Mechanism and Controlling Factors of Sandy Conglomerate Reservoir in Piedmont Thrust Belt: A Case of Kunbei Oilfield, Qaidam Basin, NW China

The Kunbei thrust belt of Qaidam Basin is a large piedmont compression-torsion thrust belt. On the belt, the thick sandy conglomerate reservoir formed in alluvial fan is developed in the lower member of the lower Ganchaigou Formation in Qie12 block of Kunbei Oilfield. Understanding of the microcosmic characteristics and the main development controlling factors of this sandy conglomerate reservoir is significant to reservoir evaluation and the comprehensive reservoir management. In this study, the microcosmic characteristics of alluvial fan sandy conglomerate reservoir were systematically studied by using casting thin sections, XRD, SEM, conventional mercury injection and constant velocity mercury injection, and the main controlling factors of reservoir development were identified. The results show that the sandy conglomerate reservoirs in the area have the petrological characteristics of low compositional maturity and low structural maturity. The reservoir space type is characterized by dual porosity medium. There are 2 types of reservoir structure modes, 4 types of reservoir space types and 4 types of reservoir pore structures. The reservoir is of low porosity, ultra-low permeability and early diagenesis stage B. Under the same geological background of reservoir dynamic diagenesis, the influence of diagenesis on reservoir reconstruction is relatively balanced. The difference of reservoir development degree is mainly controlled by the difference of clastic composition and structure caused by the difference of sedimentation. The shale content is the main controlling factor of the reservoir development degree, and the upper limit of the shale content for reservoir is 8%, while the particle sorting plays an important role in controlling the reservoir properties. The reservoir classification standard based on shale content, physical property, pore structure and productivity index was established, and the sandy conglomerate reservoir was divided into four types. The research findings have guided the comprehensive reservoir management and achieved practical application results.