Multi-Isothermal Stage Pyrolysis of the Chang 7(3)Shale OilReservoirs, Ordos Basin: Implications for Oil Occurrence States andIn Situ Conversion Exploitation

Understanding the content, component features, and controlling factors ofhydrocarbons in different occurrence states is a key element in assessing the resourcepotential, mobility, and engineering exploitability of shale oil. X-ray diffraction, bulkgeochemistry, multi-isothermal stage pyrolysis experiments, and total hydrocarbonchromatographic analyses were performed on a set of muddy siltstone (MS),finesandstone (FS), dark mudstone (DM), and black shale (BS) from the Chang 73subsection in the Huachi area to investigate the content, component features, andcontrolling factors of hydrocarbons in different occurrence forms and estimate the shaleoil engineering producibility under different production technical methods. The resultsshow 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 aswell 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 inBS and DM, while free hydrocarbons were dominant in MS and FS. The pyrolytic gaseswere chieflyCO2and methane. The (Sat. + Aro.)/(Res. + Asp.) ratios and plagioclase and chlorite contents jointly show a positiveeffect on the free/adsorbed hydrocarbon ratios, while the TOC, clay mineral, and mixed layer I/S contents were inverselyproportional. In general, geochemical features and mineral composition chieflyaffect adsorbed hydrocarbon yields to control thefree/adsorbed hydrocarbon ratio, the only exception of which is that the (Sat. + Aro.)/(Res. + Asp.) ratios affect free hydrocarbonyields. Si&Sa has good shale oil resource potential under volume fracturing because of its relatively high free hydrocarbon and brittlemineral contents and gas oil ratio. The weakly adsorbed hydrocarbons in Mu&Sh will be better transformed to the free state underpyrolysis conditions at 400 degrees C, which is more suitable for in situ heating conversion production. This study provides a new referenceand basis for the occurrence and mobility evaluation of Chang 73shale oil under different exploitation methods.