川西坳陷大邑构造上三叠统须家河组三段超致密砂岩储层成因机理OA

To clarify the genetic mechanisms of the ultra-tight sandstone reservoirs in the 3rd member of the Upper Triassic Xujiahe Formation(Xu 3 Member)in the Dayi structure,Western Sichuan Depression,systematically characterized the reservoir petrology,petrophysical properties,reservoir-space types,and diagenetic characteristics,and investigated the main factors controlling reservoir densification.The study integrated multiscale analytical methods,including core observations,cast thin-section examination,scanning electron microscopy,cathodolumine-scence analysis,X-ray diffraction,petrophysical testing,fluid-inclusion microthermometry,carbon and oxygen isotope analysis,and formation-water hydrochemistry.The results showed that the sandstone reservoirs of the Xu 3 Member in the Dayi structure were dominated by feldspathic litharenite and litharenite and had low compositional and textural maturity.Primary intergranular pores had nearly disappeared as a result of intense compaction and cementation.Secondary reservoir spaces consisted mainly of intragranular dissolution pores in feldspars and lithic fragments,together with tectonic microfractures.Porosity values were concentrated below 4%,and permeability was generally lower than 0.5×10-3 μm2,indicating strongly heterogeneous ultra-tight reservoirs with ultra-low porosity and ultra-low permeability.The diagenetic sequence showed that the reservoirs underwent intense com-paction,multiple episodes of cementation,and dual dissolution.Early rapid deep burial caused deformation of ductile lithic fragments and fracturing of rigid grains.The compaction rate ranged from 27.08%to 90.36%,with an average as high as 76.22%.This process eliminated most of the primary pores and constituted the primary fac-tor in reservoir densification.Subsequently,the diagenetic environment became highly alkaline and was charac-terized by closed,high-salinity fluids.Strong alkaline diagenesis caused cements to occlude the pores.Within this closed system,feldspar dissolution products could not be effectively removed and instead precipitated nearby and were retained in situ,further occluding pore throats and intensifying the heterogeneous densification of the reservoirs.Organic-acid dissolution and late-stage fracture development improved reservoir properties and formed a composite network of dissolution-enlarged pores and fractures.These processes locally improved reservoir proper-ties and were key to the development of the relatively high-quality reservoirs present today.In summary,intense early compaction and the in situ retention of dissolution products within the closed system are the key mechanisms of reservoir densification,whereas the superimposed effects of late-stage acidic dissolution and fracturing are important factors in improving reservoir properties.