How to find high-quality petroleum source rocks in saline lacustrine basins: A case study from the Cenozoic Qaidam Basin, NW China

Despite having made significant contributions to global hydrocarbon occurrences, source rocks in saline lacustrine basins show great mineralogical and organic geochemical heterogeneity and the controlling mechanism is weakly studied. Here, we use the Cenozoic Qaidam Basin as an example to compare the organic and inorganic parameters of fine-grained sediments in different lacustrine facies and to figure out the characterization and distribution of effective petroleum source rocks in saline lacustrine systems. Organic-inorganic database of six selected boreholes show that in the bench-margin (high-gradient) lacustrine system, fine-grained sediments in sublittoral brackish-mesosaline zones containing low chlorides (<2000 ppm) and high carbonates (25–50%) show low total organic carbon (TOC) contents (0.4–1.0%) and high chloroform extracts (0.05–0.2%); centripetally sediments in penesaline profundal zones averagely have higher chlorides (2000–15000 ppm), lower carbonates (15–35%), higher TOC contents (0.6–1.2%), and higher extracts (0.05–0.3%). These two facies can both develop valid petroleum source rocks. In the ramp-margin (low-gradient) lacustrine system, dark fine-grained sediments in penesaline centers with low carbonate (15–25%) and high chloride contents (2000–15000 ppm) show high TOC contents (0.8–2.0%) but extremely low hydrogen indexes (<100 mg HC/g TOC) (little extracts), only developing as valid gas/or biogas source rocks. Supersaline fine-grained sediments in both lacustrine systems develop invalid source rocks. Lateral superimposition maps show that fine-grained sediments with high hydrocarbon generation potential generally contain averagely 25–50% carbonates. Therefore, brackish-penesaline water salinity and bench-type basement are two necessary conditions to develop high-quality petroleum source rocks in saline lacustrine systems and average carbonate content can be a general good mineralogical indicator.