Abstract: The second member of the Lucaogou Formation in the Tiaohu and Malang Sags of the Santanghu Basin was primarily sourced from fallout volcanic ash and endogenous carbonates during its depositional period. This unique setting has resulted in a highly complex formation mechanism for high-quality fine-grained rock reservoirs. Based on petrographic and geochemical data, a systematic study was conducted on the sedimentary characteristics and diagenetic processes of fine-grained rocks in the second member of the Lucaogou Formation.The results indicate that lithology has a minimal control over the petrophysical properties, pore structure, and fluid mobility of the finegrained rocks. Regardless of lithology, high-quality reservoirs can form during diagenesis. Except for dolomite, other fine-grained lithologies are generally rich in organic matter. Cyanobacteria predominantly develop in tuff and dolomitic tuff, while green algae are observed 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 within the fine-grained rock reservoirs. Compaction is identified as the primary factor leading to the densification of fine-grained rocks in the second member of the Lucaogou Formation, whereas cementation has a relatively minor impact on reservoir quality. Dissolution is the key mechanism for forming high-quality tuff reservoirs, while dolomitization and its multiple phases have significantly enhanced the storage capacity of dolomite reservoirs. Based on the intensity of dissolution and dolomitization, the diagenetic phases of fine-grained rocks in the second member of the Lucaogou Formation were classified into five types. Among them, the strongly dissolved-weakly dolomitized phase and the moderately dissolved-moderately dolomitized phase are the most favorable. Using the random forest method in matlab, single-well predictions of diagenetic phases were conducted to determine their distribution. The findings not only provide valuable insights into identifying favorable facies zones for shale oil exploration in the second member of the Lucaogou Formation but also contribute to understanding the formation mechanisms of high-quality fine-grained rock reservoirs under this unique sedimentary background.

Key words: diagenetic phases, fine-grained rocks, salinized lacustrine, organic-rich, Luocao Formation, Santanghu Basin