Geology, Geochemical Characteristics And Mechanism Of Hydrocarbon-generating For Source Rocks From The Tertiary Salty Lacustrine Facies In The West Region Of The Qaidam Basin

The Qaidam basin, a complex basin superimposed on different stages and different types basin ,is formed from Pre-Sinian crystalline basement. Three sets of strata-Palaeozoic, Mesozoic and Cenozoic strata were developed in the Qaidam basin .Among them ,Tertiary strata is mainly developed in the west region of the basin (briefly called Qaidam basin west region).Exploration proved that the Qaidam basin west region is an oil /gas-bearing area with source rocks of Tertiary salty lacustrine facies , the source rocks are characterized by high salinity (15%~60%), high carbonate content (>20%), low organic carbon content(an average Toc <0.5 %, high conversion rate of hydrocarbon (an average "A"/ Toc is 20%, the highest "A"/ Toc is more than 60%; an average Hc/Toc is 12%, the highest Hc/Toc is more than 40% ). Moreover, the composition characteristics of group component and carbon isotopes of n-alkane are similar to that of organic matter in marine facies. In addition, the evolution of source rocks is characteristics of "two stages hydrocarbon-generating due to the particularity of organic matter bio-source composition in source rocks from the Tertiary salty lacustrine facies, the first stage of hydrocarbon-generating is at an early stage (Ro< 0.6%) which immature to low-mature oil was mainly formed by direct conversion of the soluble organic matters, and the second stage is at a late stage(Ro>0.7%) which mature oil was chiefly formed by thermal degration of kerogen.Comprehensive assessment shows that E32 source bed is the predominant source bed of the oil/gas resources for the west region of the Qaidam basin, especially for the south and north region because of its feature of wide distribution, broad thickness and moderate maturation; therefore, based on the distribution of source rocks and potential of hydrocarbon-generating, the author puts forward that Shizigou-Youshashan structure belt ,Youquanzi-Kaitemilike structure belt and Aerjing slope-belt with rich oil resource will become the exploration targets in the next step, meanwhile suggests that exploration for mature oil in deep layer should be emphasized while exploring the immature oil in shallow layer.