The Diagenesis Of Sandstones And Its Effects On Reservoir Quality From Tongbomiao Formation And Nantun Formation In The South Of Wuerxun Depression

The research area locates in the south of Wuerxun Depression in Halaer Basin, the data of exploration indicate that there appeared different degrees of oil-gas showing in this area. Tongbomiao Formation and Nantun Formation are the most important reservoirs, also are the formations of interest in this paper. The purpose of this investigation is to find out what factors influenced the higher porosity zones, by studying on the petrology, diagenesis character and physical properties of sandstones. Conclusions drawn in this paper will provide theoretical basis for predicting favorable reservoir, also will supply practical evidence for exploration and development in the future.On the basis of data which are collected from Daqing Oil-field Exploratory and Development Research Institute, the techniques applied in this study include ordinary thin-section and casting thin-section observation, SEM analysis, graphical analysis, etc.Through stating skeleton clastic constituents of 12 samples from one well, combining with 500 thin-section analysis data of 15 wells and keleton clastic constituents of 26 samples from 5 wells, it is considered that the rock types of Tongbomiao Formation are sandstone and tuff sandstone; the rock types of Member 1 of Nantun Formation are sandstone, tuff sandstone,sediment-tuff and normal volcaniclastic rock. And the sandstones include abundant detritus feldspar sandstones, minor feldspathic detritus sandstone and feldspathic sandstone; the rock types of Member 2 of Nantun Formation are sandstone, tuff sandstone,sediment-tuff and normal volcaniclastic rock, and the sandstones include abundant detritus feldspar sandstone feldspathic detritus sandstone and minor feldspathic sandstone. The diagenesis characteristics of sandstones from Tongbomiao Formation and Nantun Formation is investigated by ordinary thin-section, casting thin-section identification and SEM analysis, combining with the data collected. The types of diagenesis include compaction, cementation, replacement, dissoluteion. The paragenetic sequences of diagenesis are authigenic clay mineral coating, siderite cementation, dissolution-quartz secondary overgrowth-authigenic quartz microlite- kaolinite, replacement-illite-calcite cementation, dolomite cementation. The diagenetic facies can be devideed into early weak compaction-siderite filling-authigenic clay mineral coating diagenetic facie; strong compaction dissolution- kaolinite, quartz secondary overgrowth, authigenic quartz microlite filling pore diagenetic facie; dissolution-calcite, dolomite replacement and cementation. Acording to vitrinite reflectance, rock maxima pyrolysis temperature and authigenic clay mineral association, the diagenetic phase of sandstones from Tongbomiao Formation and Nantun Formation is mostly in middle diagenetic A phase, also sometimes is in early diagenetic A and B phase in Member 1 and 2 of Nantun Formation, sometimes is middle diagenetic B phase in Member 1 of Nantun Formation and Tongbomiao Formation. Considering geochemistry characteristics of present stratum water, authigenetic mineral sequences and the paragenetic sequences of sandstones, it is considered that the fluids in sandstones have went through such four phases as ealy acidity- alkality- acidity-alkality.The porosity and permeability of sandstone reservoir indicate that the sandstone of Tongbomiao Formation is extra low-low porosity, extra low permeability; the sandstone of Member 1 of Nantun Formation is low porosity, extra low-low permeability; the sandstone of Member 2 of Nantun Formation is low-medium porosity, extra low-low permeability. The pore types of Tongbomiao Formation and Nantun Formation include intergranular pore, filling inner pore, crack pore, solution pore by means of casting thin-section observation and SEM analysis. Through the data of mercury intrusion, it is determined that the pore structure of Tongbomiao Formation is medium (ⅡB), the pore structure of Member 1 of Nantun Formation is mostly medium (ⅡB), minor bad (ⅢA ) and (ⅢB ), the pore structure of Member 2 of Nantun Formation is medium (ⅡB). The porosity of Tongbomiao Formation and Nantun Formation is decreasing as depth increasing, four higher porosity zones are recognised at 1500m, 1850m-2050m, 2300m and 2600m-2800m. According to Sneider Plate, the original porosity of studying area is 33%. Regression analysis is proceeded on porosity and depth,the content of carbonate, equations of regression[φ=39.45*e-0.0006*h,φ=(30.142-0.203*CC-0.008*h)/100] are gained. According to the equations of regression, the porosity decreases about 8-9% as depth increasing 1000m, decreases about 2% as the content of carbonate increasing 10%. Using porosity,depth,the content of carbonate three parameters, the quality of reservoir of signal well is evaluated, and the results with precipitation facies and diagenesis facies are exhibiting on plane and longitudinal direction.On plane, the quality of reservoirs in Tongbomiao Formation is mostlyⅢ,these reservoirs mainly locate in lacustrine bottom fan, shore-shallow lacustrine, deep lacustrine-half deep lacustrine. The diagenetic facies include carbonate cementation, solution, silica cementation.ⅠandⅡreservoirs of Member 1 of Nantun Formation mostly locate in braided channel delta front, with the diagenetic facie of solution and silica cementation,Ⅲreservoir locates in deep lacustrine -half deep lacustrine.ⅠandⅡreservoirs of Member 2 of Nantun Formation mostly locate lake bottom fan, braided channel delta front, also locate deep lake-half deep lake. The diagenesis facies are characteristics of solution. In longitudinal direction,ⅠandⅡreservoirs mainly locate in Member 1 and 2 of Nantun Formation, the sedimentary facies are mostly fan-delta front,braided channel-delta front,lacustrine bottom fan. The diagenesis facies are characteristics of solution. Two types of reservoirs(ⅠandⅡ) has been found in Tongbomiao Formation.According to the atatistics of physical properties of reservoir from Tongbomiao Formation and Nantun Formation, the quality of reservoir in fan-delta front and braided channel-delta front is best, in lacustrine bottom fan is better, in shore-shallow lacustrine and deep lacustrine-half deep lacustrine is the worst. The sedimentary facies are the important reason to cause the four higher porosity zones to form, but not the only one. The diagenetic facies have relationship with higher porosity zones.The weaker of compaction and cementation,the existence of authigenic clay mineral coating and authigenic quartz microlite avail for conservation of primary porosity, and the solution leads to formation of secondary porosity due to the thermal evolution of organic mass,conversion of clay mineral and leaching of meteoric water. In addition, siderite is growing surrounding grains, which can inhibit further compaction, primary porosity can be preserved. Also under given condition, siderite can react with kaolinite and siderite itself can be decomposed, with CO2 generating. The forming carbonic acid during CO2 dissolving in water, affects on unstable minerals, the secondary porosity forms. Conservation of primary porosity and formation of secondary porosity can form higher porosity zones of sandstones from Tongbomiao Formation and Nantun Formation.