou, North Tarim Basin. With an entrance of 4.8 m wide and 20 m high, the Liuhuanggou ancient karst cave extends to NW 315° and has been filled by sediments, which can be classified into three types according to their nature and origin: clastic sediments (gravels), chemical sediments (palaeotravertine), and mixed sediments. Among them, the Paleotravertine and gravels are most common. The palaeotravertine is laminated type and yellowish brown or black in color due to oil inside. When saturated with oil, the palaeotravertine is black. Under microscope, oil mainly accumulates in the cracks between calcite crystals. The filling period of the Liuhuanggou karst cave can be divided into three stages: mixed filling, chemical filling, and clastic (palaeotravertine breccia) filling. Numerous fractures are distributed on one side of the karst cave. The fractures can be as wide as 40 cm and over 40 m long. Fractures of different periods and sizes constitute a fracture network and are all filled with calcite veins. Oil can be found inside the fractures. In the chemical fillings, sulfur is also found, which can indicate the magma influence on karst caves. The existence of sulfur also confirms that the Liuhunggou karst cave was formed in ancient times rather than recently or nowadays. Different from normal sedimentary limestone, the ancient Liuhuanggou karst cave has a distinct isotope fractionation and the depletion of heavy carbon and oxygen isotope. Controlled by the northwest oriented vertical strike-slip shear fault, the karst cave is actually a dissolved vertical fault belt.