Study On The Evolutionary Characteristics Of Saline Lacustrine Basin During Sedimentation Of Qingshuiying Formation And Hydrocarbon Geological Condition In Ningnan Basin

Several geological factors such as climate,tectonism and sedimentary environment results in the development of saline lacustrine basin.Therefore,the research on the evolutionary characteristics of saline lacustrine basin is of great importance on the reconstruction of regional tectonism,palaeoclimate and sedimentary environment as well as the discussion of its relations and the interaction process.The lithology of Paleogene Qingshuiying Formation was mainly the interbedding of Gray-green,purple mudstone and gypsum in Ningnan Basin locate in Liupanshan tectonic belt,which is the typical saline lacustrine basin.Development and evolution of bioorganic matter,the hydrocarbon generation and expulsion characteristics in saline lacustrine basin and hydrocarbon accumulation is one of the important scientific issues.Research on the development characteristics,formation mechanism and accumulation condition of the Cenozoic hydrocarbon is of great significance on the theoretical study of hydrocarbon accumulation in saline lacustrine basin and the practical exploration of the Cenozoic hydrocarbon.The Hejiakouzi section of Qingshuiying Formation in Ningnan basin was chosen as the research target.The evolutionary characteristics and formation mechanism of saline lacustrine basin during sedimentation of Qingshuiying Formation were analyzed by the research of sedimentary stratigraphic features,the major elements in Qingshuiying Formation and Sr,S isotopic characteristics combined with former studies on regional foundation geology.The formation mechanism on the hydrocarbon source rock of Qingshuiying Formation was summarized based on its evaluation and particularity.On the basis of hydrocarbon inclusion features and oil-source correlation,the accumulation condition and resources potential of Cenozoic hydrocarbon in Ningnan basin were evaluated.The lithology of Qingshuiying Formation is mainly gray-green,purple mudstone and gypsum in Ningnan basin,which is the typical saline lacustrine basin.basin.The sedimentary environment can be divided into four types,namely deep saline lake,semi-deep saline lake,shallow saline lake and brackish lake.Among the major elements in Qingshuiying Formation,the contents of Al2O3,MgO,Na2 O,K2O,SiO2 and P2O5 reflect the sedimentary effects of terrestrial detrital material,while CaO indicates the chemical sedimentation of evaporate in the saline lacustrine basin.The regional climate conditions can be evaluated by Sr snd S isotopes of gypsum in Qingshuiying Formation.When the climate is humid,Sr isotope will accelerate and S isotope will decrease,and vice versa.During sedimentation of Qingshuiying Formation in Ningnan basin,the regional climate is generally dry and the evolution of saline lacustrine basin is not enough which cannot reach high concentration stage for sylvite sedimenting.Based on sedimentological and geochemical records,the evolution of saline lacustrine basin during sedimentation of Qingshuiying Formation at 38-34 Ma,34-30 Ma,30-27 Ma,and 27-23 Ma were deep saline lake,semi-deep saline lake,shallow saline lake and brackish lake respectively.The evolution of saline lacustrine basin is controlled by the resources of saline materials,the closure of lacustrine basin and the dry-humid climatic condition.The saline materials from the atmosphere water cycle offer abundant material basis for the formation of saline lacustrine basin.The evolution of four climatic types,namely humid-hot(45-40Ma),dry-hot(40-30Ma),dry-cold(30-27Ma)and humid-cold(27-23Ma),occurs under the evolution of the Tibet Plateau and the Global climate cooling,which is the key of the evolution of saline lacustrine basin.In addition,the closure of lacustrine basin is the necessity either.The mudstone of Qingshuiying Formation in Ningnan basin,with low abundant and dominated type ?of original matter,is in the lowe maturity level.The partial layer of Qingshuiying Formation is potential hydrocarbon source rocks in Cenozoic,which has a high conversion rates and early time on hydrocarbon generation and expulsion.The formation mechanism of hydrocarbon source rocks of Qingshuiying Formation is as follows.During the evolution of deep saline lake,Plankton like halobacteria from the brackish lakes and terrestrial plant from inflowing rivers were the organic matter resources.The concentration,salinization and stratification of the lake water form strong reductive sedimentary environment in the lacustrine basin,which provide beneficial conditions for the conservation of sedimentary organic matter.The study of hydrocarbon inclusions of Qingshuiying Formation and oil-source correlation in Cainozoic show the mudstone is also the potential hydrocarbon source rocks in Cenozoic of Ningnan Basin,and the Cenozoic is the vital stratum of hydrocarbon exploration because the oil and gas from Cretaceous hydrocarbon source rocks can migrate to the overlying stratum along the faults.There are two kinds of source-reservoir-caprock association in Cenozoic of Ningnan Basin,which are the ?lower-generation and upper-storage? and ?self-generation and self-storage?.The key of hydrocarbon accumulation is the development of the faults connecting to the hydrocarbon source rocks and the traps in the ?lower-generation and upper-storage?,therefore the surrounding of faults becomes the favorable area for hydrocarbon accumulation.While the key of hydrocarbon accumulation in ?self-generation and self-storage? is high quality hydrocarbon source rocks and the corresponding effective traps,so the effective traps near the high quality hydrocarbon source rocks is the favorable area for hydrocarbon accumulation.