| This paper selected Chang4+5member, Triassic Yanchang Formation in the Ordos Basin as research target. According to a great deal of research on the regions’s geological data and absorbing the anterior research fruit of forefathers, on the basis of core observation description, core analysis test data and well log, combined with field profiles, drill and log data, ect, applying sedimentology, petroleum geology and geophysics theories, the sedimentary system, sedimentary microfacies, sand bodies morphology and its distribution, reservoir characteristics and the distribution of Chang4+52member Yanchang formation reservoir in Baiyushan area were studied systematiclly..Based on above, the oil reservoirs of this region were evaluated, and provided a base for the further exploitation.The research area was located in the middle-north of Shanbei slope structure belt in Ordos basin, which a was westward monocline and altitude was east higher than west. The change of structure was simple. It hadn’t a fault and large basement uplift. A series of low amplitude western-leaning nose-like uplifts slope developed from east to west, whose sizes were different. However, the up-downs forms and the tendency of slope dip direction of them reached an agreement. Source, reservoir, cap rock matched, and the area of direction dip was sheltered from nonpermeabile layer, which formed a good lithologic trap and was meaningful in the control of oil-gathering late. The purpose exploration target was the Chang4+5member, Triassic Yanchang Formation in the Ordos Basin, which was the main oil reservoir. In the later Triassic, the Ordos basin was an asymmetry downwarped basin with the east slow toward west steep. Major oil and gas resources of the Yanchang formation distributed in the east of Shanbei slope belt which was flat, shallow-water, wide-distribution area. The density of river which carried a large number of sediment was high than lake’s, then the river still had a high velocity along the flat bottom flowing forward far in the lake, where formed a broad delta front deposition, and research area was just located in this delta front edge area.According to the sedimentary cycle and grading control principles, through dividing and correlation the295wells of the Chang4+5reservoirs, from top to down, there were Chang4+51, Chang4+52, and Chang4+52was target beds(further layers: Chang4+521, Chang4+522, Chang4+523, Chang4+524). It did40sections correlation, including18east-wests,22south-norths. Stratum distribution had a certain of continuity, and deposition thickness overall by northeast to southwest had a trend of gradually thickening with a little change. Combined with regional geological settings, through analyzing electrofacies, and sedimentary facial, it got clearly characteristics of Chang4+52reservoir in Baiyushan area. Based on determination of delta front facies, it further divided into three microfacies including underwater distributary channel, river lateral margin, and interdistributary bay. And even it explained its space-time evolutionary regular pattern. The micro face of underwater distributary was the main reservoir of this area. The sand of Chang4+5distributed well and continually. Main oil layers was Chang4+52, then Chang4+522and Chang4+523were main layers, Chang4+524was the next, Chang4+521was out of oil. Thickness of sand body in Chang4+522was mainly between8-12m and surface exhibition was wide, multi-paragraphs and multi-layer distribution were apparent in vertical.By means of the casting thin sections and scanning electron microscope Chang4+52reservoirs in Baiyunshan area, detrital feldspar sandstone were majoriry, carbonate rock such as calcite, authigenicmineral such as kaolinite, hydromica, chlorite were the major chink. Textural maturity of Chang4+52reservoirs was mediuma. Intergranular pore and intragranular pore were mainly types; intercrystal pore and micro fracture were local. Pore structure types gave priority to the medium hole, and contained a number of big holes. Fine throat pores and micro-fine throat holes were major. The types of pore constriction were pore narrow, neck shrank, flake, curved flake and bundle, which among them neck shrank and the flake larynx way had the priority. According to the mercury injection material, based on pore constriction grading standards, Chang4+52reservoir was fine throat, otherwise has a certain amount of crude throat, however, the distribution was different. On the comprehensive analysis of reservoir, sedimentary microfacies and diagenesis were the main factors of controlling and affecting reservoir development and distribution.The main distribution of porosity and permeability of Chang4+52reservoirs were12%～12%,2.0～4.0×10-3μm2Chang4+52reservoirs was low permeability and low porosity. Variation of porosity and permeability was little and stable, which was related with thickness of sand distribution. Physical heterogeneity of the reservoir’s lithology was dominated by the sedimentary microfacies type of Chang4+52reservoir, and the degree of reservoir heterogeneity in vertical and in space was stronger by later diagenesis transformation. According to the evaluation standard, based on the variation coefficient, dash coefficient, difference of permeability, it could evaluate that heterogeneity of Chang4+52reservoir in study area was high.The core observation,the status of field development by racer test results show that fracturing direction of Chang4+5mainly extend along the East-West, At the same time it has the characteristics of diversity.The distribution of the hydrocarbon in the area was extremely malconformation in the plane direction. The lithologic characteristics, migration distance and local structure had a great effect on the distribution of the hydrocarbon. However, the mainly controlling factor was the sedimentary micro facies of the reservoir sand body. |
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