Geological Study On Chang 4+5 And Chang 6 Reservoirs In Jiyuan Area Of Ordos Basin

The Chang 4+5 and Chang 6₁ reservoirs of Yanchang Formation in JiYuan area, Ordos Basin has beneficial of oil. And its structural traps have few been discovered, the sedimentary facies has been changed comparatively and vertically with strong reservoir heterogeneity. The distributary sand bodies in the submerged delta frontier are the most favorable face belt for reservoir development.The sand reservoir's characteristics of petrology, pores structure and physical property have been studied by means of many techniques, including of microscope, EMS, cathodophosphorescence, pressure mercury intrusion and thin section. It is showed that: the Chang 4+5 and Chang 6₁ reservoir rock types are mainly as clastic feldspar sand and feldspathic clast sand; its pore types are mainly intergranular pores and second dissolution pores; pore structures are mostly the type of middle class of discharge pressure and throat and the type of high class of discharge pressure and thin throat. As a whole, the reservoir in this area belongs to the types of low porosity and extremely lower permeability reservoir.The main factors affecting reservoir performance are sediment, structure, diagenesis and reservoir heterogeneity. The sediment controls the development and distribution of reservoir rock bodies. Structure changes the physical property by making the rock fractured and coming into cracks in this area. The main diagenesis in this area included the mechanical compaction, chemical compaction, cementation, metasomatosis and dissolution. According to the analysis of Ro of mudstone, the contacts of clastic grains, pore types, homogenization temperature of fluid inclusions, the development and the type of authigenic minerals, the diagenetic stage of the reservoir sandstone of Chang 4+5 and Chang 6₁ is between the late A period of the Terminal and the late B period of the Early. In study area, the reservoir has the strong heterogeneity; but in low permeability reservoir, the relative high permeability areas with strong heterogeneity are sometimes the enriched oil sites.The evaluation of pore space and reservoir types have been evaluated according to the evaluation of sand body characteristics, the contents of infills and carbonates, as well as diagenesis and physical property. There are three types of reservoir formation in this area. The type of I and II are the favorable ones and mainly distributing in MaJiashan-JiYuan and TieBiancheng areas of Chang 4+5₂ reservoir.