Abstract: In Tiaohu and Malang sags of Santanghu Basin, the sediment sources were mainly fallout volcanic ash and endogenous carbonates
during the deposition of second member of the Permian Lucaogou Formation (Lu 2 member). This unique sedimentary setting resulted in a highly complex formation mechanism of high-quality fine-grained reservoirs. Based on petrological and geochemical data, a systematic study has been conducted on sedimentary characteristics and diagenetic process of Lu 2 member fine-grained rocks. The study results indicate that the lithology had a minimal control over physical properties, pore structure, and fluid mobility of the fine-grained rocks, and high-quality reservoirs were formed after diagenesis of fine-grained rocks regardless of lithology. Except for dolomite, organic matter was generally rich in fine-grained rocks with other lithologies. Among them, cyanobacteria were predominant in tuff and dolomitic tuff, while green algae were developed in tuff, dolomitic tuff, and tuffaceous dolomite. The widespread organic matter underwent significant hydrocarbon generation during the middle diagenetic stage A, which not only altered the diagenetic environment, but also effectively and extensively dissolved soluble minerals in the fine-grained reservoirs. Compaction was the primary factor leading to the tightness of fine-grained rocks, while cementation had a relatively minor impact on reservoir quality. Dissolution was the key mechanism for forming high-quality tuff reservoirs, and multi-stage dolomitization significantly enhanced the storage capacity of dolomite reservoirs. Based on the intensity of dissolution and dolomitization, five types of diagenetic facies have been classified for Lu 2 member fine-grained rocks. Among them, the intense dissolution–weak dolomitization facies and the moderate dissolution–moderate dolomitization facies were the most favorable diagenetic facies. Using Matlab random forest method, diagenetic facies in a single well has been predicted, and the distribution area has been determined. The study achievements not only provide valuable insights into identifying favorable facies zones for shale oil exploration in Lu 2 member, but also contribute to understanding the formation mechanism of this unique type of high-quality fine-grained reservoirs.

Key words: diagenetic facies, fine-grained rocks, saline lake facies, organic-rich, Lucaogou Formation, Santanghu Basin