Micro-charging and accumulation of shale oil in lamellar shale interbedded with siltstones, Dongying Depression, Bohai Bay Basin

Abstract

The lithofacies of matrix-type shales are diverse. Interbedded siltstones and shales, a combination that is known for excellent reservoir properties, are primary targets in terrestrial shale. We investigated the generation of shale oil and the charging processes in the interbedded shale/siltstone succession of the upper fourth member of the Shahejie Formation (Dongying Depression, Bohai Bay Basin). The charging process is categorized into four stages: (A) initial stage, (B) charging stage, (C) maximum charging stage, and (D) late charging. During stage A, the production of shale oil is minimal, the mobility is limited, and the stage division curve remains consistent. During the B stage, there was a rapid increase in shale oil production, OSI, and filling rate, accompanied by significant changes in the slope of the curve. The production of shale oil, along with the OSI and filling rate of the C stage, achieved peak values, and the stage division curve exhibited stability. During stage D, there is a rapid decline in shale oil production, accompanied by a reduction in the filling amount. By using petroleum geochemistry, microscopy and micro-CT detection we analyze the evolution of the properties of the shale oil, mineral compositions, reservoir spaces and shale oil occurrence in these different stages. A combination of thermal evolution degree, shale oil properties and mineral composition controls the enrichment of shale oil. During the medium to high thermal evolution stages, the shale oil dominated by light to medium-weight oil components predominantly occurs in a free state within abundant dissolved pores, intergranular pores, inter-crystalline pores and microfractures; this is the most favorable stage for the enrichment of shale oil. On this basis we develop a model for shale oil enrichment in matrix-type shales that are interbedded with siltstone layers; this may be useful in the exploration of shale oil in similar lithofacies.