The Reservoir Characteristics Of Chang8Formation And Its Impact On Oil Production Capacity In Heshui Area Of The Southwestern Ordos Basin

The characteristics of low porosity and permeability reservoirs and its distribution of oil and gas gather was the concerned difficult issues of petroleum geology research, it is also the hotspot of oil and gas exploration in upper Triassic in Ordos Basin recently. This paper takes the low porosity and permeability reservoir of Chang8formation in HeShui area, southwestern Ordos Basin as the research object. According to geological drilling, logging and production test data, on the basis of the formation fine classfying and sedimentary microfacies studies, using scanning electron microscopy, cast thin section, high pressure mercury, petrophysical and NMR rock samples test of Chang8main producing layers, comprehensive reveals the characteristics of the reservoir rock, such as main pore types, reservoir physical properties and heterogeneities. Then analyzed the oil and gas distribution in main producing layer and producing capability, made comprehensive discussion on the producing capacity and geological characteristics of main production layer, and their affective factors, in order to provide an important foundation for increasing reserves, exploring potential and developing efficiently. Mainly got following main outcomes:(1) Using the analysis methods of sedimentary facies restricting logging facies explanation, studied the sedimentary microfacies types and its distribution, and the sand thickness distribution that controlled by facies. The results shows that Chang8main producing layers mainly developmented two sedimentary microfacies of underwater distributary channel and shunt bay of braided river delta front. The underwater distributary channels shows north-south trending and have good connectivity in plane. By making the map of top interface structure of Chang811and Chang812layer, revealed the overall gently west-dipping structure face of Chang8main producing layer. The average gradient was (8-10) m/km and the dip about0.5°～1°.(2) The debris rock and pore structure study show that Chang8main producing layers detrital framework grain composition mainly was feldspar (31.4%), quartz (29.2%), and lithic (21.7%), interstitial material mainly ankerite (4.9%),water mica (2.4%),chlorite (2.3%) and calcite(2.1%). Lithology is mainly dark gray and gray-green medium-small particles lithic feldspathic sandstone and feldspathic sandstone, fine sorting, subangular roundness, the cementation is mainly increase-porosity, pore, the film-pore.(3) The study of cast thin section shows that diagenesis is mainly mechanical compaction, pressure solution, cementation, metasomatism and erosion, in A period of medium diagenetie stage. There are seven diagenetie facies in the area and chlorite film-feldspar dissolution facies was the best diagenetie facies to reservoir properties in study area.(4) Using experience statistics and mercury intrusion experiment, identified porosity6%, permeability0.1×10-3μm2as the limit of reservoir properties of Chang8main producing layers. Reservoir shows medium-strong heterogeneity. Reservoir comprehensive evaluation results show that Chang811layer is mainly type Ⅲ reservoir wile Chang812layer is mainly type Ⅱ reservoir, and the reservoir properties of Chang822layer is better than that of Chang811layer.(5) The result of oil reservoir distribution, oil production capacity and its controlled factors of Chang8main producing layers shows that oil reservoir distribution and high productivity relatively block more than4t/d was mainly controlled by diagenetie facies types, reservoir properties and heterogeneity, that controlled by main sandstone belt. Chlorite film-feldspar dissolution facies was the best diagenetie facies for Chang8main producing layers in study area. As to conventional reservoirs that permeability above1×10-3μm, oil production capacity and permeability shows negative correlation, but, it shows positive correlation in unconventional reservoirs.