Research On Characteristics Of Upper Paleozoic Tight Sandstone Reservoirs In Bahannao Area Of Northern Ordos

Located in the transition zone between Yimeng uplift and northern Shanxi slope in northern Ordos, Bahannao area will be an important production area of Sulige gas field in the future due to its rich natural resources in upper Paleozoic reservoirs. In order to clarify the problems about poor physical properties, strong heterogeneity, low and anstable production of the tight sandstone from upper Carboniferous Benxi formation (C2b) to the eigth member of middle Permian Shihezi formation (P2h8), we have completed a comprehensive study including petrology characteristics, pore structue characteristics, diagenesis, physical properties and reservoir heterogeneity. This research has achieved a fine reservoir description from microscopic scale to macroscopic scale and summed up the causes of tight sandstone and good reservoirs. Finally it has completed a comprehensive classification and evaluation of the reservoirs.In the period of late Paleozoic, Bahannao area developed a large area of continental sedimentary with south epicontinental sea receded. Clastic sediments of meandering rivers and braided rivers filled in lower Permian Shanxi formation (P1s) and P2hs. These river sandbody which are mostly medium-coarse lithic sandstone and lithic quartz sandstone, are thick and connected, but poorer than the paralic sandstone from C2b and lower Permian Taiyuan formation (Pit) in sorting and psephicity.By microscopic observation, the levels of cements in sandstone are high, such as the clay minerals (most are illite and kaolinite) content of sandstone is between6.64%-12.55%. Clay minerals, siliceous cements and carbonate minerals filled most residual intergranular pores which are only preserved well in the third member of lower Pis and upper P2H8. The remaining layers mainly developed dissolve pores and intergranular pores which combined flaky throats and tubular throats as a large hole-tiny throat structure type. Constant speed mercury injection method proved throat radius which influenced permeability most is an important parameter to the tight sandstone reservoirs.After a long diagenetic evolution, most sandstone from C2b to P2h8in Bahannao area have entered middle diagenetic stage B, as a few are still in middle diagenetic stage A. Since the sandstone of the target layers suffered compaction, pressure solution, cementation and dissolution, reservoir properties have changed a lot. The average porosity of different layers except C2b is between5.38%--8.66%, and the average permeability is between (0.23-1.74)×10-3μm2. The above data show the sandstone own typical low porosity, low permeability and moderate to strong heterogeneity characteristics. According to rock particles, cements, pore structure, diagenesis and other features, reservoirs of Bahannao area are divided into5diagenetic facies. Quartz overgrowth and intergranular pore facies as well as kaolinite intergranular pore and dissolution facies own better physical condition while porosity and permeability conditions from illite cementation account microporous facies to carbonate minerals cementation facies and compaction and pressure dissolution dense facies turn worse.After multi-aspect analysis, sedimentation, diagenesis and tectonic are the main reasons for upper Paleozoic reservoirs densification. Quartz content, particle size and development of good sedimentary microfacies and diagenetic determine whether reservoirs are good. Referring to various types of research results and experimental parameters, the tight sandstone reservoirs of upper Paleozoic in Bahannao area are divided into four categories, of which Ⅰ, Ⅱ class reservoirs mainly developed in the third member of lower P1s and the second member of lower P2h8as well as the second member of upper P1s while III, IV class distributed vastly in the remaining layers so as that the reservoirs quality is poor.