Petroleum geologic characteristics of medium-small continental basins in five provinces in South China

In 5 provinces (Hainan,Guangdong,Yunnan,Guizhou and Guangxi) in South China,about 63 medium-small continental basins generated from the Late Cretaceous to Cenozoic.6 basin zones and 2 basin generating environments were divided according to tectonic backgrounds.Combined with sedimentary and filling process,4 patterns were concluded.In the east and center of the study area,source rocks generated early (usually during Paleogene) and were featured by big thickness (400-1000,100-900m) and lacustrine mudstones.In the composite and superimposed basins in the west of the study area,source rocks generated late (mainly during Neogene) and were featured by smaller thickness (125-700m) and carbonate mudstones and coal beds.TOC values were high in these basins,but the organic types were complicated and decreased from east to west.The thermal evolution of source rocks also decreased from east to west.The eastern and central basins were during the immature and low-mature stages,and the exploration targets should be focused on immature biogas and low-mature oil.The western basins were during the immature stage,and immature biogas should be explored.As the result of abundant depositional system in these basins,physical property of sandstone reservoirs were fine,usually characterized by mid-high porosity and permeability.What's more,during the fading stage of basin,shallow lacustrine and swamp facies mudstones filled regionally and worked as seal.According to exploration data,petroleum pools in the study area were classified into 4 types including tectonic (mainly in the eastern basins),lithologic (mainly in the western basins),stratigraphic and composite (mainly in the central basins).