Study On The Diagenesis-pore Evolution Difference And Mechanism Of Tight Sandstones In Coal Measures

Tight sandstones of coal measures in the second member of the Permian Shanxi Formation(herein referred to as Shan 2 sandstones)in the eastern Ordos Basin are one of the main gas-bearing intervals of the Upper Paleozoic gas reservoirs.However,these tight sandstones are characterized by diverse mineral compositions,complex microscopic pore-throat features,and significant changes in physical properties,which seriously restricts the reservoir quality evaluation and prediction.To reveal the diagenesis-pore evolution difference and mechanism of tight sandstone reservoirs deposited in similar sedimentary facies,we have carried out detailed research on petrological characteristics,microscopic pore-throat characteristics,and diagenesispore evolution characteristics of the Shan 2 sandstones.This study can provide a basis for reservoir quality evaluation and high-quality reservoir prediction.The following innovative points can be obtained through research:The three types of pore-throat configuration mode and physical properties of Shan 2 sandstones were revealed in this study.Sandstones were divided into three types according to the pore-throat configuration mode.Sandstones classified as Type ? were intergranular pore-dominated reservoirs,with multiple types of throats coexisting.The permeability of sandstones classified as Type ? was generally greater than 0.5mD,which were usually identified as gas layers.Sandstones classified as Type ? were intragranular pore-dominated reservoirs,accompanied by slice-shaped throats and cluster throats.The permeability of sandstones classified as Type ? was generally greater than 0.3mD,which were usually identified as gas-bearing layers or gas layers.Sandstones classified as Type ? were intercrystalline pore-dominated reservoirs,accompanied by cluster throats.The permeability of sandstones classified as Type ? was generally less than 0.3mD,which were usually identified as gas-bearing layers or dry layers.It was found that the pore-throat configuration mode of Shan 2 sandstones has a significant effect on pore structure and pore-throat size distribution.From Type-I to Type-? sandstones,the average pore-throat radii of sandstones decreased and the sorting of pore-throats tended to be poor.Also,the average throat radii of sandstones decreased significantly,and the throat radius distribution curves presented multimodality-low peak type,bimodality-low peak type,and unimodality-high peak type in turn.Furthermore,the pore-dominated stages of the total mercury intrusion curves became shorter while the throat-dominated stages became longer,indicating that throats' increasing control of sandstone seepage.The diagenesis-pore evolution difference and mechanism of Shan 2 sandstones were revealed in this study.The compaction caused by ductile grains was the major control factor of primary pore reduction for the sandstones.Due to the distribution of ductile grains content,the degree of compaction gradually increased from Type-I to Type-? sandstones.The fluid released by the adjacent mudstone was dominated by compaction-driven flow and rich in organic acid derived from the organic matter,which then migrated to the sandstones and dissolved silicate minerals.The primary intergranular pores acted as the main channels for fluid migration.Thus,due to the distribution characteristics of primary pores,dissolution mainly occurred in Type-I and Type-? sandstones,with intergranular dissolution and intragranular dissolution respectively.However,the dissolution products were difficult to migrate,resulting in a weak pore-increasing effect of Shan 2 sandstones.Cementation was the second factor causing the loss of pores in Shan 2 sandstones,showing strong differentiation.Clay mineral cementation was the main source of intercrystalline pores.Diagenesis-controlled pore evolution model of Shan 2 sandstones was established in this study.The original mineral components of Shan 2 sandstones(detrital quartz grains,ductile rock fragments,volcanic rock fragments,and clay matrix)showed strong heterogeneity.Under the specific diagenetic environment and fluid background in the study area,the original mineral composition significantly affected the early compaction and cementation degree of the sandstones,which determined the distribution of residual primary intergranular pores.Thus,it restricted the type and degree of later dissolutioncementation,ultimately affected the distribution of secondary dissolved pores and cement,forming the current pore-throat configuration mode.Provenance control and sedimentation transformation jointly determined the distribution characteristics of the original mineral composition of Shan 2 sandstones.According to this distribution characteristic,combined with the diagenesis-controlled pore evolution model of the sandstones,the distribution of various sandstone reservoirs can be predicted.There are 100 figures,28 tables,and 273 references in this dissertation.