Characterization of Crude Oil, Formation Water, and Fluid Inclusions of Hydrocarbon-Bearing Strata and Their Hydrocarbon Geological Significance in the Wuliyasitai Southern Sub-sag of the Erlian Basin, China

[Image: see text] The hydrocarbon-bearing inclusions, formation water, and crude oil contain a wealth of information regarding the process of accumulation of oil and gas. Their research has been one of the hotspots in the field of oil and gas geology. Chemical analysis data was systematically employed for the first time to identify the properties of liquid fluids in the oil-bearing system of the Wuliyasitai southern sag in the Erlian Basin. A biased lacustrine algal input was shown by the stable carbon isotopes (δ(13)C) of whole hydrocarbons, and the morphological characteristics of n-alkanes, isoprenoids, steranes, and terpenes were examined by gas chromatography–mass spectrometry. The δ(13)C values were determined by the salinity of the sedimentary water body and input of land-sourced materials. The signatures of the n-alkanes’ molar fraction arrangement suggest that water washing, crystallization, and wax precipitation are regular occurrences in oil samples. The salinity of formation water and the proportion of characteristic biomarkers (gammacerane, β-carotene) demonstrate that the salinity of the ancient water body ranges from fresh water to brackish water. The correlation between the stratigraphic water’s characteristic coefficients and the total dissolved solids (TDS) is strong, particularly for the desulfurization coefficient, which illustrates a better hydrocarbon storage environment when TDS is >6 g/L. The metamorphic coefficients demonstrate that the existing reservoir preservation circumstances are not optimal, and the TDS and concentration degree of formation water are positively connected to the increasing depth, indicating poor longitudinal connectivity. The influence of atmospheric water infiltration is responsible for the fluid inclusions’ temperatures of homogenization (60 °C). However, due to deep burial, crude oil has not undergone secondary significant biodegradation despite the percolation of atmospheric water. Besides, the distribution of reservoirs is highly correlated with formation water parameters such as the TDS, which can provide scientific guidance for further exploration of oil and gas in the study area or even in the Erlian Basin.