| The 6th section of the upper Triassic Yanchang formation is the most important reservoir in the Ordos basin. Baibao area located in the western part of the Shanbei incline, Ordos basin. This paper focus on the upper Triassic Yanchang formation is a typical compact oil reservoir that has abundant structure cracks, which led to the oil reservoir heterogeneousness and the difficulty of reservoir evaluation. Previous studies indicate that this reservoir is characterized by: complex lithology-electricity correlation; protean physical property and unreadable log response and interpretation parameter. Then, through systemic analysis of the log response of the lithology, physical and particular the crack-developed reservoir, log interpretation of the oil-water distribution patterns, create a suitable evaluation criterion and explain model of the studied reservoir have great significance for the oil field exploration.Based on the detailed information of the basic geological character of the reservoir( sandstone distribution pattern, lithology, character of crack-filled and cementation material, lithogenesis, physical character and its change in plane), this paper focus on the evaluation of the role of cracks in reservoir, hole texture in oil-gas congregate process. We use data of well logging, log, drilling core, oil-test, thin-section observation and scanning electron photomicrography to evaluate the 6th section of Yangchang formation reservoir in the Baibao area and obtain following five conclusions:1. The 6th section of the Yangchang formation in the Baibao area mainly comprise gray feldspathic litharenite with brown and black clay minerals in pores. Compaction and pressure solution is the main process that controls the lithogenesis. The pores include primary, secondary pores and cracks with secondary pores are dominated(63%). Porosity is well correlated with the permeability, data of piezo-hydrargyrum indicate a complex pores textures and bad reservoir property. Oil test data indicate that the reservoir is low in porosity, permeability and yield.2. The data of imaging, nuclear magnetic resonance(NMR) and conventionality well log of the cracks and drilling core observation, reservoir property analysis and drilled log to indicate that cracks mainly occurred in north part of the Baibao area, and rarely developed in the southern part. Summarize the develop rule of the cracks and find the influencing factors. In the plane, the cracks-bearing stratum becomes younger from south to north and display a feature of diachronism. The cracks were restricted in the same lithology stratum from east to west and without diachronism. These features imply that the cracks are increasing from north to south, and mainly occurred in the thin-beded, compacted pelitic siltstone from the delta front.3. Use lithology cores, well logging, oil-test data to summarize the well logging response character of different lithology and cracks. The data suggest that beside some conventionality sandstone, there are also many radioactivity reservoirs with high GR. 216 data from 12 well from the 6th section display a good correlation between measured porosity and sound wave time difference. The different correlation in different section suggests the vertical difference in the 6th section of the Yanchang formation in the Baibao area.4. The reservoir in 6th section from Baibao area are fine-granied and compacted with different mineralized formation water, inhomogeneous pore textures and complex lithology-electricity correlation; protean physical property and unreadable log response and interpretation parameter. Then, based on the study of lithology, physical property, diagrams of (Δt×(USP/SSP)×(1-ΔGR)versus electric-resistivity (Rt) and sound wave time difference (AC)versus electric-resistivity (Rt) can clearly divide the field of oli, water and dry layer. Then, this type of diagram is suitable for the well logging interpretation of the 6th section of Yanchang formation in Baibao area.
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