The Origin Of Authigenic Illite Of Xujiahe Sandstone In Xiao-xin-he Area, Western Sichuan Depression

The analyses of authigenic illite content, type and distribution regular of T3x2 Member and T3x4 Member in the Xiao(quan)-Xin(chang)-He(xingchang) Area of Western Sichuan Sag are based on X-ray Diffraction(XRD), Scanning electron microscope(SEM), Thin-section analysis and Carbon and oxygen isotope analyses. The analyses lay a foundation for discussing the formation mechanism and the influence to reservoir, forcasting and assessing the quality of the reservoir.Authigenic illite has average content 0.3% of sandstone reservoir in Xujiahe Formation, the content is higher in T3x4 Member than in T3x2 Member. It exist as network, membrane of granules, the authigenic illite filling in intergranular pore and the authigenic illite with the feature of the conversion of Kaolinite.The experiments prove that:①The system is relatively open from syngenetic period to the early stage of diagenesis, the acid fluids coming from T3x3 Member and T3x5 Member, which are coal measure stratas, drives solution of feldspar of underlying strata (T3x2 and T3x4) to accumulate Kaolinite; marine originated fluids provide extra K+ to T3x2 Member, and start illitization of Kaolinite by overcoming barriers of the reaction, at the same time, a portion of K-feldspar is preserved;②The system is relatively closed since early buried diagenesis and before the paleotemperature reached at 120℃～140℃, illitization of smectite turns to be a reaction closely associated with albitization of K-feldspar at the time, the process does not stop until smectite has completely consumed out.③The system approaches to be closed in deeply buried diagenesis with temperatures higher than 120℃～140℃, illitization of Kaolinite eases dynamics barriers of dissolution of K-feldspar through removing K+, the process does not stop until K-feldspar or K-feldspar has completely consumed out.Reticular authigenic illite and envelope illite have protection action for secondary pores forming from dissolution of feldspar; illitization of Kaolinite is propitious to dissolution of K-feldspar and the formation of secondary pores; the reaction of K-feldspar and Kaolinite generate illite is a reducing bulk course.