Petrologic and geochemical characteristics of high-quality saline lacustrine source rocks in western Qaidam Basin

Abstract

The main oil source in western Qaidam Basin is the Paleogene saline lacustrine source rocks. Traditionally, it was considered that these source rocks were characterized by low abundance and poor type of organic matter as well as limited oil generation potential, but this poor source rock condition was very inconsistent with good hydrocarbon exploration results. A previous study has just confirmed that this area develops high-quality source rocks, which is of great significance for recognizing potential of hydrocarbon resource and consolidating confidence to search large and medium-sized oil and gas fields in western Qaidam Basin. Based on fine geochemical analysis of source rocks, petrology, element geochemistry and organic geochemistry are applied to discuss development conditions of high-quality source rocks in this area. The results showed that the saline lacustrine sediments in western Qaidam Basin were deposited in relatively shallow and quiet water with relatively high salinity and low nutrient content; influenced by the drought and cold climate, the terrigenous supply of fresh water was limited, and few aquatic organisms were developed. Good preservation condition is favorable for rapid preservation of oil generating biomass (such as aquatic organisms), to form source rocks rich in hydrogen element with high soluble organic matter content; but relatively lower paleo-productivity makes organic matter abundance lower than that in the eastern saline lacustrine basins. The semi-deep lake, with relatively low salinity and abundant nutrients, is the most favorable are for development of high-quality source rocks; the source rocks have large thickness in the sedimentary depression areas, but due to high water salinity and far away from the provenance area, the water stratification was obvious, so it have moderate to high organic matter abundance. Keywords: Saline lacustrine source rocks, TOC, Major and trace elements, Biomarker, Paleo-productivity, Preservation condition, Qaidam basin