Sedimentary System Research Of Benxi Formation In East Ordos Basin

Exploration and development in recent years of the Erdos basin indicates that it is one of the important basins in china having the greatest potential in gas exploration. The Upper Paleozoic gas reservoirs in the eastern part of the basin (study area), however, is poorly understood. Based on outcrop observation, core observation and description plus geochemical analysis and identification, data statistics, univariative and comprehensive analysis of samples, we conducted detailed investigation on sedimentary facies analysis, spatial distribution, and evolutionary history of the sedimentology of the Benxi Formation in the eastern part of the Ordos Basin. This study reveals the horizontal distribution of sedimentary facies and sand body of the Benxi Formation from a macro perspective, which would offer an important theoretical basis for further exploration and development of the gas reservoirs in the study area.On the basis of marker beds, rock electric characteristics, and depositional cycles, gas reservoir-bearing strata in the study area are stratigraphically correlated. Considering the actual situation of geological stratification and development of the strata, the Benxi Formation is sub-divided into Ben1(an equivalent of the previous Jinci Member) and Ben2(equivalents of the previous Hutian+Pangou members) members, according to medium-to long-term cycles.A systematic summary of the signs of various types of sedimentary facies in the study area provides a strong basis for the identification and classification of sedimentary facies. Vertical evolution analysis at the representative section leads to the sedimentary sub-division of the Upper Paleozoic Carboniferous Benxi Formation as have deposited in two major depositional systems:barrier coast and continental shelf. These two depositional systems are further divided into seven sedimentary microfacies:the sand flat, mixed flat, mud flat, marl lime flat, lagoon mud, and muddy shelf. Sand barriers of the Benxi Formation lanes a north-south distribution, with the sand flat was spread in a NNW-SSE belt.Sandstone of the Benxi Formation is mainly composed of quartz sandstone, followed by lithic quartz sandstone. Pores are primarily intergranular+intergranular+dissolved types. The diagenetic history falls in an A-B phase, with some of them at the late stage. Sandstone generally is extra low porosity to low permeability reservoirs, and reservoir properties were controlled by the depositional and diagenetic processes. Barrier islands and sand flat quartz sandstone, occur commonly in the study area, generally have well reservoir properties.Reservoirs are divided into four categories. Classification and planar distribution of the gas reservoirs were investigated, on the basis of a comprehensive analysis of the reservoir porosity and permeability parameters in the study area. Reservoirs having higher exploration potential are predicted, based on the finds of sand body with high porosity and high permeability. This would be favorable for further exploration and development of gas reservoirs in the study area.