Characteristics And The Formation Mechanism Of High-quality Reservoir In Nanbeier Depression Of Tamuchage Basin

With the development of oil and gas exploration at home and abroad, the exploration target changes gradually from high permeability reservoir to low permeability reservoir. How to find relatively good quality reservoirs, the so-called desert, under the background of low porosity and permeability has become one of the focus that scholars have paid full attention to. In this paper, the author takes the Nanbeier depression of Tamuchage basin as an example and studys on the overall characteristics of the reservoir and the formation mechanism of high-quality reservoir.Based on suming up and sorting out the collected data, observation of routine thin sections , casting thin sections and SEM, image analysis and X-ray diffraction were adopted on the study.According to the collected data of thin section and the observations of routine thin sections, the rock types of Tongbomiao and Nantun formation in the northern part of east sub-depression of Nanbeier Depression include massive tuff and tuffaceous sandstone, a spot of sandstone and sedimentary tuff. From the Tongbomiao formation to the Nantun formation, the content of pyroclastic tephra increases gradurally and the rock types transforms from sandstone and tuffaceous sandstone to tuff.Observations from polarization microscope and scanning electron microscope show that the diagenetic types of reservoir consist of compaction, compaction and authigenesis, dissolution, devitrification, alteration and so on. Among the diagenesis above, devitrification and alteration exists mainly in the pyroclastic rock, espescially in the tuff and sedimentary tuff. The symbiosis sequence of diagenesis in this area is in following order: alteration, devitrification, compaction—chlorite cladding—overgrowth quartz—smectite, illite—microcrystalline quartz—siderite—dissolution, kaolinite, albite—ferrocalcite—ankerite. On the basis of vitrinite reflectance, the maximum pyrolysis temperature of rock, clay mineral assemblages and so on, the diagenetic phase is mainly in the A period of the middle diagenetic phase, and the remaining small part belong to the B period of the early diagenetic phase. As for the research of diagenetic facies, two types of classification scheme are adopted: firstly, the longitudinal direction classification scheme of single well includes porosity, the rock type and the type of diagenetic facies; secondly, the transverse classification scheme of each formation includes calcite cementation diagenetic facies, pore filling diagenetic facies, cladding diagenetic facies and dissolution diagenetic facies.Through the observation of routine thin sections and cast thin section and the porosity character analysis, the pore types of Tongbomiao and Nantun formation in the northern part of east sub-depression of Nanbeier Depression includes intergranular pore, the pore in interstitial material, fracture pore, dissolved intergranular pore, dissolved pore in interstitial material, dissolved fracture pore. Among these types of pore, various kinds of dissolved pore are abundant, especially dissolved pore in the tuffaceous interstitial material and dissolved intergranular pore. The level of pore structure in the study area is mainly in the middle level. The numbers of the poor level and the good level of pore structure are almost equal.According to plenty of measured data of rock core porosity, the reservoirs of Tongbomiao formation and the Nantun-Ⅱformation are low porosity and permeability reservoirs. However, the reservoir of Nantun-Ⅰis middle porosity and low permeability which are better than the other two reservoirs of Tongbomiao formation and the Nantun-Ⅱformation. In the longitudinal direction, there are five zones with the abnormal high pore, which are all belong to Nantun-Ⅰformation. On the plane, the distribution of porosity and permeability in the Tongbomiao formation seems like concentric ring. The reservoir physical properties becomes better from the core to the edge. The distribution of porosity and permeability in the Nantun-Ⅰformation is nearly banding. The reservoir physical properties becomes better from the northwest to the southeast.Through the observation of cast thin section and the porosity character analysis and examination of typical wells, the chief reason of the formation of high-quality reservoir is dissolution. Abundant tuffaceous material, feldspar and rock debris provide dissloved materials; the existence of chlorite in the pore and kaolinite in the early phase proves that there was some fluid in the pore; ; chlorite, illite and illite smectite mixed layer on the particle surface could inhibit the later compaction in some extent, and protect the primary pore; less carbonate cementation created favorable conditions for the development of high quality reservoir; the depositional environment of fan delta front is the main reason of dissolution,which is beneficial to the migration of acid fluid and the formation of chlorite; plenty of unconformity surfaces and faults was used as migration channels.