Multi-Isothermal Stage Pyrolysis of the Chang 73 Shale Oil Reservoirs, Ordos Basin: Implications for Oil Occurrence States and In Situ Conversion Exploitation

Understanding the content, component features, and controlling factors of hydrocarbons in different occurrence states is a key element in assessing the resource potential, mobility, and engineering exploitability of shale oil. X-ray diffraction, bulk geochemistry, multi-isothermal stage pyrolysis experiments, and total hydrocarbon chromatographic analyses were performed on a set of muddy siltstone (MS), fine sandstone (FS), dark mudstone (DM), and black shale (BS) from the Chang 73 subsection in the Huachi area to investigate the content, component features, and controlling factors of hydrocarbons in different occurrence forms and estimate the shale oil engineering producibility under different production technical methods. The results show that the residual hydrocarbon content in FS, BS, MS, and DM decreased in turn. Mudstone and shale (Mu&Sh) samples featured lower quartz and chlorite contents as well as more pyrite and mixed layer I/S than siltstone and sandstone (Si&Sa) samples. Free and weakly adsorbed hydrocarbons constitute the mainstay of the hydrocarbons in BS and DM, while free hydrocarbons were dominant in MS and FS. The pyrolytic gases were chiefly CO2 and methane. The (Sat. + Aro.)/(Res. + Asp.) ratios and plagioclase and chlorite contents jointly show a positive effect on the free/adsorbed hydrocarbon ratios, while the TOC, clay mineral, and mixed layer I/S contents were inversely proportional. In general, geochemical features and mineral composition chiefly affect adsorbed hydrocarbon yields to control the free/adsorbed hydrocarbon ratio, the only exception of which is that the (Sat. + Aro.)/(Res. + Asp.) ratios affect free hydrocarbon yields. Si&Sa has good shale oil resource potential under volume fracturing because of its relatively high free hydrocarbon and brittle mineral contents and gas oil ratio. The weakly adsorbed hydrocarbons in Mu&Sh will be better transformed to the free state under pyrolysis conditions at 400 °C, which is more suitable for in situ heating conversion production. This study provides a new reference and basis for the occurrence and mobility evaluation of Chang 73 shale oil under different exploitation methods.