Development Mechanism Of Alkaline Lacustrine Source Rocks In The Mahu Sag,Junggar Basin

Compared with other source rocks,the alkaline lacustrine source rocks are of high quality with advantages of high abundance of organic matter,good type and strong hydrocarbon generation capacity.However,the development mechanism of the alkaline lacustrine source rocks is still unclear.Fengcheng Formation in the Mahu Sag of Junggar Basin is the most typical alkaline lacustrine deposits with a complete alkaline-forming sequence.The Fengcheng Formation(FF)source rocks are the main oil source of the lower assemblage reservoir forming system in Junggar Basin.Therefore,the alkaline lacustrine source rocks in the Mahu Sag of Junggar Basin are very representative.It is of great practical significance to study and enrich the development mechanism of alkaline lacustrine source rocks to guide oil and gas exploration of the alkaline lacustrine deposits.In this paper,source rock core samples were collected systematically and the geochemical characteristics of them were investigated by means of organic carbon content,Rock-Eval pyrolysis,gas chromatography-mass spectrometry of saturated hydrocarbon and carbon isotope analysis.Based on the analysis of tectonic setting and palaeoclimate,the relationship between paleoproductivity,preservation of organic matter and the development of source rocks was comprehensively analyzed.Further,the main controlling factors of the development of alkaline lacustrine source rocks was clarified,the development model was established,and the development mechanism was revealed.The abundance of organic matter in the alkaline lacustrine source rocks in Mahu Sag varies with members,areas and lithofacies.The abundances of organic matter of the Lower FF in each paleogeomorphic areas are relatively low,while those of calcareous mudstone in salt rock facies,tuff,tuffaceous dolomite,tuffaceous mudstone,dolomite and dolomitic siltstone in mudstone facies are high.The organic matter abundances of the Middle FF in the slope area are the highest,followed by the sag area,and the foreland thrust fault area is the lowest.The abundances of organic matter of dolomitic mudstone,argillaceous dolomite and soda-bearing mudstone in salt lithofacies,argillaceous dolomite and tuff in dolomitic lithofacies,and tuff,tuffaceous dolomite,marly mudstone and mudstone facies are high.The organic matter abundance of the Upper FF in slope area is higher than that in the paleoplatform area.The organic matter abundances of mudstone and dolomite in the salt rock facies are the highest.There are three types of alkaline lacustrine source rock development patterns in the Mahu Sag.The results show that “beginning alkali-forming model” of the Lower FF,“main alkaliforming model” of the Middle FF,and “ending alkali-forming model” of the Upper FF.During the initial alkali-forming stage of the Lower FF,the climate gradually became dry,and salinetolerant and basophilic algae and bacteria began to bloom,and the salinity stratified to form a reducing environment.During the main alkali-forming stage of the Middle FF,the climate was mainly arid and intermittent humid.In this situation,a large number of alkaline minerals and organic matter interbedded,saline-alkali tolerant and basophilic algae and bacteria developed.The salinity stratification of the water was stronger,and the reductivity was enhanced.During the ending alkali-forming stage of the Upper FF,the climate had been gradually humid,the alkaline minerals have not developed,and the saline-alkali tolerant and basophilic algae and bacteria have reduced.Meanwhile,water was gradually desalinated,the salinity stratification of water was weakened,and the reductivity became worse.