Occurrence State of Soluble Organic Matter in Shale Oil Reservoirs from the Upper Triassic Yanchang Formation in the Ordos Basin, China: Insights from Multipolarity Sequential Extraction

Quantitative characterization of different occurrence states of soluble organic matter is the basis and key for shale oil resource evaluation and movable oil and producibility assessment. A series of integrated methods, including basic geochemistry, X-ray diffraction, multipolarity sequential extraction and chromatographic analysis were conducted on 15 Ch-7(3) Triassic Yanchang Formation samples, which included four kinds of lithology, from well CY-1 in the Huachi area, Ordos Basin, to characterize quantitatively the content and components of soluble organic matter in different occurrence states, evaluate shale oil mobility and recoverable potential, and analyse constraints and controlling factors. The results show that, as the main reservoir type of shale oil, residual hydrocarbon contents in black shale (BS), fine sandstone (FS), muddy siltstone (MS) and dark mudstone (DM) decreased sequentially. In DM and BS, medium and heavy components of adsorbed and miscible kerogen and mineral surface adsorption hydrocarbons were dominant, while the proportion of free and fracturing free hydrocarbons that were mainly composed of light components was larger in MS and FS. The movable hydrocarbon contents of the four lithologies were FS > MS > BS > DM. The brittleness index, TOC content and (Sat + Aro)/(Res + Asp) ratios were correlated positively with shale oil mobility. Clay minerals were basically inversely proportional to movable hydrocarbon content, while chlorite can improve it to some extent. MS and FS have great potential for shale oil exploitation under horizontal well volume fracturing technology, while the relatively low brittle mineral content and movable hydrocarbon content as well as high clay mineral content may limit the artificial fracturing exploitation effect of DM and BS. This study provides new revelation for the evaluation of physical status, mobility and producibility of shale oil.