Control factors for the high-permeability reservoirs of Chang 101 Reservoir Group of Yanchang Formation in northern Shaanxi Province, Ordos basin

Northern Shaanxi area in central Ordos basin is one of the richest Mesozoic hydrocarbon accumulation zones, where the previous studies of Mesozoic oil-bearing series focused on Chang 6 and Jurassic Yan'an Formation. The Chang 101 reservoir group is a new layer explored and evaluated in the past few years. In this paper, sedimentary characteristics and diagenesis of Chang 101 reservoir group were studied and control factors for the high-permeability reservoirs were analyzed by using various modern analytic techniques and mathematical statistics. The research results indicate that the distribution of the high-permeability reservoirs was controlled by the sedimentary environment, and the formation of high-permeability reservoirs was mainly affected by diagenesis. The sand body of the main channel is characterized by great thickness, coarse grain size, large porethroat and good connectivity between the pores, whereas the distributary channel has thin thickness, fine grain size, small porethroat and poor connectivity between the pores. Therefore, there existed more intergranular pores in the main channel than in the distributary channel after compaction, which led to more dissolution and less cementation in the main channel than in the distributary channel; as a result, the reservoir property of the main channel is much better than that of the distributary channel. The Chang 101 high-permeability reservoir was developed in the main channel and the interface of interdistributary rivers which were influenced by sedimentation and diagenesis.