lacustrine organic-rich black shale laminated facies and its reservoir significance - a case study of the Shahejie Formation shale in the Jiyang Depression

Key words: 
• Shale /
• Lamina type and combination /
• Reservoir characteristics /
• Shale oil occurrence pattern /
• Bohai bay basin

Abstract: China possesses abundant terrestrial shale oil resources, exhibiting significant potential for exploration. This study focuses on the black shale of the Shahejie Formation(Es1) in the Jiyang Depression as its research subject. By employing XRD, rock pyrolysis, fluorescence analysis, scanning electron microscopy, and low-temperature gas adsorption techniques, we comprehensively investigate the laminated black shale layer types, layer combinations, pore types, and structural characteristics of the Es1 section within the southern Bohai Bay Basin. The findings reveal a diverse range of mineral types present in the Es1 section black shales. Based on their distinct compositions, these black shale layers are classified into five categories: feldspar-quartz lamina, clay mineral-rich lamina, aragonite lamina, micritic calcite-dominated lamina, and organic-rich lamina. Furthermore, the vertical stacking relationships allow for their division into three binary layer combinations: "organic-rich + micritic calcite", "organic-rich + aragonite" and "organic-rich + feldspar-quartz". Among these, the porosity of "organic-rich + aragonite" and "organic-rich + feldspar-quartz" binary lamina combination shales was relatively higher, with superior pore structure and connectivity compared to those of "organic-rich + micritic calcite" binary lamina combination shales. In the "organic-rich + aragonite" binary lamina combination shale, shale oil is present in a free state within interlayer crevices, exhibiting optimal mobility. The coexistence of free oil and adsorbed oil in the "organic-rich + aragonite" binary lamina combination shale results in relatively poor mobility. However, in the "organic-rich + feldspar-quartz" binary lamina combination shale, shale oil primarily exists in an adsorbed form, demonstrating the lowest mobility. The research findings elucidate the heterogeneity in reservoir properties among different lamina combinations and establish the pivotal influence of diverse lamina combinations on balck shale's porosity, pore structure, and hydrocarbon storage capacity. Based on these discoveries, we have developed shale oil storage models tailored to various lamina combination black shales, thereby providing a scientific foundation for future extraction of shale oil and gas.