Classification and Control Factors of Reservoir Types in Hybrid Sedimentary Rocks: A Case Study on Lucaogou Formation in the Jimusaer Sag, Junggar Basin

Hybrid sedimentary rocks (HSR) represent a significantreservoirtype in fine-grained sediments. However, the classification and understandingof HSR reservoirs, including their storage mechanisms and identificationof optimal "sweet spots," have been limited due to thelack of clarity regarding the multiple sources of components and theirmixing processes. This study focuses on the Lucaogou formation ofJimusaer Sag and aims to highlight the reservoir classification principles,controlling factors, and evolutionary patterns associated with thecomponents of HSR, beginning with examining the microscopic pore structure.The analysis of the microscopic pore structure characteristics revealsthe presence of five distinct reservoir types within the HSR. Thequality of these reservoirs is governed by various factors, includingthe composition and support mode of particles, diagenesis, provenance,and sedimentary microfacies. In regions near a provenance with stronghydrodynamic conditions, the HSR predominantly exhibits type I andtype II reservoirs, characterized by numerous coarse-grained componentsand a granular-support mode. As the distance from the provenance increases,transitioning into medium hydrodynamic conditions, the HSR shiftsto an interbedded-support mode, primarily developing type III reservoirs.In areas far from the provenance with weak hydrodynamic conditions,HSR reservoir types primarily consist of type IV and type V. Additionally,diagenetic effects such as compaction and calcite cementation furtherdeteriorate intergranular and dissolution pores, consequently diminishingreservoir quality. Notably, during the mixing deposition processesof sand and dolomite, the developmental mode of HSR shifts from typeI to type II and type III. Likewise, in the mixing deposition of mudand sand, the HSR transitions from type II to type III and type IV.Similarly, the mixing deposition of dolomite and mud leads to a changein the developmental mode of HSR from type III to type IV and typeV. Moreover, this study effectively predicts the occurrence of "sweetspots" using reservoir classification, which reveals theircontinuous distribution. These findings provide a geological foundationfor evaluating "sweet spots" and testing the oil productionin HSR reservoirs.