TERTIARY SALINE LACUSTRINE HYDROCARBON SOURCE ROCKS OF THE WESTERN QAIDAM BASIN, NORTHWEST CHINA

Based on numerous salinity and organic geochemical tests of lacustrine shales, this paper studies influences of salinity on organic matter richness of hydrocarbon source rocks and builds the organic matter accumulation model for Tertiary salt lakes of the western Qaidam basin. The relationship between total organic carbon content (TOC) and salinity of source rocks indicates: (1) under the condition of chloride concentration less than 1000ppm, TOC distribution characteristics are similar to those of typical onshore source rocks and TOC may attain higher value while its mean is not high; (2) under the condition of chloride concentration of 1000-8000 ppm, TOC increases with the growth of salinity and reaches the peak when chloride concentration is 4000ppm-6000ppm; (3) under the condition of chloride concentration more than 8000 ppm, TOC decreases with the increase of salinity. The organic matter accumulation and preservation model of Tertiary Saline lakes of the western Qaidam basin shows: (1) at the early stage of salt lakes, a large amount of diverse organisms live in fresh and brackish water, where there are high productivities of organic matters. Preservation conditions of organic matters improve with the increase of salinity. Water body conditions are beneficial to organic matter accumulation and preservation; (2) at the middle stage of salt lakes, saline water provides ideal living conditions for halophilic organisms and organic matter productivities increase with the growth of salinity until chloride concentration attains 5000ppm. When chloride concentration exceeds 5000ppm, organic matter productivities decrease with the increase of salinity. Preservation conditions continue to turn better with the growth of salinity. Accumulation and preservation conditions of organic matters at this stage are better than those of the early stage; (3) at the late stage of salt lakes, organisms rapidly die away in hypersaline water and organic matter productivities decrease with the increase of salinity. Sulfate ion enrichment and its oxidation weaken improvement of preservation conditions of organic matters with the increase of salinity. Totally, water body conditions at this stage are unfavorable for accumulation and preservation of organic matters.