Reservoir Characteristics and Main Controlling Factors of Paleogene Sandy Conglomerate in Western Segment of Altun Piedmont, Qaidam Basin

The Paleogene strata in the western segment of the Qilian Mountains foreland basin in the Qaidam Basin developed a fan-delta-lobe sedimentary system. The conglomerate reservoirs exhibit high matrix content, strong cementation, generally poor physical properties, and pronounced heterogeneity. The dominant reservoir-controlling factors remain unclear, severely constraining current oil and gas exploration deployment and breakthroughs. Utilizing extensive analytical data from cores, thin sections, and scanning electron microscopy, a detailed study was conducted on the upper E32 sandstone-conglomerate reservoirs of the Lower Gandaigou Formation in the study area. Results indicate that the reservoir rock types are primarily feldspar sandstone, feldspar-clastic sandstone, and feldspar-clastic sandstone with low compositional-structural maturity. Reservoir spaces are primarily composed of primary intergranular pores, dissolution pores, and fractures. Sedimentary characteristics and diagenesis are the primary factors influencing reservoir properties. Key constraints on reservoir properties include: ① Rapid accumulation near the source led to “inherent insufficiency” of reservoir space; ② Carbonate-gypsum matrix cementation caused early reservoir compaction, while riverine freshwater influx played a crucial role in preserving favorable reservoir properties; ③ Differential compaction and tectonic stress-strain interactions promoted fracture development, contributing constructively to later reservoir modification; ④ A saline lake basin + alkaline diagenetic environment + low TOC resulted in insufficient “post-formational” dissolution porosity development. Guided by dominant factors favorable for reservoir development, combined with overlay analysis of sandstone distribution, reservoir properties, fault structures, and known reservoir types/distributions, this approach predicts and optimizes favorable hydrocarbon accumulation zones, providing scientific decision-making support for subsequent exploration.