| Hujianshan Oilfield is located in Yishan Slope which lies in the central west part of Ordos Basin. As a large-scale continental sedimentary basin, Ordos Basin is250,000km2. This paper took an area of695km2in Dingbian County of Shaanxi Province as its research objective. The terrain in this research area is rather complex, while the geological structure is relatively simple. Faults ceased to develop in this area.Hujianshan is among the most important oil-gas producing areas in Ordos Basin. Chang6formation of Triassic system is the major oil-bearing series in western Ordos Basin. Under the guidance of theories in relevant subjects, such as Continental Sedimentary Petrology, Sequence Stratigraphy, Petroleum Geology, Logging Geology and Reservoir Geochemistry, this paper analyzed the geological characteristics of Chang6formation based on the geologic settings and geologic characteristics of reservoirs in Hujianshan area, laying a solid foundation both for future exploration and for the study of sedimentary system, microfacies and sand body distribution in the area. Though over100diverse wells were covered in the research, such as exploratory wells and appraisal wells, this paper took Chang6formation as its research objective, since it is the major productive strata in the area.Based on coring data, logging data and physical property data, this paper divided Chang6formation into3layers, namely Chang61, Chang62and Chang63, among which Chang61was further divided into three sand layers, namely Chang611, Chang612and Chang613. Taking marker bed K4as the top layer, this paper set up9standard sectional drawings for comparison. No major change of sand body thickness was detected in Chang6formation.Results show that:the rocks in Hujianshan Oilfield are mainly light-colored. Besides sand shale, a small amount of conglomerate and coal seam can also be detected. The sandstones in Chang6formation are mainly micro-fine calcium-mud arkoses of dark grey color, lithic arkoses, litharenite, alternating layers of peliticsiltstone of dark grey color and grey color, siltstone, and argillite of ash black color. The beddings of sandstones developed in varied ways, among which cross bedding, horizontal bedding, parallel bedding, bed scour and fossils of plant stems and leaves took the majority. Through a thorough research to the porosity, thickness, permeability of sandstones and distributional characteristics of microfacies, it is concluded that:Chang6formation mainly belongs to sedimentary system of Meandering River Delta and lake sedimentary system. The research area is situated in the delta front, where microfacies of underwater distributary channel, mouth bar, distributary interchannel were developed.The research area is basically monoclinic in structure, with nose-shaped microstructures developed in some parts. The sand body goes from northeast to southwest, bearing similar distributive characteristics as microfacies. Different parts of the sand body join closely, mainly in a banded shape. Since the structure of the sand body is relatively simple, the dominating factor of the reservoir property is the types of microfacies.Results indicated that:the sandstones of the reservoirs are mainly composed of lithic arkoses and feldspathic litharenite. The major interstitial matters are kaolinite, hydromica, chlorite, calcite and ferrocalcite, with a small amount of siliceous matters mixed in. Three types of pores can be found in the reservoirs, which are intergranular pores, intergrain pores and intragranular pores, mainly in large-medium structure with micro-throat. Generally, Chang6is a low-porosity formation with low and extremely low percolation. Rock formation in this area is mainly influenced by compaction, cementation and dissolution. The dominating factors of the physical properties of the reservoirs are microfacies and diagenesis.Through research to the petrologic features and physical properties of the research area, it is concluded that the controlling factors of reservoir development in Chang6formation are deposition and diagenesis, the former being the primary one. The underwater distributary channel and the river mouth bar in delta front are the potential developing areas of sedimentary facies. Based on analysis of relevant parameters, such as porosity, permeability, oil saturation, microfacies and pore structure, combined with study of the actual situations of the production units, this paper proposed a set of criteria for the classification of reservoirs in this area, and evaluated the reservoirs according to the criteria proposed. The paper concluded that Chang6formation is mainly composed of reservoirs of Type Ⅰ and Type Ⅱ; comparatively, reservoirs of Type Ⅲ can be rarely found. Basically, reservoirs of Type Ⅰ and Type Ⅱ are distributed in the central part of underwater distributary channel in the river delta, and are of the best physical properties. Reservoirs of Type Ⅲ are distributed on both sides of the river channel and the dense areas inside the river channel. Generally speaking, potential areas are distributed in the central section, the southwest part and the northwest part of this region.Through a comprehensive analysis to the research area, this thesis predicted the potential areas for oil and gas accumulation in Hujianshan Oildfield. The prediction was based on a variety of data in the area, including sedimentary facies belt,sandbody distribution, reservoir characteristics, interpretation of logging curves, oil and gas shows, actual oil testing data and production status, etc., among which the former is the most important one. The sandbody in the underwater distributary channel of Chang6formation is the advantageous area of reservior development. The development of sandbody is the major basis in the prediction of potential areas. Different areas of sandbody in the reservoirs of this research area linked closely together in the surface, and the linkage penetrated to the earth, which formed transportation channels and accumulative place for oil and gas. In some cases, uplifts are the suggestive areas for the transportation and accumulation of oil and gas. The uplifts formed in the research area, such as nose-shaped upheavals, are the suggestive areas. Besides, the current situation of exploration is a vital basis for the prediction of potential areas. Some other data, such as oil testing data, oil extracting data and oil analysis data, are also considered for scientific prediction. |
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