The Occurrence Characteristics and Enrichment Patterns of Shale Oil in Different Lithofacies: A Case Study of the Fengcheng Formation in the Mahu Sag.

The supply of shale oil is of significant importance to the sustainability of energy resources. However, due to the diversity of shale lithofacies, the occurrence characteristics and enrichment patterns of free oil and adsorbed oil remain unclear. Based on this, taking the shale oil reservoir of the Fengcheng Formation in the Mahu Sag as the research object, a series of experiments were conducted, including X-ray diffraction analysis (XRD), total organic carbon (TOC) content analysis, high-pressure mercury injection (HPMI), and field emission scanning electron microscopy (FE-SEM). In addition, multitemperature pyrolysis was used to preliminarily determine the contents of free oil and adsorbed oil in different lithofacies, and a laser scanning confocal microscope (LSCM) was employed to visually characterize the occurrence characteristics of shale oil at the lamina scale. Principal component analysis (PCA) was combined to assess the effect of pore structure parameters on the contents of free oil and adsorbed oil in different lithofacies. The results showed that the samples could be categorized into three lithofacies: continuous laminated felsic shale, discontinuous laminated felsic shale, and agglomerated mixed shale. Based on sedimentary structural characteristics, the shale oil in the continuous laminated felsic shale indicated obviously laminated distribution, while for the shale oil in the discontinuous laminated felsic shale, the laminated distribution of shale oil was weakly distributed, exhibiting a district distribution along the fracture. The histogram counts of the two lithofacies adsorbed oil fluorescence were higher than those of the free oil. The agglomerated mixed shale oil was widely distributed and exhibited a platy pattern, and the histogram counts of the free oil fluorescence were higher than those of the adsorbed oil. Analysis of the pore principal component parameter (including neighborcount, rugosity, aspect ratio, and symmetry) revealed that the pores in the continuous laminated felsic shale and the discontinuous laminated felsic shale could provide more effective adsorption sites, resulting in higher contents of adsorbed oil. The pore type of the agglomerated mixed shale was quite different from the other two lithofacies, with higher free oil content compared with adsorbed oil. Combined with the sedimentary structure and the occurrence characteristics of shale oil, the shale oil enrichment patterns were finally categorized into lamina-pore controlled type, lamina-pore-fracture controlled type, and fracture-pore controlled type, respectively. Overall, this study aims to facilitate efficient exploration and production of shale oil and gas resources.