Dolomite Genesis Of Qixia Formation, Middle Permian, Western Sichuan Basin And Contrast To Dolomite Genesis Of Feixianguan Formation, Lower Triassic, Northeastern Sichuan Basin

Dolomite reservoir in Qixia Formation, Middle Permian, Western Sichuan Basinhas well exploration potential, cause depositional setting is marine transgression,lacking of evaporate rocks deposition, evaporation dolomitization is not applicable,while mixed water dolomitization are being questioned for the past few years. So it isnecessary to research the dolomitization mechanism and mode in this area, based onthe geological background and dolomite character. Researchers had done lots ofdetailed work about dolomite genesis and mechanism of Feixianguan Formation inNortheastern Sichuan Basin, which is approximate to technically maturation andtheoretically perfect. That’s why using dolomite of Qixia Formation contrast toFeixianguan Formation, incluing the following aspects: rock fabric, geochemicalcharacteristic, fluid inclusion homogenization temperature and salinity, dolomitegenesis and dolomitization mode, to make further explanation of dolomite genesis ofQixia Formation.On the basis of our predecessors’ work on basic geology, such as tectonic,sedimentary facies, lithofacies paleogeography, diagenesis, etc. We gainedtremendous depth and insight form all of those work and influenced us to achievedgreat results. Using a series of methods to gain rock fabric and geochemicalcharacteristic, included field section measurement, core observation, sampling,ground sample, thin section examination, cathodoluminescence analysis, electronmicroprobe analysis, scanning electron microscopy analysis, energy spectrum analysis,carbon and oxygen isotopic analysis, x-ray diffraction analysis, fluid inclusionhomogenization temperature test, to invert dolomitization fluid and dolomite genesis. The category of Qixia Formation dolomite is very mono, there is only one singlecrystalline dolomite, lack of micrite dolomite, and also lack of gypsum deposition.Trace element of dolomite changed contrast to pioneer limestone, showing the contentof Manganese (28.4~117ppm) and Ferrum (199.4~381.4ppm) increase, Sr(35.9~110.5ppm) decrease. Cause Pyrite and bitumen can often be seen at outcrop andcore, with the benefit of thin section examination, scanning electron microscopyanalysis, energy spectrum analysis, etc., there also have many hydrothermal mineralsdeposited in dolomite of Qixia Formation. These hydrothermal minerals includingquartz, fluorite, illite, barite and apatite filled in the corroded pores and vugs.Over-dolomitization means that the dolomite precipitated in the pores and vugs andthus made the rock more compact. After that the saddle dolomite dissolution, calciteprecipitate in the pores and vugs formed by dolomite dissolution. Dolomite of QixiaFormation has typical saddle crystal morphology and curved crystal faces, and appearwavy extinction in thin section under crossed-polarized light, also has dissolution,brecciation, all these features are associated with hydrothermal fluid.The δ13C(PDB) of dolomite and limestone of Qixia Formation lie in the range of-0.66~3.84‰, the δ18O(PDB) of dolomite lie in the range of-3.42~9.16‰，reflectingthe fluid comes from marine. Homogenization temperature of matrix dolomite fluidinclusions show a range of67~243℃, homogenization temperature of saddle dolomitein a range of91~223℃, and the homogenization temperature of calcite in a range of54~215℃, demonstrating that the temperature dolomite precipitated higher more than10℃than calcite precipitated. And the homogenization temperature reach up to243℃,which is not only the burial condition can be reached.Adopting oxygen isotope of minerals and the homogenization temperature offulid inclusions to invert the oxygen isotope of fluid. The δ18O of fluid correspondingwith matrix dolomite, void-filling dolomite and saddle dolomite lie in the range of0~12‰(SMOW), the δ18O of few samples’ corresponding fluid lie in the range of-4~0‰(SMOW), and there are2samples’(one is matrix dolomite, the other is saddledolomite) δ18O of fluid lie in the range of12~13.5%(SMOW). The δ18O of fulid thatcalcite precipitate lie in the range of-4~0‰(SMOW),0~8‰(SMOW),12~14.5‰(SMOW), and recorded the multi-periods of50~100℃,100~170℃,190~250℃, and the salinity of fulid is higher or much higher than the coeval seawater.The87Sr/86Sr ratios of coeval sewater in Permian lies in0.7085~0.7067, while the87Sr/86Sr ratios of dolomite of Qixia Formation lies in0.7076~0.7069and is lowerthan the coeval seawater. Thermal history recorded the temperature anomaly between Middle and Late Permian, the maximum paleo-heat flow reached in the early stage ofLate Permian at approximately259Ma, after that the paleo-heat flow decreased topresent. Meaning the dolomitization fluid mainly derived from the coeval seawaterwhich influenced by the abnormal heat.The crystalline dolomites are the mainstream dolomite phases in FeixianguanFormation of Lower Triassic, Northeastern Sichuan Basin. And there also exist somefabric-retentive dolomitic grainstones and micritic dolomites. The dolomites inNortheastern Sichuan Basin have low manganese contents (average of63.47ppm),low Ferrum contents (average of1019.67ppm) and high strontium contents (averageof132.39ppm). The crystalline dolomite phases have high strontium contents, and theassociated calcite cements have extreme high strontium contents. The δ13C values areclose to the compositions of coeval seawater. The precipitating fluids of thesedolomite phases have the relatively high δ18O values (ranges between2to7‰;SMOW), and have the higher87Sr/86Sr ratios than the ratios of coeval seawater. Fromthe earlier Changxing Formation to the later Leikoupo Formation, the differencesbetween strontium isotope ratios of dolomites and of coeval seawater decreasedgradually. The precipitating fluids of dolomites were getting closer and closer tocoeval seawater. The homogenization temperatures of fluid inclusions in dolomitesare high (ranges from85.85to142.24℃). And the salinities of fluid inclusions indolomites are double as much as the salinity of seawater (ranges from3.01to12.61%NaCl). Thus, the dolomitizing fluid is a kind of marine-derived fluid, andoriginated from downward-migrated refluxing evaporated and concentrated seawaterof Jialingjiangian. The dolomitization occurred in a closed diagenetic system duringdeep burial, and was the product of brine infiltration during deep burial.