Comparative Study On Accumulation And Enrichment Law Of Shale Gas And Tight Gas In Sichuan Basin

As unconventional natural gas,shale gas and tight sandstone gas have physical characteristics of low porosity and permeability in their reservoirs.Their hydrocarbon source characteristics,reservoir space characteristics,migration and preservation characteristics have many similarities,but there are also significant differences.The theory of shale gas and tight gas accumulation and enrichment still needs to be deepened.Shale gas and tight gas coexist in different formations in the same area of the Sichuan Basin.The spatial unified development of shale gas and tight sandstone gas also requires the formation of a comparison between shale gas and tight sandstone gas in terms of reservoir conditions,reservoir formation conditions,and accumulation patterns,in order to achieve unity in the exploration and development process.Based on existing exploration and development data,combined with previous research results,this study has conducted a general discussion on shale gas from the Wufeng Formation to Longmaxi Formation in southern Sichuan Basin and southeast Chongqing,and tight sandstone gas from the Shaximiao Formation in Qiulin area and its adjacent areas through research on sedimentary facies and reservoir distribution.The differences of source rocks and the control of source rocks on reservoir formation are compared by geochemical analysis.The differences in petrology characteristics,pore genesis,and control factors between shale and tight sandstone are compared by reservoir petrology analysis,thereby achieving reservoir physical property comparison.Through structural analysis,a comparison of migration and preservation was made,and the main modes of migration and preservation of shale gas and tight gas in the study area were identified.Based on the analysis of the occurrence forms,reservoir heterogeneity,and potential connections between shale gas and tight gas,the main controlling factors for the accumulation and enrichment of shale gas and tight gas are identified,and the accumulation and enrichment models of shale gas and tight gas in the study area are proposed.The following understanding has been achieved:(1)The deep-sea shelf microfacies control the large area spatial distribution of organic rich shale,and the underwater distributary channels control the spatial superimposed distribution of channel sand bodies;Shale and tight sandstone are characterized by low porosity and low permeability.The porosity of the tight sandstone in the Shaximiao Formation is generally between 1.0%and 16.0%,and the permeability is basically less than 0.1m D.The porosity of high-quality shale of Wufeng and Longmaxi Formation is generally between 0.7%and 9.2%,and the average permeability is less than0.1m D.(2)Both shale gas and tight sandstone gas have good hydrocarbon source conditions.The TOC of the marine shale of the Wufeng Formation and Longmaxi Formation is on average greater than 2.0%,and the thermal evolution(R_o)of the organic matter is basically greater than 2.0%.The gas generation ability is far superior to the shale of the Da’anzhai section and the shale of the Xujiahe Formation.The high-quality hydrocarbon generating capacity of the shale in the Wufeng Formation and Longmaxi Formation makes it capable of self reservoir formation,while the hydrocarbon generating capacity of the shale in the Da’anzhai section and Xujiahe Formation is insufficient.(3)The pore differences between tight sandstone and shale are mainly caused by genetic differences,and diagenesis has similar effects on their pores.The main difference lies in the role of organic matter in pores.The pore structure of tight sandstone is mainly affected by compaction and dissolution,with high homogeneity.Due to the abundant presence of organic pores in shale,its pore morphology is affected by various factors and has strong heterogeneity,which creates the most intuitive difference between shale and tight sandstone reservoirs.(4)There is secondary gas migration of shale gas mainly in the form of intra layer migration.The difference in fluid potential also provides a certain possibility for shale gas migration within the formation.The fractures formed by tectonic compression provide channels for the intra layer migration of shale gas.Tight sandstone gas migrates over long distances,and faults are the dominant channels connecting hydrocarbon source rocks and sand bodies.Structural uplift and formation denudation will destroy the preservation of shale gas and tight gas,and the negative effect of faults on shale gas reservoir formation is far greater than the positive effect.(5)Compared with tight sandstone gas,adsorbed gas has become a unique occurrence form of shale gas,which enrichment is jointly controlled by diagenesis,burial depth,organic matter content,and thermal evolution.Both shale and tight sandstone reservoirs have strong heterogeneity.The more complex mineral composition of shale,the difference in the content and evolution degree of organic matter,determine that shale has stronger microscopic and vertical heterogeneity.The distribution of tight sandstone is mainly controlled by sedimentary factors,and the physical properties of reservoirs in different sedimentary regions vary greatly,which also leads to stronger macroscopic and lateral heterogeneity.Shale and tight sandstone usually coexist,and the interlayer between sandstone and shale constitutes the form of mutual conversion between shale gas and tight gas.(6)The accumulation and enrichment of shale gas in the Wufeng Formation and Longmaxi Formation in southern Sichuan Basin are mainly controlled by preservation conditions.Structural styles,fault,uplift and denudation affect the preservation of shale gas.Luzhou area develops barrier type structure with wide gentle syncline type and broom shaped anticlinal type gas reservoirs,Weiyuan area develops updip pinchout single slope type gas reservoirs,Changning area develops slope type gas reservoirs,southeast Chongqing area develops anticlinal type gas reservoirs,residual syncline type gas reservoirs,and residual slope type gas reservoirs.The tight sandstone gas reservoirs from West Sichuan to Central Sichuan are mainly controlled by the spatial combination of"source-fault-sandstone",and are divided into two types of reservoir formation modes:non fault reservoir formation and fault reservoir formation.