Upper Paleozoic Tight Sandstone Diagenesis and Its Relationship with Reservoir Formation in Linxing-Shenfu Area of Ordos Basin
The Upper Paleozoic tight sandstone gas reservoir in Linxing-Shenfu area of Ordos Basin has good exploration prospects.However,the tight and non-homogeneous characteristics of the Upper Paleozoic sandstone reservoirs in the Linxing-Shenfu area are se-verely limiting the exploration results under the strong diagenetic modifications.A systematic study of the diagenesis of the tight sand-stone reservoirs in the study area was carried out based on the cast body thin section and scanning electron microscopy.The microscopic pore structure of the tight sandstone in the study area was characterized by combining the core pore permeability test and high-pressure mercury pressure test.The theoretical lower limit,lower reservoir formation limit,lower effective percolation limit and lower buoyancy equilibrium method of the tight sandstone reservoirs in the study area were determined based on the water film thickness method,filling power method,test gas capacity method and buoyancy equilibrium method in combination with the gas production characteristics of the exploratory wells.The lower theoretical limit,lower reservoir formation limit,lower effective percolation limit and upper tightness limit of the tight sandstone reservoir in the study area were determined based on the water film thickness method,test gas production capacity method and buoyancy balance method.The results show that the tight sandstone reservoir in the study area is now in the middle-forma-tion A-B stage of evolution,and compaction and various types of cementation are the main control factors leading to the decline of reser-voir quality.Five diagenetic phases are classified according to the differences between diagenetic transformations of tight reservoirs:chlorite inclusions,dissolution,quartz cementation,mixed cementation and tight phases.According to the reservoir formation bounda-ry,there are conventional reservoir,Ⅰ level tight reservoir,Ⅱ level tight reservoir and Ⅲ level tight reservoir.Between conventional reservoir and Ⅰ level tight reservoir corresponds to the upper limit of tight,conventional reservoir is mainly dominated by chlorite in-clusions and soluble phase,Ⅰ level tight reservoir is dominated by soluble phase,followed by mixed cementation phase,between Ⅱ level tight reservoir and Ⅲ level tight reservoir corresponds to the lower limit of effective seepage,Ⅱ level tight reservoir is dominated by soluble phase and mixed cementation phase,Ⅲ level tight reservoir is dominated by mixed cementation phase and tight phase.Among them,conventional reservoirs and class Ⅰ tight reservoirs are the most promising exploration targets.
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