Sedimentary characteristics of Benxi Formation of the upper Carboniferous in Shenmu-Jiaxian area of the north-eastern Ordos Basin

The Sedimentary features of Benxi Formation of the upper Carboniferous in Shenmu-Jiaxian area and the distribution pattern of sedimentary microfacies and evolution of the Section One and Section Two were studied in depth based on the comprehensive analysis of the rock type, rock texture, sedimentary structure, rock electric and the thickness and the exhibition features of sandstone, mudstone, limestone, coal seam and ratio of sand and stratum of 1500 wells. The results show that the delta and barrier coastal sedimentation systems dominate the study area. The delta system is primarily composed of delta front subfacies, which are further divided into underwater diversion channel and diversion bay microfacies. The barrier coastal system is divided into three subfacies: barrier island, tidal flat, and lagoon, which are further subdivided into various microfacies, such as sand flats, mud flats, and coastal marshes. Lagoon-barrier islands are developed in the west of the study area in Section Two and Section One of the Benxi Formation and the barrier islands distribution are sporadic. In the eastern part of the study area, tidal flat deposits is the main Sedimentary subfacies in Section Two of the Benxi Formation, which include mud flat and sand flat mainly, and only a small scale delt front developed in the north of the study area. But in section one of the Benxi Formation, the tidal flat evolved into delta front which with a distinct north-south zoning features. The sedimentary facie is delta controlled by river in north of Shuangshan-Shuang 99 well, and the sedimentary facie is tidal modification delta in southwest of Shuangshan-Double 99 well line. Because of the high degree of tidal modification, the rock of modification delta in southwest are coarse granularity and higher content of rock debris and cement, therefore, the reservoir is more physical property and higher natural gas production because it had higher content of intergranular hole, debris dissolution hole and intercrystalline pore.