Characteristics Analysis Of Tertiary Saline Lacustrine Source Rocks In The Western Qaidam Basin

Qaidam Basin has special geographical location, climatic conditions and depositional environment. A set of saline lacustrine source rocks was developed in the Western Qaidam Basin. In present study, the organic abundance, kerogen type and maturity of the source rocks from saline depositional environment were analysed. The results derived will be beneficial for petroleum exploration in Western Qaidam Basin.Total organic carbon (TOC) content, chloroform bitumen "A" content can be used to analyze the abundance of organic matter in western Qaidam. On the base of relationship between pyrolysis parameters and TOC, the effective source rocks have minimum TOC content of 0.4%. The abundance of organic matter in the Eogene strata is higher than its in Neocene strata, abundance of organic matter also differs laterally and vertically. The organic abundance of source rocks in Western Qaidam is generally low, however, the high quality intervals are still exist, especially in Eogene strata, where high abundance of organic matter and high soluble organic matter exist. Depositional environment of salt lake in Western Qaidam Basin has certain impact on the generation and preservation of organic matter. High salinity is beneficial to the preservation of organic matter, but extreme high salinity is harmful for the development of source rocks.On the base of elemental analysis of kerogen and rock pyrolysis analysis, organic matter in source rocks is dominated by typeⅡkerogen in Western Qaidam. Organic maceral component analysis shows a higher proportion of sapropel content, which is mainly deried from aquatic organisms and bacteria. Algae is the best hydrocarbon precussors in Western Qaidam.Vitrinite reflectance (Ro%) is the best parameters to determine the organic maturity of source rocks, according to vitrinite reflectance values, rock pyrolysis maximum temperature (Tmax) and steranes isomerization C29S /S+R value, the maturity of organic matter is determined in saline lacustrine source rocks in the Western Qaidam Basin. The evolution of the organic matter in source rocks in the Tertiary strata is in the oil window, and the maturity of source rocks differs from region to region. The maturity of source rocks in the Eogene strata is higher than its in Neocene strata, source rocks remain immature in basin margin area. Characteristics and significances of biomarker in the Tertiary saline lacustrine source rocks in the western Qaidam Basin were discussed in terms of n-alkanes, isoprenoid alkanes and steranes, terpenes and carbon isotopic composition.The source rock potential of each formation in the Western Qaidam Basin were evaluated. The source rocks in E3 layer were widely distributed, and it has a large thickness, which can serve as the main source of oil and gas, especially in south and central area. N1 and E1+2 stratas contain minor source rock potentials, while the source rocks potentials in the N21 and N22 layers are poor.