Enrichment and movability of lacustrine tight shale oil for the first member of the Upper Cretaceous Qingshankou Formation in the Sanzhao Sag, Songliao Basin, NE China: Insights from saturated hydrocarbon molecules

The Sanzhao Sag is one of the main hydrocarbon generation sags with good shale oil exploration and exploitation potential in the northern Songliao Basin, and the first member of the Upper Cretaceous Qingshankou Formation (K2qn1) is regarded as the most important target layer. In this paper, the complete K2qn1 rock cores from the first shale oil parameter well named the SYY3 well in the study area were taken to investigate tight shale oil enrichment and movability from saturated hydrocarbon molecules combined with bulk organic geochemical characteristics. The results show that the K2qn1 mudstones/shales in the SYY3 well are of high organic matter abundance (1.28–5.62 wt% TOC), good organic matter types (I and II1 kerogens) and medium maturity (0.82%–0.98% Ro), creating excellent material basis of hydrocarbon generation for shale oil enrichment. The molecular compositions of saturated hydrocarbons suggest that the K2qn1 were deposited in reducing brackish lacustrine water with mainly prokaryotes and secondly eukaryotic algae as the biotic input, and are characterized by obvious two-stage distribution with buried depth. The lower unit (>2015.00 m) is generally more enriched in shale oil than the upper unit (<2015.00 m). Relative to the upper unit, the lower unit has lower ratios of pristane/phytane, gammacerane/C30 hopane, C19-20/C21-26 tricyclic terpanes, C24 tetracyclic terpane/C26 tricyclic terpanes, 4-methylstane/C27-29 αααR steranes, higher ratios of regular steranes/17α-hopanes, C27/C29 and C28/C29 αααR sterane, and higher contents of diadrimanes, diahopanes and diasteranes. This indicates that the lower unit can be deposited in a more reducing, lower salinity and more clay-rich sedimentary environment, and contains more eukaryotic algae and less prokaryote biotic input than the upper unit, which can be conducive to the formation and retention of shale oil in the lower unit. The S1 values showing certain positive correlations with regular steranes/17α-hopanes and C27/C29 αααR sterane indicates that the biotic input contribution of eukaryotic algae can be an important positive factor influencing free oil content. The negative correlation between (nC21 + nC22)/(nC28 + nC29) and OSI (S1 × 100/TOC) can be due to diffusion fractionation effect of hydrocarbon molecules in micro-nano reservoir spaces. Low (nC21 + nC22)/(nC28 + nC29) values usually indicate good shale oil movability and reservoir space connectivity under the similar thermal maturity.