Lithofacies assemblages of shale and dense shell limestone: implications for shale oil exploration

Abstract

Assemblages of limestone and shale have been proven favorable shale oil-producing layers in North American and Chinese basins. However, the Jurassic Da’anzhai Formation in the Sichuan Basin did not meet these expectations. The relationship between the lithofacies assemblages and shale oil enrichment was investigated based on FE-SEM, XRD, N₂ adsorption, HPMI, NMR, and rock pyrolysis experiments. The results showed that ten main lithofacies developed in the Da’anzhai Formation. The undeveloped dissolution pores in the shell limestone led to clay intergranular pores and microfractures constituted the main oil reservoir space. With increased shale content, the physical properties and oil content gradually improved. These special geological conditions indicate that the breakthrough of shale oil in the Da’anzhai Formation depends only on the developed fracture system. A favorable exploration target was the layered lithofacies with developed veined calcite and bedding fractures. The results obtained in this study are significant for shale oil exploration of the same type of lithological assemblage worldwide.

Graphical abstract

Keywords

shale oil / lithofacies / pore structure / Da’anzhai Formation / Sichuan Basin

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