The Mechanism Of Feldspar Dissolution And Conservation In Upper Triassic Xujiahe Formation In Xinchang Of Western Sichuan Depression

XinChang is one of the most important gas industrial bases, which has wide explorationprospect. The ways of casting microsection, cathodoluminescence, XRD, scanning electronmicroscope and C&O isotope analysis are used to research the mechanisms which includepetrological characteristics, types of authigenic mineral and porosity, feldspar dissolution andconservation.The most rock types include sublitharenite, litharenite and feldspathic litharenite, whichpercentage composition is 53.54%, 24.24% and 6.40%, respectively. The content of feldspar inXu2(which average value is 10.4%) is greater than that in Xu4. The average value of authigenicmineral is 7.58% in Xujiahe formation, which has low to moderate level of authigenic mineralcontent. Carbonate is the most main authigenic minerals, which percentage composition is80.08%. Other authigenic minerals include quartz, kaolinite, chlorite and siderite and so on.Kaolinite is only in Xu4, but the average value of chlorite in Xu2 is higher than that in Xu4.Secondary porosity is the most type of porosity.Based on the simulation of feldspar dissolution kinetics,it is showed that: (1) it is foundpositive correlation relationship between Si:Al rations in aqueous solution and feldspardissolution rates. The relationship between Si:Al rations and K-feldspar dissolution rates iscloser than that of albite. (2) It is found negative correlation relationship between pH value andfeldspar dissolution rates. The greater the pH value has, the lower the feldspar dissolves. Thehigher the temperature is, the stronger that pH value effects to the feldspar dissolution rate. (3)The higher the temperature is, the faster the feldspar dissolves. (4) All Gibbs free energy ofK-feldspar is larger than albite, it is showed that the trend of albite dissolution is stronger thanK-feldspar. Secondary mineral precipitation kinetics controls feldspar dissolution rate. The fluids of feldspar dissolution are atmospheric water and acid water in coal bed. Thetime of strongest dissolution is the end of Xu4. There are many geology evidences: Kaolinite isonly in the top of Xu4 in Xinchang, where the content of feldspar, plagioclase and orthoclase arereduced remarkablely. Cathodoluminescence images show that the reduction of feldspar isascribe to dissolution, which occurred in Early diagenesis before effective compaction occurred.Secondary porosity is distributed mostly in the top of Xu4, the porosity which feldspar isdissolved is the most important, and according with kaolinite developed zones and feldsparreduction zones. The isotope geochemistry characteristic of carbonate shows that organic carbontake part in the process of carbonate cements precipitation, which source is relate to feldspardissolution.After the temperature is greater than 120℃, K-feldspar dissolution occurs in Xujiaheformation, the reaction is: K-feldspar + kaolinite = illite + SiO₂+ H₂O, which have no use oforganic acid in mudstone. H+ comes from other reaction: kaolinite+K+ = illite + H++ 3H₂O, thereaction turn the acidophil clay mineral to basophile. Therefore, H+ is released. However, thereaction: K-feldspar + H++ H₂O = kaolinite + SiO₂ + K+ drive the previous reaction. So, there arecirculated reactions in close system. Therefore, the relative content of K-feldspar and kaolinitecontrols the quantity of dissolved feldspar and the relationship among K-feldspar, kaolinite andillite after reaction. There is only kaolinite in the top of Xu4, it is showed that the content ofkaolinite is greater than or equal to K-feldspar before the reaction between kaolinite andK-feldspar occur, so kaolinite is surplus after reaction. More H+ are stored in the top of Xu4, andmore unstable feldspars are dissolved in the top of Xu4 before the temperature don't reach to120-140℃. Different from Xu4, there is only K-feldspar in Xu2. It is showed that thecontent of kaolinite is less than or equal to K-feldspar before the reaction between kaolinite andK-feldspar occur, so K-feldspar is surplus after reaction. Limited H+ are stored in Xu2. Lessunstable feldspars are dissolved in Xu2 before the temperature don't reach to 120-140℃.Feldspar dissolution which leads to the increase of porosity is the most importantintensive process, but because of the lack of source and active organic acid, feldspardissolution is very limited.