Coupling Relationship Between Sedimentary Facies And Shale Gas Related Parameters Of The Lower Silurian Longmaxi Formation In Sichuan Basin

In this paper,the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation in the Lower Paleozoic marine shale gas strata in Sichuan Basin and its peripheral areas where a breakthrough has been achieved in shale gas industry in China,is taken as the main research subject.Based on a large number of domestic and foreign research data,combined with theories of stratigraphy,paleontology,sedimentology,petrology,geochemistry,petroleum geology,geostatistics and unconventional petroleum geology,made full use of outcrop,drilling,logging and analytical data via microchip,scanning electron microscope,X-diffraction,argon particle polishing and log interpretation,the coupling relationship between sedimentary facies and parameters related shale gas has been studied.As a result,the following ideas and progress have been achieved:The Longmaxi Formation in Sichuan Basin and its peripheral areas is characterized by complete strata,easy lateral tracing and correlation,clear top and bottom boundaries and easy division.The Longmaxi Formation is in conformity with the underlying Wufeng Formation(Guanyinqiao Section)while in the denudation area it is in unconformity with the overlying Liangshan Formation of the Lower Permian,and in the non-denudation area it is also in conformity with the overlying Shiniulan Formation,Luoreping Formation and Xiaoheba Formation respectively.Longmaxi Formation can be divided into Long(one)and Long(two)Formation in turn from old to new while Long(one)Formation can be subdivided into Long1 and Long2 Sub-Formation.The Longl Sub-Formation is the main shale gas production layer in Sichuan basin at present.Thickness of Longl Sub-Formation is stable,ranging from 20 to 50 meters while the overall thickness of Long(one)and Long(two)Formation vary,but with obvious inheritable characteristics,presenting a trend of gradual thinning from the two sedimentary centers of Luzhou deep-water area and WK-C7 well area to Leshan-Longnusi paleo-uplift and central Guizhou-Xuefeng paleo-uplift area.A large set of mudstones and shales are deposited in the sedimentary center area while closer to the surrounding paleo-uplifts,the more silty and calcareous components are deposited.The study area of Longmaxi Formation was located in the shelf facies zone where the water circulation was restricted.The facies zone can be divided into two subfacies:deep-water shelf and shallow-water shelf.The deep-water shelf subfacies consists of three microfacies:siliceous shelf,siliceous mud shelf and silty mud shelf.The shallow-water shelf subfacies consists of three microfacies:shallow-water silty mud shelf,(argillaceous/calcareous)silty shelf and argillaceous lime shelf;longitudinally,there are two cycles from the rapid transgression to the slow regression during the deposition of the Longmaxi Formation in the study area,the Rhuddanian stage had the largest transgression,and the whole area is in a siliceous mud shelf,during the sedimentary period from the end of The Longl Sub-Formation 3-4 layer to the end of Long(one),the microfacies evolved into siliceous mud shelf,silty mud shelf and shallow water silty mud shelf,some areas include silt/calcareous mud shed,the second rapid transgression was in the early stage of Long(two),then the sea-level regressived slowly,the sedimentary facies evolution in the non-sedimentary center area was in turn shallow silty mud shelf,muddy/calcareous silt shelf and argillaceous lime shelf,the sedimentary facies evolution in the deep water area of Luzhou and the well area of WK-C7 in eastern Sichuan is in turn silty mud shelf and shallow silty mud shelf,some areas include deep flow microfacies;horizontally,subjected to the Leshan-Longnushi paleo-uplift and the central Guizhou-Xuefeng paleo-uplift and influenced by the enclosure structure of Kangdian paleo-continental shelf and by rise and fall of sea level during the Longmaxi stage,the Sichuan Basin as a whole presents a trumpet-like sedimentary pattern.Deep-water shelf environment was developed in deep-water areas far from provenance areas.The deepest water is siliceous mud shelf while in turn silty mud shelf,shallow-water silty mud shelf and mud/calcium shelf were evolved toward paleo-uplift and paleo-continent.Through the analysis of the close relationship between different microfacies and TOC,mineral components,gas-bearing property,pore and permeability conditions of Longmaxi Formation,it is shown that,except the siliceous shelf,the siliceous mud shelf,the most favorable sedimentary facies belts,has the highest TOC,siliceous component,gas-bearing property,porosity and permeability,the organic pore the most developed and the organic pore size the largest,silty mud shelf and shallow-water silty mud shelf the next.10 favorable exploration zones in Sichuan Basin are predicted concerning single factor of sedimentary facies belt,combined with other decisive conditions of favorable exploration area.