Research On Lithology Characteristics And Reconstruction Of Petrogenetic Minerals Of Sandstone Reservoir Of The Xujiahe Formation In Yuanba Area Sichuan Basin

The exploitations have been proved successful for obtaining industrial natural gasin many wells incurrently in member2,3,4of Xujiahe Formation in Yuanba areaSichuan Basin so far, indicating a good prospect of exploration of Xujiahe Formationreservior. However, due to the difficulties in solving problems-about ultra-deep burial,low porosity and permeability, it is extremely challengable for exploration anddevelopment. Based on the collaborative project "the Diagenesis of the reservoir ofXujiahe Formation in Yuanba area research" sponsed by Sinopec exploration southerncompany., using the methods including thin section analysis, grain size analysis,technical methods cathodoluminescence, scanning electron microscopy etc., thisstudy investigated the lithology characteristics and reconstruction of petrogeneticminerals of major sandstone reservoirs in Member2,3,4of the Xujiahe Formation inYuanba Area.Sandstone reservoir of Xujiahe Formation in Yuanba area Sichuan Basinwas selected for the study in the paper, which combined with previous research, and theconclusion may provide an important reference for the distribution of relative qualityreservoirs with the low porosity and permeability characteristics. Several achievementshave been obtained as follows:The sandstone of Xujiahe Formation in Yuanba area are mainly lithic-richsandstone, feldspar lithic sandstone, and quartz lithic sandstone. Lithic-rich andfeldspar-poor is an important feature of mineral constitute of sandstone reservoirs inresearch area.In the major member of Xujiahe Formation, Member2has the highest feldsparcontent. There is the highest lithic content, the lowest feldspar grain content andcompositional maturity in Member3. Member3have almost the same quartz and lithicContent with that of Member2, but less feldspar content. Potassium feldspar is the most common feldspar type. Sedimentary rock lithics is dominant in lithics. Member2has higher magmatic and metamorphic lithic contents and Member3has highestcontent of carbonate rock lithics.Overall, quartz content increased and feldspar, lithic content decreased from westto east, showing the provenance and sedimentary facies influenced the mineralconstitution of sandstones. According to the contour map of the distribution of theminerals, there may be still a secondary provenance in addition to the northwesternancient Longmen Moutain and the northeastern Dabashan Mountain.In the interstitial material, content of the cement is significantly higher thancontent of the matrix. In the main reservoirs, cements are dominated by carbonateminerals, and authigenic quartz are also widely occured and a small amount ofauthigenic clay minerals. In Member3, the content of carbonate cements significantlyhigher than it of the other two members, may be associated with carbonate minerals"homegrown". In Member4content of siliceous cement is higher than it in Member3.In the chemical diagenesis, the cementation in Member3is stronger than that in theother members, hence most of the pores were filled. In Member2,4, dissolution aremore developed, and it may be related to acidic fluid generated by thin coal bedswithin the Formation. Thin section porosity of Member2is less than Member4, but theporosity is higher than that in Member4. The higher porosity of Member2must beassociated with the high permeability due to development of fractures.After a long and complex burial history, the content of petrogenetic minerals ofsandstone reservoirs have changed greatly. By using cathodoluminescence analysis ofthe thin sections, it is easy to identify the changes of content and analysises ofprovenances, and it helps to confirm the diagenetic process. In the researchclassification statistics of luminous colors of quartz, feldspar, carbonate rock lithics andother nimerals, morphology, content, and so was finished, and indicates the influence ofdifferent provenances at the different members on components of sedimentarypetrogenetic minerals, and at least by3%to12%feldspar minerals were dissolved byacidic fluid during the process of diagenesis, thus high compositional maturity ofsandstone and larger "intergranular pore volume" are not a native. Thecathodoluminescence of residual feldspar grains indicate that during the burialdiagenesis, dissolved feldspar is mainly potassic feldspar, most unstable feldspar grainswere dissolved in the early stages of burial. Compared with feldspar, dissolution ofcarbonate lithic fragment is relatively more difficult, mainly occurred during early diagenesis, and often associated with the dissolution due to atmospheric water ofunconformity or sequence boundary near an opened system (There may also beinvolved in other acidic mediums).As a result of the deep burial in Xujiahe Formation, compaction, pressure solution,cementation of authigenic carbonate and quartz are the main destructive diagenesises,and dissolution of feldspar and lithics is the most constructive diagenesis, Butdifferential compaction resulted in the generation of cracks, and to improve thephysical properties of reservoir pore permeability relationship is also a constructiveeffect. Compaction that reducing the porosity of the reservoir in Xujiahe Formation inthe research area is the most important physical diagenesis, cementation of carbonate isthe most important chemical diagenesis caused the decline of reservoir porosity inXujiahe Formation.