Sedimentary characteristics and reservoir properties of the Shahejie Formation braided river delta in Raoyang Sag

Abstract：The Shahejie Formation in Raoyang Sag mainly comprises a set of terrigenous clastics such as conglomerate, conglomeratic sandstone, sandstone, siltstone and mudstone. A study of sedimentary characteristics led to the recognition of typical braided river delta deposition, whose characteristics are distinct and significantly different from other types of deltas resulting from river tractive current sedimentation which are generally controlled by torrential flood and seem to be seasonal sediments. The most notable feature of the braided river delta deposition in this area is the existence of a three-layer structure, which is composed from bottom to top of pre-braided river delta subfacies, braided river delta front subfacies and braided river delta plain subfacies, showing an obvious progradational sequence. There are several main microfacies, such as braided distributary channel, flood plain, subaqueous distributary channel, subaqueous inter-distributary channel, mouth bar and sheet bar. According to lithologic characteristics, sedimentary structure, grain size and sedimentary sequence of the braided river delta, a sedimentary model for the braided river delta was established. From lithologic characteristics of deposition, it is thought that the thickly-bedded conglomerate and conglomeratic sandstone developed in the braided river delta are widely distributed and constitute favorable oil and gas reservoirs. The sand bodies of the subaqueous distributary channel and mouth bar make up the most beneficial reservoirs and can form useful lithologic reservoirs. These places deserve much attention in future exploration.