Characteristics and Influencing Factors of Unconventional Hydrocarbon Accumulation in Saline Lacustrine Fine-Grained Sedimentary Rocks in the Northwestern Qaidam Basin

Although Cenozoic saline lacustrine fine-grained sedimentary rocks with great potential for unconventional oil and gas resources are widely deposited in the northwestern Qaidam Basin, there is still a lack of investigation on their reservoir conditions, owing to complex saline sedimentary environments, relatively low maturity, and strong heterogeneity. Previous studies have shown that the source rocks have a relatively high hydrocarbon generation conversion rate, and several oil-bearing structures have been found in the study area. In this study, core samples of the Neogene saline lacustrine fine-grained sedimentary rocks from representative wells in the study area were collected and investigated. Based on the testing of organic matter abundance, type, and maturity, combined with X-ray diffraction, high-pressure mercury injection (HPMI) and low-temperature liquid nitrogen adsorption tests (N₂-GS), scanning electron microscopy (SEM), and thin section observations, the hydrocarbon accumulation conditions and pore-development-influencing factors of the samples were studied in detail. New research suggests that (1) Neogene fine-grained sedimentary rocks have relatively low total organic carbon (TOC) content, and the organic matter distribution is controlled by the salinity in the northwestern Qaidam Basin. The salinity and type I organic matter content of the fine-grained sedimentary rocks increase from the Shangganchaigou formation (N₁) to the Shangyoushashan formation (N₂²), whereas their maturity tends to decrease. (2) Mesopores are developed dominantly in these samples. The SEM observations show that most of the pores are interparticle, intraparticle, and intercrystal pores, which are mainly open (parallel plate pores), semiopen, and a certain amount of “ink-bottle” pores. This is not conducive to macropore development in high-salinity samples because schistose salt minerals cover their surfaces. (3) Salinity, mineral content, TOC content, and thermal maturity play important roles in the pore distribution of Neogene fine-grained sedimentary rocks in the northwestern Qaidam Basin, which have salinity as the core-controlling factor. Within a certain salinity range, the Brunauer–Emmett–Teller (BET) specific surface area and mesopore volume of the samples increase gradually with the increasing salinity, whereas the macropore volume changes irregularly. When salinity becomes very high, the BET specific surface area, mesopore volume, and particularly the macropore volume of the samples decrease. This is unfavorable for pore formation and preservation in fine-grained sedimentary rocks. (4) The contents of quartz, feldspar, and pyrite in the samples decrease with increasing salinity, whereas the contents of carbonate mineral and clay mineral increase. The specific surface area of the samples shows a certain positive correlation with the clay mineral content and a negative correlation with the carbonate mineral content. The mesopore volume is positively correlated with TOC content, the sum content of quartz, feldspar, and pyrite, and clay mineral content but negatively correlated with the thermal maturity and carbonate mineral content. Therefore, the fine-grained sedimentary rocks with moderate salinity, relatively high organic carbon content, mature stages, and well-developed pores should be considered for unconventional oil exploration and development in the saline lacustrine basin.