Petrological Characteristics Of Dolomites In The Qixia Formation Of Middle Permian In Nanjiang, Sichuan Basin

Dolomite reservoir is one of the main reservoir types of carbonate rocks. Currently, the Petro China Southwest Oil and gas field company has found gas fields of Permian dolomite reservoirs with industrial value in Sichuan Basin, indicating good potential of dolomite reservoir in oil and gas exploration. Previous studies of Qixia Formation mainly focus on fossils, sedimentary facies and sequence stratigraphy and so on, and the research results of dolomite in Qixia Formation are mainly gained from the western Sichuan Basin but relatively lack in northeastern Sichuan Basin. Therefore, this paper takes dolomites of the Middle Permian Qixia Formation in Nanjiang-Qiaoting area in northeastern Sichuan Basin as the research object. Base on field section observation, thin section examination, cathodoluminescence analysis, scanning electron microscopy analysis, energy spectrum analysis, x-ray diffraction analysis, the pertological characteristics, the order degree of dolomites and information of diagenetic fluid are studied and the origin of dolomite of Qixia Formation in Nanjiang-Qiaoting is discussed by combined with previous studies of geological background, stratigraphy, sedimentary facies, buried history and thermal history.Comparative study of the order degrees of dolomites of the Qixia Formation fromthe Nanjiang Qiaoting section, Guangyuan Changjianggou section and Well Wujia 1, Well Kuang 2 and Well Hanshen 1 indicates that: ①The order degree of dolomite is related to its crystallinity. For example, dolomite with ethereal and relatively large crystals has better order degree than that of micritic dolomite; ②the higher degree of dolomitization, the better order degree of cdolomite. ③The content of Fe2+, Mn2+ and Sr2+ is negatively related to the order of dolomites. Dolomite samples from three drilling wells show more thoroughly dolomitization and better order degree of dolomite than those from Nanjiang-Qiaoting and Guangyuan Changjianggou sections. Dolomites from Well Hanshen 1 located in the southwest have the highest order degree, while the order degree of dolomites from Nanjiang-Qiaoting section in the northwestern Sichuan Basin is the lowest. This trend reflects the influence of thermal events that farther away from the heat source(Emei Basalt), weaker dolomitization.The dolomitization of the Qixia Formation is not completely, and the main lithology is calcareous dolostone. Dolomites are lack of original structures and are mainly with fine crystalline crystals in size. The crystals mainly consist of planer subhedral-nonplaner anhedral, planer porphyritic cryatals as well as typical saddle dolomite crystals. Some special fabrics are developed in dolomites, such as stylolite and fractures that cut through dolomites formed in shallow-buried environment, the dissolution of nonplaner dolomite, the precipitation of saddle dolomite. And authigenic non-carbonate minerals related to hydrothermal activities are also found in Qixia Formation, such as fluorite, fluorapatite, sodium feldspar and pyrite. All the above fabrics and minerals are evidences of the hydrothermal origin of the Middle Permian dolomites in Nanjiang area.The dolomite exhibits a weak pink luminescence and the calcite exhibits dark red or no luminescence, indicating the diagenetic fluid from sea water mixed with exogenous fluid. Combined with previous studies of the burial and thermal history, we believe that the dolomite genesis of the Qixia Formaton in Qiaoting section in Nanjiang area is related to the eruption of the Emeishan basal, and the high temperature overcomes the kinetic barrier of the formation of dolomites, while the short time of thermal effect leads to incomplete dolomitization.