Formation mechanism of Gulong shale oil: Insights from semiclosed hydrous pyrolysis

As a typical lacustrine shale oil, Gulong shale oil has enormous resource potential. The current lack of understanding regarding the formation process of this oil has affected its development. This study uses a low-mature shale sample from the first member of the Qingshankou Formation in the Songliao Basin and analyzes the hydrocarbon generation, expulsion, and retention characteristics by conducting hydrous pyrolysis in a semiclosed system. The results show that the formation process of the Gulong shale oil can be divided into three stages. The slow hydrocarbon generation stage occurs when organic matter maturity (Ro) < 0.8 %, and only a small amount of hydrocarbons is generated by kerogen cracking. The products are mainly heavy components (C15+), with retained oil composing the majority of them. The rapid oil generation stage occurs when Ro is of 0.8 %-1.1 %, where a large amount of kerogen is cracked to generate hydrocarbons and reaches its peak when Ro is 1.1 %. Most of generated light component oil (C-6-C-14) is expelled, and retained oil is mostly composed of heavy components (C15+); Ro > 1.1 % indicates the stage of oil cracking and rapid gas generation. Light component oil (C-6-C-14) and gaseous hydrocarbons (C-1-C-5) are formed when heavy components oil (C15+) begins to crack. The light-to-heavy ratio of the hydrocarbon products (C-1-C-14/C15+) rapidly increases with maturity. The hydrous pyrolysis experiment in a semiclosed system is more in line with actual geological conditions of Qingshankou Formation compared to other experimental systems and can simultaneously consider the influence of geological factors such as temperature, pressure, hydrocarbon expulsion, and formation water.