Abstract: Previous understandings on types, evolution rules and formation mechanisms of the Yanchang Formation microfacies in delta estuary of the Ordos Basin are different. In this paper, a systematic study was conducted on microfacies of Chang 4+5 pay zone in Fanxue area of the Ordos Basin and its formation mechanisms based on core and well logging data. The study provides the following conclusions. First, the study area is dominated by distributary channel, mouth bar and interdistributary bay in terms of microfacies. Second, the sedimentary model for the Fuxue area with co-existence of rivers and bars is established according to the microfacies distribution in key layers, and thin blanket mouth bars and narrow zonal distributary channels were mainly developed during the early sedimentation stage of Chang 4+5 pay zone, while thick bulk mouth bars and wide zonal distributary channels during the late sedimentation stage. Third, a semi-quantitatively statistical analysis was carried out on the spatial forms of mouth bars using the data of dense well pattern. Fourth, it is concluded after analysis on main controlling factors for evolution of sedimentary microfacies that, due to the sustained falling of the sedimentary base level, the autogenetic cycle processes of distributary channels, such as bifurcation, convergence and lateral migration, were intensive to constrict the lateral growth of mouth bars, thereby resulting in a reciprocal relationship between mouth bars and distributary channels. Fifth, more attentions should be made to the mouth bar sand bodes which are predominant reservoir beds in the delta front.

Key words: Fanxue area, Chang 4+5 pay zone, mouth bar, distributary channel, allogenetic cycle, autogenetic cycle, Ordos Basin