The Oil Reservoir Main Controlling Factors Of The Lower Yanchang Formation In The Middle Of West Ordos Basin

In recent years, the lower Yanchang formation exploration in the middle of west Ordos basin is making a breakthrough and displaying preferable prospect. Many scholars studied the lower formation of Yanchang formation reservoir of the area tectonic units, stratigraphic framework, sand spread characteristic, but the less study of the distributive regularity and control factors of the lower formation of Yanchang formation reservoir and which restricts the exploration process. The results of these programs improve the understanding of Ordos Basin in Mesozoic accumulation theories, but also direct the exploration of the area.Utilizing the latest drilling data, the research analyses the main controlling factors the lower formation of Yanchang formation reservoir in the middle of west Ordos basin. The comprehensive research includes two aspects which were static elements controlling reservoir formed and dynamic elements which destructed and transformed reservoir. Static elements are formed by deposition phase, characteristics and source rock conditions. These dynamic elements are consist of diagenesis, hydrocarbon migration, accumulation reservoir, trap formation, and the reservoir destruction and transformation.Static factor analysis, the sequence stratigraphy method is used to divide Yanchang formation reservoir into sedimentary cycles, establish the lower Yanchang formation reservoir3D geological model. Used light and heavy mineral assemblages, trace elements, imaging logging paleocurrent direction to analysis provenance direction. The conclusion is that the provenance direction of in the middle of west Ordos basin subject to the control of the direction of the northwest and northeast provenance, and the northwest provenance is the mainly control. Combination the basin evolution and sedimentary facies characteristic comprehensive analysis that early Late Triassic lake basin base was in descending stage. The middle of west Ordos basin is closer to provenance, Chang9was developed braided river delta sedimentary system, and Chang8was developed shallow water delta. Analysis of the statistics shows that the lower Yanchang formation mainly sandstone is Lithic arkose, feldspathic lithic sandstone. The Grain Size of Chang9sandstone is coarse, the average porosity of the sandstone is11.27%, and the average permeability is3.8OmD. Chang8sandstone is fine sandstone, The dominating pore type of Chang8reservoirs is intergranular pore and feldspar dissolved pore, the average porosity is8.5%, and the average permeability is0.61mD.Dynamic factor analysis, the lower Yanchang formation sandstones are in the A period of the late diagenetic stage. This set of sandstone experienced compaction, cementation, metasomatism and dissolution diagenesis. Used the single factor analysis and multifactor comprehensive method to analysis diagenetic facies zone of the area. The result is that the feldspar dissolution diagenetic facies, chlorite intergranular pore diagenetic facies is a constructive diagenetic facies. Two different phases of hydrocarbon inclusions have been identified according to the microscopic characteristics of sandstone type U-deposit and homogeneous temperature distribution characteristics and referring to the tectonic evolution history of the Ordos Basin. It represents Late Jurassic and end of the Cretaceous tectonic uplift process hydrocarbon accumulation event. Based on the characteristics of tectonics evolution, the well drilling, log information and microscopic, high resolution boreholemicro resistivity image logs were used to investigate the flat distribution regulation and genesis of the fractures.The research shows that the E-W and NW-SE regional fractures are developping well. It is found that E-W fractures developed well where closed to Yishan Slope, and NW-SE fractures developed well neared Tianhuan depression or faults.The directions of the present tectonic stress fields in the basin were NEE-SWW which controlled the fractures expanded and permeability. Hydrocarbon source rocks of Chang7increased pressure from hydrocarbon generation expanding which was the dynamic force for the oil transverse and vertical migration.The oil migrated from higher pressure area to lower pressure area and accumulated in lower pressure area. Chemical characteristics of formation water were used to study the condition about the oil and gas migration and preservation of the lower Yanchang formation in the middle of west Ordos Basin. It is found that the formation water type is CaCb, the formation water salinity is high, ratio of Na+and Cl-and coefficient of desulphuration is low where develop shallow-water deltas in sandstones and closed to Yishan Slope. The formation water salinity is low, ratio of Na+and Cl-and coefficient of desulphuration is high where neared Tianhuan depression or faults. Conclusion The formation water distribution was influenced by Yanshan movements, closed geological environment is good for oil to be preserved where the formation water salinity is high, ratio of Na+and Cl-and coefficient of desulphuration is low.Analysing those static elements and dynamic elements of the controlling factors of the lower assemblage of Yanchang formation reservoir, four targets were selected in Chang8reservoir to provide the orientations of exploration.