Pore system of the multiple lithofacies reservoirs in unconventional lacustrine shale oil formation

Unconventional lacustrine shale formations are characterized by strong heterogeneity and intensive lithological variations in the vertical direction. Nevertheless, most studies focused merely on the organic-rich intervals due to the successful exploration and development of marine shale gas resources. This study systematically analyzed the overall pore system of the Triassic Chang 7 lacustrine shale formation in the Ordos Basin, China, using geochemical, petrological, mineralogical and petrophysical methods. In addition, the statistical analysis method of spearman rank correlation was employed for the intensive data analysis. The results showed that rock components and diagenetic processes have significantly different effects on the development of multi-lithologic pore systems in lacustrine shale formations. Organic-rich laminated shale (ORLS) and argillaceous siltstone (AS) show a bimodal, while the medium organic matter massive shale (MOMMS), fine sandstone A (FSA), and coarse sandstone B (CSB) exhibit unimodal pore size distribution. Organic matter negatively affects the pore volume and positively influences the pore throat, while clay minerals generally play a negative role except for ORLS. Quartz is conducive to the pore development of the reservoir except that it blocks some pore space in ORLS. Meanwhile, feldspar boosts the pore volume of all reservoirs and inhibits the pore throat of ORLS and MOMMS. Calcite and dolomite have negative effects on the entire pore system. Pyrite and siderite could facilitate pore throat preservation but inhibit pore space development. The diagenetic analysis shows that thermal maturation of organic matter and mineral dissolution play important roles in promoting pore development, while cementation and mechanical compaction have counteracting effects. The assemblage of ORLS and CSB acts as the most favorable reservoirs for lacustrine shale oil exploration under ideal conditions. The pore size distribution of the lacustrine shale oil reservoir with random multi-lithologies shows that macropores and mesopores are the main reservoir space. This study contributes to a better understanding of sweet spot prediction in the notoriously complicated lacustrine shale formation.

Automated summary

This study systematically analyzed the overall pore system of the Triassic Chang 7 lacustrine shale formation in the Ordos Basin, China, and identified the different effects of rock components and diagenetic processes on the development of multi-lithologic pore systems. The results showed the negative impact of organic matter on pore volume and the positive influence on pore throat, while clay minerals generally played a negative role except for in organic-rich laminated shale. Quartz and feldspar were found to contribute to pore development, while calcite and dolomite had negative effects. Pyrite and siderite facilitated pore throat preservation but inhibited pore space development. This study contributes to a better understanding of sweet spot prediction in lacustrine shale formations.