Sedimentary facies of sub oil-bearing formation of Chang 8₁ and their influence on physical properties of reservoir, Heshui, Ordos Basin, NW China

Basing on the data of drilling cores, logging records and testing of physical properties of reservoir, this paper discusses the relationship between sedimentary microfacies and reservoir physical properties of the sub oil-formation of Chang 8₁of the Upper Triassic Yanchang Formation in Ordos Basin.Four types of microfacies, including underwater distributary channel, mouth bar, frontal sheet sand and inter-distributary bay, occur in the sub oil-formationof Chang 8₁, which is with sedimentary facies of shallow delta front. The multi-stage superimposed underwater distributary channel is the main type of microfacies controlling the development of sand bodies. According to its thickness ratios of sandstone and mudstone, it can be divided into the core, the middle and the flank of the channel.The subaqueous distributary channels occur frequently in small layers of Chang 8₁², whose sedimentary periods are more often than Chang 8₁¹ and Chang 8₁³,and its connectivity is better.The structural maturity, which is determined by sedimentary microfacies, controls the initial porosity of the reservoir and then affects the current porosity of the reservoir to a certain extent. The reservoir in the core of underwater distributary channel is with the best physical properties, and it is the favorable microfacies type. It is suggested that the small layers of Chang 8₁², is the potential target layers for future exploration in studied areas.