Analysis of the main controlling factors and the distribution rules of effective reservoirs in sandy braided river delta plain — A case study of Ct3 area, Ordos Basin, China

Abstract

To clarify the main controlling factors of effective reservoirs in the sandy braided river delta plain, the eighth member of Xiashihezi Formation in the Ct3 area, Ordos Basin, is taken as an example. We describe and analyze approximately 600 cores in detail and correspond the cores on the logging curve. We summarize the logging facies mode suitable for the Ct3 area, and finally investigate the types and spatial distribution characteristics of sedimentary microfacies. The P2h8 member of Ct3 area in Ordos Basin is braided river delta plain subfacies. Sedimentary microfacies are divided into three types: distributary channel, channel bar, and distributary interchannel. The sand body in the h84 of the P2h8 member is the thickest, and the number of channel bar is the largest. All gas reservoirs (effective reservoirs) are developed on the sand body of the channel bar, which is the most favorable microfacies type. Poor gas reservoirs are mostly developed in distributary river channel sand body, whereas gas-bearing reservoirs (gas reservoirs and poor gas reservoirs) are undeveloped in distributary interchannel. According to the hydrodynamic conditions and sedimentary mechanism, the channel bar is divided into the bar head, the bar tail, the bar main body, and the bar wing. It is found that 63% of the gas reservoirs in the channel bar are developed in the bar main body, followed by the bar tail. The number of gas reservoirs developed at the bar head and the bar wing is the least. The results found that the most effective reservoirs are developed in the bar main body. Sedimentary microfacies, sand-body thickness, and grain size are the main controlling factors for effective reservoirs. The research results can guide the gas field exploration and later well location deployment in the sedimentary environment similar to the Ct3 area.