Origin and controls on shale oil enrichment of bedding-parallel fractures in the Chang 73 lacustrine shales, Yanchang Formation, Ordos Basin

Abstract

As the main fracture type of lacustrine shales, bedding-parallel fractures (BPFs) play a key controlling role in diagenetic mass transfer as well as hydrocarbon migration and accumulation. To reveal the origin of BPFs and their controls on shale oil enrichment, this study integrates core, thin section, scanning electron microscopy (SEM) observations, and fluid inclusion analysis, focusing on the BPFs of the lacustrine shales of the Chang 7₃ sub-member, Yanchang Formation, Ordos Basin. The results show that the variations in mineral composition, arrangement, and size on both sides of the laminae interface induce significant mechanical anisotropy, which become the essential geological prerequisite for the generation and extension of BPFs along the laminae interface. Furthermore, influenced by the mineral planar fabrics, BPFs can also develop within the laminae. Compared with laminated shale, massive mudstone does not show significant BPFs development characteristics due to the lack of lamina fabric. As a consequence of disequilibrium compaction, the vertical seepage force gradient caused by hydrocarbon generation overpressure periodically breaks through the pressure threshold of the closed shale system, resulting in the episodic opening of weak interfaces, thereby constituting the major formation mechanism of BPFs. In addition, the tectonic compression effect near the basin margin can also promote the development of BPFs to a certain extent. In different lithofacies of shales, the differences in the development characteristics and degree of BPFs are mainly caused by the variations in laminae attributes including laminae combination, linear density, and average thickness. High total organic carbon (TOC) content is conducive to increasing hydrocarbon generation overpressure. However, excessive TOC content reduces the compressive strength of rocks and has an adverse effect on the development of BPFs. The analysis of shale oil production indicates that, compared with non-bedding-parallel fractures (NBPFs), BPFs play a dominant role in the enrichment of shale oil and can significantly increase the daily oil production of individual wells. These findings provide a theoretical basis for revealing the origin, development pattern of BPFs and their influence on fluid activity in lacustrine shales.