HIGH-QUALITY SOURCE ROCKS IN THE LOWER COMBINATION IN SOUTHEAST UPPER-YANGTZE AREA AND THEIR HYDROCARBON GENERATING POTENTIAL

There are good oil and gas exploration prospect in the southeast of Upper-Yangtze area,but the geologic conditions are complicated and the degree of exploration is low there.The potential hydrocarbon source rocks in Xianfeng and Dayong areas were studied to determine the characteristics of their organic geochemistry,petrology,carbon isotope composition and thermal evolution,etc.Two series of high-quality hydrocarbon source rocks were discovered.They are the Lower Cambrian black shale and the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation black shale in the Lower Palaeozoic.Both of them have characteristics as follows: 1)good organic types(sapropel type),high abundance of organic materials and hydrocarbon potentials;2)large thickness and extensive distribution;3)good hydrocarbon generating potential and evidences of oil(bitumen) and gas.At the Late Jurassic-Cretaceous,the thermal evolution degree of the hydrocarbon source rock in the Lower Cambrian has come to the early stage of over-mature at which the dry gas was generated.And the hydrocarbon source rock in the Upper Ordovician-Lower Silurian has come into the late stage of high-mature to the early stage of over-mature,which was the peak period of gas generation.These high-quality hydrocarbon source rocks provide abundant organic material basis for the hydrocarbon generation and accumulation in the Lower Combination in the west of Hunan and Hubei,the east of Chongqing and the Xuefengshan nappe zone.