Organic Geochemical Characteristics Of Source Rocks In Continental Rift Basins,for Example,the Permian Lucaogou Formation In The Jimusar Sag

The Permian Lucaogou Formation was formed in an intracontinental rift lacustrine basin with strongly volcanic activities in Jimusar Sag in Junggar Basin.Those activities provided tuffaceous materials to processes of deposition and diagenesis.A special type of source rocks formed in this environment during Lucaogou period.In this paper,through a large number of rock thin section observation,combined with rock pyrolysis,kerogens microscopy,vitrinite reflectance,chromatographic-mass spectrometry,electron probe and whole rock X-ray diffraction,the characteristics of petrology,organic geochemistry,and sedimentary environment were researched,and the influences on forming high quality hydrocarbon source rocks by tuffaceous materials were discussed.Four main conclusions are as following.(1)The Lucaogou Formation not only developed a series of source rocks formed in normal lacustrine environment,but also a type of source rock associated with volcanic materials that mainly compose of tuff,sedimentary tuff,and tuffaceous dolomites.According to the Ritman index of feldspars in tuffaceous rocks using electron microprobe,the sediments in the study area are derived from deeply alkaline magma,which suggest that the study area is affected by significant volcanic activities.(2)According to organic geochemical experiments,Lucaogou Formation developed a set of good-excellent hydrocarbon source rocks with mainly type ? 1,and less type ? organic matters.Which are mainly composed of sapropel group,and little sapropelic amorphous bodyand rare sapropel clastics.Hydrocarbon generation parent material is mainly composed of algae and lower aquatic organisms.Maturity is low mature-mature,only a small amount of sample showed immature and high maturity,heterogeneity is strong in the plane and longitudinal.(3)According to the characteristics of biomarker compounds,the n-alkanes show multiple superior single-peak type and pristane advantage.The C24/C23 in the tricyclic terpane in relatively high,while that of C22/C21 is low,which reflects the characteristics of lacustrine source rocks.The content of gamma paraffin wax is low,and the C24 in the fourth ring road terpane is also less,reflecting the water medium for the brackish water-salt water.The comprehensive analysis shows that the study area of Lucaochou Formation is a weakly oxidized-weakly reduced brackish-saline lake sedimentary environment in intracontinental rift basin.(4)The tuffaceous matter provided by volcanic activities has an important effect on the formation of high quality source rocks in the study area,mainly due to the fact that the heat provided by the tuffaceous materials can make the source rocks mature.Furthermore,the reducing environment created by the tuffaceous materials is also beneficial to preservation of hydrocarbons source rocks.