Evaluation Of Reservoir Section 8^th Of Upper Triassic Yanchang Formation In Huanxian Area, Ordos Basin

The reservoir in Ordos basin is a typical low porosity and permeability reservoir among China’s major reservoirs of the basin. The chang 8 section of Yanchang Formation in upper Triassic is the aim of this study. After years of exploration and studies on reservoir evaluation, it is confirmed that most of the chang 8 section’s depositional environment is fluvial facies and delta facies. Study area is the mixing zone of many provenances, the depositional environment is delta front, fan delta plain, fan delta front, and the whole physical property is poor.Sand bodies develop from west to east and northeast to southwest in study area. The plane distribution of porosity, permeability and shale content is same with the distribution of sand bodies. The physical property of reservoir in braided channel and subaqueous distributary channel’s sand bodies is good, the ones in interchannel is contrary. Reservoir property mainly affected by sedimentary microfacies, diagenesis can also affect it in great extent and the distance of provenance, grain size, compositional maturity, type and content of interstitial material can also affect it in some extent.The chang 8 section of Huanxian area mainly comprise lithic arkose and feldspathic litharenite. The grain gradation is moderate, psephicity is mainly subangular, and grains are point-lineal contact, part of them is point contact. Support types more to grain-supported, and the cementation types are mainly contaction and pores. The pore types are mainly intergranular pore and feldspar dissolution pore, followed by debris dissolution pore, intergranular dissolution pore and intercrystal pore and a small amount of fracture pore. There are many types of diagenesis including compaction, cementation, dissolution, metasomasis, etc., mainly in the A,B period of middle diagenetic stage.Heterogeneity of reservoir is same with the distribution of sedimentary facies, the strong heterogeneity distribute to the main channel and subaqueous distributary, the heterogeneity of interchannel and some of the edge of sand in channel is weak. The areas with strong heterogeneity often have good physical property in chang 81 and chang 82 section.The reservoir with high porosity and permeability is mainly controlled by sedimentarymicrofacies, diagenesis, tectonic stress and so on. The porosities of reservoir with high porosity and permeability mainly including retained primary intergranular pores and development of secondary pores. The development of chlorite is beneficial to retain the original porosity; secondary pores are related with the dissolution of organic and inorganic acid during the thermal evolution, fractures formed by tectonic stress, early hydrocarbon filling.In the paper we select some parameters which have a great impact on reservoir and also reference to other micro-factors to comprehensive evaluate reservoir by gray system theory, and the reservoir is divided into three types in the end. The class I and class II reservoir have better properties, most of them are locate in western and central parts of braided channels and subaqueous distributary channel, and class III reservoir is maily located in parts of interchannel and the edge of subaqueous distributary channel, poor reservoir properties.