Study On Reservoir Characteristics And Hydrocarbon Accumulation Regularity Of Chang8Oil Bearing Beds In Zhuang36Well Area Of Xifeng Oilfield, Ordos Basin

Ordos Basin is a large Mesozoic inland depression one. It is rich oil-gas resources. Ordos basin goes across five provinces of Shanxi, Gansu, Ningxia, Niemen and Shanxi. It is the second largest depositional basin. Ordos basin has many kinds of characters, that is, oil-gas distribution wide, many formations with oil-gas, thickness, porosity and low permeability. Besides, there are many sorts of sedimentary system types in Ordos basin Mesozoic Yanchang formation, such as, alluvial fan sedimentary system, fluvial sedimentary system, lacustrine sedimentary system, fan-delta sedimentary system, braided-river delta sedimentary system and meandering-river delta sedimentary system. These sedimentary systems are good for source-reservoir-seal assemblage in Ordos basin Yanchang formation so that it became a gas enrichment area.Zhuang36well area located in the slope zone of Ordos Basin, Heshui city, Shanxi province. It is complicated topography, simple structure and fault undevelopment in this area with the study area of59km2. There are298wells about exploratory wells and producing wells in the study area. The Chang8formation in Yan chang formation of Jurassic is the key horizon with good development prospect in this study. It has the excellent characters of rich reserves, sufficient oil source and development of sand body.The reseach on the core data, logging data and physical data of Chang8formation, the paper carried out the thin layer division and contrast of298wells in the area, with the marker bed of tuff on the bottom of Chang7formation. It divided Chang8formation as Chang81and Chang82.Based on the core observation, according to lithology, physical property, and sedimentary structure, and the character of well logs, the sedimentary facies in research area is identified as braided river delta front subfacies, and four kinds of micro facies are recognized:distributary channel, mouth bar, channel flank, and interdistributary bay. The distributary channel sand body is main reservoir sands.The main sandstone is fine-grained arkose, it shows that tie diageness mainly include compaction, pressure solution, cementation, packing action and dissolution. It is concluded that the diagenetic evolution of Chang8sandstone has mainly been in the stage A of late diagenesis. The reservoir mainly develops intergranular pore and solution pores, the pore throat structure is the type of small pore and microthroat. The average porosity is10.14%, and the average permeability is0.91μmd, it belongs to low pore-especially low pore and low permeability-especially low permeability reservoir.By the analysis of the sedimentary facies and reservoir characteristics, it shows that deposition is the basic reason controlling of reservoir spreading, diagebaesis mainly determines the reservoir quality, the mechanical compaction and cementation which result in primary porosity loss almost are the main factor causing the lowest permeability in early diaagenetic phase. The dissolution of feldspar and debris that is dissolved improves the porosity and permeability. The reservoir heterogeneity is serious and it controls the scale and distribution of oil reservoir. Sedimentation and diagenesis have impacted on the the reservoir together, which leads to the physical properties of subsea distributary channel and mouth bar were better than that of channel frank.According to the physical properties and pore structure parameters of reservoir, the studying reservoirs can be classified into four types, The type I belongs to the best reservoirs, The type Ⅱ belongs to the good, the typeⅢbelongs to the bad.and the type Ⅳbelongs to the worst, and the type Ⅱ andⅢreservoir are the most widely distributed type in Zhuang36well area.Through the analysis of regularity of accumulation oil and gas, lithologic reservoir was the main reservoir type. The sedimentary environment and good physical property of thick sand bodies were the main factors of reservoir accumulation and forming. The sedimentary environment and reservoir heterogeneity had an obviously effect on water cut rising.