Sweet spot evaluation and feasibility of cube development of continental shale oil in the Permian Lucaogou Formation, Jimusar Sag, Junggar Basin

Continental shale oil and gas have emerged as an important contributor to petroleum reserve growth and production addition in China. However, their efficient development of multiple reservoirs simultaneously in a three-dimensional space, that is, cube development, requires fine-scale geological sweet spot evaluation. The Lucaogou Formation in the Jimusar Sag, Junggar Basin represents one of China’s three major demonstration areas of shale oil production capacity construction. This study aims to ascertain the shale oil resource prospects and sweet spot distribution of the Lucaogou Formation to provide theoretical support for its future cube development. Based on a detailed analysis of the geological characteristics of the formation, we assess its “four key properties”, that is, oil-bearing property, oil mobility, reservoir property, and fracability. Accordingly, the concept of sweet spot index is proposed to quantitatively assess the resource quality of shale oil plays. Furthermore, we systematically identify the lithofacies and lithofacies assemblages of shales, quantitatively evaluate the resource potential of varying lithofacies assemblages, and determine the vertical distribution of sweet spot layers. The results indicate that 11 lithofacies can be identified in the Lucaogou Formation based on variations in mineral composition, organic matter abundance, and sedimentary structures. These lithofacies can be further classified into four lithofacies assemblages according to the source rock-reservoir configurations. The most favorable lithofacies for shale oil exploration is identified as the lamellar organic-rich argillaceous siltstone within two lithofacies assemblages: (1) an assemblage consisting of lamellar organic-rich mudstones and limy dolomites and lamellar organic-rich siltstones and (2) an assemblage composed of lamellar organic-rich mudstones and limy dolomites, lamellar organic-rich siltstones, and massive organic-rich to organic-lean siltstones and mudstones. Within the lithofacies assemblage of lamellar organic-rich mudstones and limy dolomites, parts of lamellar organic-rich felsic mudstones and lamellar organic-rich mixed mudstones exhibit considerable potential for shale oil exploration. Shale oil hosted in oil plays characterized by adjacent and integrated source rock-reservoir configurations represents a favorable exploration target in the Lucaogou Formation within the Jimusar Sag. This formation contains multiple sweet spot layers with significant resource potential. Overall, the study area holds promising resource prospects. For continental shale oil systems with multiple superimposed sweet spot layers, the cube development should be adopted based on a fine-scale geological sweet spot evaluation to reducing costs.