Geochemical features of source rocks and oil in saline and freshwater lake environments: A case study in the southwest Qaidam Basin

Abstract

The Qaidam Basin is a Cenozoic continental salt-lake basin in China. The lakes within this basin develop various salinities. This paper studies the geochemical characteristics of source rocks and petroleum in different salinity environments. Our results indicate that from the Paleogene to the Neogene, the salinity in the western Hongshi Sag gradually decreased and that the salinity in the eastern Zhahaquan Sag gradually increased, reflecting the eastward movement of the sedimentary center of the Paleogene basin. However, the salinities of western strata are still higher than those of the eastern strata. Due to the salinity differences between sedimentary water sources, the geochemical characteristics of source rocks and their products are also different. We can categorize these oils into two types according to their geochemical characteristics: Type A and Type B. Type A materials are produced around the Hongshi Sag and are the product of salt water deposition, while Type B materials are produced around the Zhahaquan Sag and are the products of freshwater deposition. Compared with Type B, Type A has the advantages of a high gamma paraffin index, a low Pr/Ph value, and a uniform carbon number of n-alkanes. Simultaneously, some geochemical parameters (Ts/Tm, OEP, Pr/Ph, etc.) lose their significance due to high salinity during formation. Geochemical differences between these sedimentary environments make it relatively easy to distinguish the source rocks for each type: Type A materials come from the E32 source rock in the Hongshi Sag and Type B materials come from the E32 source rock in the Zhahaquan Sag. This study is significant for revealing the geochemical characteristics of source rocks and crude oil, refining the law of oil and gas migration and further characterizing the sedimentary environment in Salt-Lake Basin.