Diagenesis of Sandstone and Genesis of Compact Reservoirs in the EastII Part of Sulige Gas Field,Ordos Basin

Based on the experimental methods of slice identification,casting slice,scan electron microscope and X-ray diffractions,characteristics of petrology and diagenesis on reservoir in Shan1 section of Shanxi Formation and He 8 section of Shihezi Formation of Permian system in the east Ⅱ part of Sulige gas field were studied.The result shows that the main lithology of sandstone in this area is dominated by lithic sandstone and lithic silicarenite with low grade of maturity.Diagenesis of sandstone in this area mainly include compaction,cementation,corrosion and alteration.Diagenetic minerals mainly include siliceous mineral(chalcedony,quartz secondary overgrowth,and authigenic quartz small crystal),clay minerals(chlorite,illitesmectite formation,kaolinite,illite) and carbonate minerals(calcite,ferrocalcite,ankerite) etc.The highest diagenetic stage reach to period B of middle diagenetic stage on the basis of maturity of organic matter,diagentic minerals,homogenisation temperature of fluid inclusion etc.diagenetic symbols.Diagenesis of sandstone in the east Ⅱ part of Sulige gas field underwent syndiagenetic stage,early diagenetic stage and middle diagenetic stage.Diagenetic environment succession went through faintly acidic environment in period A of early diagenetic stage,relative highly acidic environment in period B of early diagenetic stage,highly acidic environment in period A of middle diagenetic stage,and faintly alkaline environment in period B of middle diagenetic stage. Fast decreasing of porosity could be seen in evolution curve of porosity caused by cementation from period B of early diagenetic stage to period A of middle diagenetic stage. The early diagenetic compaction is one of main factors to decreasing porosity of Shan1 section of Shanxi Formation and He 8 section of Shihezi Formation by means of quantitative calculation.Cementation is the most important factor to decreasing porosity in the studied area.Cementation in period B of middle diagenetic stage is the most important factor leading to poor physical property of sandstone reservoirs especially.The secondary pore formed by corrosion in acidity medium condition in period A of middle diagenetic stage can distinctly ameliorate poor reservoir capability of sandstone.Consequently,erosion play a key role in promoting of physical property of sandstone reservoirs in the east Ⅱ part of Sulige gas field.Secondary pore formed by corrosion of calc-alkali volcanics rock,feldspar and calcite cements in acidity medium condition in period A of middle diagenetic stage is main reserving space in the studied area.Corrosion of quartz particle was discovered in this area for the first time,which implies properties of fluid medium transferred gradually from acidity to alkaline since period B of middle diagenetic stage.