GEOCHEMICAL CHARACTERISTICS AND SOURCE OF PALEO- RESERVOIR SOLID BITUMEN IN SHIWANDASHAN BASIN

Systematic geochemical analysis such as bitumen occurrence,group composition of dissolved organic matter,saturated hydrocarbon chromatography,biomarker compounds,carbon isotope and bitumen thermal maturation is applied to the solid bitumen of Laohushan and Shangtun paleo-reservoirs from Permian limestones in Shiwandashan Basin of Guangxi Province,China.And combined with geochemical characters of hydrocarbon source rocks and reservoir-forming mechanism,the source and genesis of above two solid bitumen has been discussed.The results show that the content of dissolved organic matter and saturated hydrocarbon is very poor and the content of aromatic hydrocarbon(the highest being 58.60%)and nonhydrocarbon is rich.The gas chromatogram analysis of saturated hydrocarbon show that bitumen is from reduction environment originating from lower grade aquatic geology with higher thermal evolution degree.The stable carbon isotope of bitumen is -27.8‰~-28.7‰,reflecting that the main source of input of source rock is lower aquatic biology.The lower content of sterane and terpane(dominated by low molecular compounds),higher content of pregnane in sterane series and tricyclic terpane predominated in terpane indicate that the bitumen was characterised by high evolution.Like the argillaceous source rock of Permian,there is nearly no aromatic sterane in bitumen.The reflectance of bitumen is 1.9%～2.5%,and the equivalent vitrinite reflectance is 1.6%~2.0%,associated with the results of reservoiring and evolution history,it shows that the bitumen experienced 230℃ temperature.Finally combined with relative geochemical characters of hydrocarbon and certain information of reservoir-forming,it is considered that the paleo-reservoir bitumen has closer genetic relationship with Upper Permian argillaceous source rock,and it is one of the final pyrolysis products of crude oil in paleo-reservoir,(that is pyrobitumen).