Organic Geochemical Characteristics of Saline Lacustrine Source Rocks: A Case Study from the Yingxi Area, Qaidam Basin, China

Based on the systematic analyses of samples collected from the western Qaidam basin, the organic geochemical characteristics of representative Paleogene saline lacustrine carbonate source rocks were synthetically studied. The results show that the carbonate source rocks are characterized by total organic carbon (TOC) content varied in the range of 0.07 to 1.57%, with an average of 1%, and hydrocarbon generation potential (S-1 + S-2) is range from 0.2 to 9.61 mg HC/g rock with an average of 4.6 mg HC/g rock, and organic matters of Types II1 and Type I, which indicated that it belongs to high-quality carbonate source rocks. Meanwhile, the T-max of source rock range from 409 to 438 degrees C and productivity index (PI) varied in the range of 0.09 to 0.38, which imply that the source rock in immaturity to low-maturity phase. The biomarker compositions are characterized by the Pr/Ph ratio of the source rocks is 0.36, and the gammacerane/alpha beta-C-30 hopane ratio up to 1.25, these data reflect that the presence of a highly reducing saline environment. Meanwhile, a high abundance of C-27 sterane and bicyclic terpane indicates that aquatic algae are the main organic parent material. The identification of microscopic organic components of samples also reveals an abundance of the Botryococcus braunii fossil of up to 30% with strong yellow fluorescence. These results, together with comparison research of crude oil and source rocks, show that the saline lacustrine carbonate source rocks from the Yingxi area deposited under an anoxic and saline environment at the stage of low maturity. This study provides guidance for predicting the distributions of high-quality source rocks in the western Qaidam basin.

Automated summary

Based on systematic analysis of samples from the western Qaidam basin, the organic geochemical characteristics of Paleogene saline lacustrine carbonate source rocks were studied. The results show that these source rocks have high total organic carbon content and hydrocarbon generation potential, indicating their high quality. The source rocks are in a state of low maturity and were deposited in an anoxic and saline environment. Aquatic algae are the main organic parent material, as indicated by biomarker compositions. The presence of Botryococcus braunii fossils was also identified. This study provides guidance for predicting the distribution of high-quality source rocks in the western Qaidam basin.