Sedimentary Facies Analysis And Source Rock Evaluation Of The Late Cretaceous Jinzhushan Formation In The Nima Basin,Tibet

The Nima basin is a Cenozoic continental basin located on the abdomen of the Qinghai-Tibet Plateau.The Lunpola basin,adjacent to the basin,is the only sedimentary basin in Tibet which is the only industrial oil flow.In recent years,the discovery of asphalt oil seedlings in the Nima basin shows that it is an oil and gas bearing basin.At the same time,55 km long oil and gas display belt have been discovered in the Jinzhushan Formation of the Kanggale region,Nima Basin,which further indicates that the basin has a significant oil and gas exploration prospect.Based on the theory of sedimentology and petroleum geology,we used sedimentary facies analysis,organic geochemistry and organic petrology to make a systematic sedimentology study and comprehensive evaluation of source rocks of the Late Cretaceous Jinzhushan Formation in the Nima Basin.In combination with the above two,the controlling factors of the lacustrine source rock in the Nima Basin were discussed.Four main types of lithofacies were identified in the measured section of the Late Cretaceous Jinzhushan Formation in the Nima Basin,which can be classified into three types of lithofacies sequences and multiple sets of sedimentary tectonic signs.The sedimentary facies of each section was analyzed using the lithofacies compositional characteristics.The results show that the Sangqian section is a lacustrine facies,which can be divided into shallow lake-deep lake subfacies;the Kanggale section identifies lacustrine facies and divides the Lakeside-Shallow lake-deep lake subfacies.The analysis of saturated hydrocarbon mass spectra indicates that the source of source rocks in the study area is mainly the contribution of algae,but the input of higher plants is not excluded.At the same time,the organic matter of hydrocarbon source rock is formed in reduction,strong reduction environment and high salinity conditions.The source rocks of the late Cretaceous Jingzhushan formation are mainly distributed in the dark mudstone of the shallow lake subfacies and dark mudstone and black shale of the deep lake subfacies,in which the shale organic matter abundance is between 0.04¹.73%,with an average of 0.27%.Organic matter type is type II₁,in which some samples reach the standard of hydrocarbon source rocks.The cumulative thickness of the Kanggale section is 556m,of which the effective source rock thickness is 13.4m,but the high abundance and high quality source rock data are scarce.Further exploration and research should be carried out.The organic carbon content of the shale of the Late Cretaceous Jingzhushan Formation was high,ranging from 0.34 to 5.29%with an average of 3.04%,and TOC can reach to 5.29%.The type of organic matter is better,which is characterized by type I.Ro was between 0.71 and1%,and the mean value was 0.86%.It was in the mature stage and reached the standard of high-abundance high-quality.Source rocks with a thickness of 5.43m.It is shown that the black shale formation in the Late Cretaceous of Nima basin is the key stratum for the development of hydrocarbon source rocks.