The lacustrine shale samples were selected from the second member of the Funing Formation in Gaoyou Sag, Subei Basin, to precisely and quantitatively evaluate the free oil content and its controlling factors of lacustrine shale. The mineral composition, hydrocarbon generation capacity, pore structure, oil-bearing properties, and other characteristics of the samples were analyzed using X-ray diffraction tests, TOC content measurement, multistep rockeval pyrolysis, high pressure mercury injection, organic matter extraction and 2D NMR. The results show that the shale is rich in quartz, feldspar, calcite, dolomite, clay, and other minerals. The TOC is between 0.61% and 3.70%. The R_o is between 0.70% and 0.72%, indicating the source rocks being in the mature stage. The reservoir space is mainly composed of intergranular and organic pores. Two-dimensional NMR, which can efficiently detect and characterize the occurrence of free oil, was employed to help extracting free light and heavy hydrocarbons as well as soluble organic matter from the samples, and performing spectra analysis before and after the extraction, on which light oil zone, solid organic matter zone, hydroxyl compound zone, and water zone were identified. Post-extraction solid organic matter content was found to have a good correlation with TOC. Pre-extraction NMR-based light hydrocarbon content and NMR-based differential light hydrocarbon content between pre-extraction and post-extraction show a relatively strong linear connection with pyrolysis free oil content. The free oil content rises initially and then fall as the felsic mineral content rises, and it is positively correlated with TOC as a whole. The free oil contents in type Ⅰ and Ⅱ₁ kerogen source rocks are higher than those of other kinds. The right degree of thermal evolution of source rocks increases the ratio of light hydrocarbons and mobility of kerogen-generated hydrocarbons, as well as free oil content. Shale reservoirs with high porosity, specific pore volume, and median value of pore throat radius are conducive to free oil enrichment.

To accurately investigate the evolution characteristics and generation mechanism of retained oil, the study analyzed organic-rich lacustrine shale samples from the Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin. This analysis involves Rock-Eval pyrolysis, pyrolysis simulation experiments, Gas Chromatograph Mass Spectrometer (GC–MS), and reactive molecular dynamics simulations (ReaxFF). The results revealed the retained oil primarily consisted of n-alkanes with carbon numbers ranging from C₁₄ to C₃₆. The generation of retained oil occurred through three stages. A slow growth stage of production rate was observed before reaching the peak of oil production in Stage I. Stage II involved a rapid increase in oil retention, with C₁₂–C₁₇ and C₂₄–C₃₂ serving as the primary components, increasing continuously during the pyrolysis process. The generation process involved the cleavage of weak bonds, including bridging bonds (hydroxyl, oxy, peroxy, imino, amino, and nitro), ether bonds, and acid amides in the first stage (R_o = 0.50%–0.75%). The carbon chains in aromatic ring structures with heteroatomic functional groups breaks in the second stage (R_o = 0.75%–1.20%). In the third stage (R_o = 1.20%–2.50%), the ring structures underwent ring-opening reactions to synthesize iso-short-chain olefins and radicals, while further breakdown of aliphatic chains occurred. By coupling pyrolysis simulation experiments and molecular simulation technology, the evolution characteristics and bond breaking mechanism of retained oil in three stages were revealed, providing a reference for the formation and evolution mechanism of retained oil.