Carbonate Cements From Xujiahe Sandstone And Its Forming Mechanism In Xinchang Area

T3x2 to T3x5 zone were the gas main producing and reservoir formation,in the west of Sichuan basin. There are quite plenty of natural gas resources and huge potential for exploration. Years of exploration and development to confirm that is extremely dense typical reservoir, with ultra-deep, ultra-low porosity and permeability. The quantitative statistics show that the mechanic compaction is the main cause resulting in the disappearance of primary porosity, and the next one is cementation.Xinchang area increasingly becoming the most important positions of the gas exploration and development of the Xujiahe formation in the west of Sichuan basin since this century. However, the corresponding study of diagenesis still weak in the area. So, we tried to select Xujiahe reservoir of Xinchang area, to study primarily cabornate cements and its foming mechanism in the reservoir sandstones. We expect the results to give out certain theory foundations of the prediction about distribution of relative high-quality reservoirs in the general context of low porosity and low permeability.At present, it has been confirmed that carbonate cements of different types or different stages in oil and gas reservoirs is an important densification factor. They are hot issues of the study of low-permeability reservoir about carbonate cement formation mechanism, mode of occurrence, material sources and its impact on reservoir properties. Paper in view of the petrology characteristic in Upper Triassic clastic rock reservoir in Xinchang area, on the basis of analysis indoor data, combined with the petrology and sedimentary theories, by means of various analytical techniques including impregnated thin section analysis,scanning electron microscope(SEM),X-ray Diffraction(XRD) and so on. Based on those experiments,the petrology charater, mineral character and geochemistry character of authigenic carbonate mineral is analysed, the effect on the on reservoir quality for carbonate cements is evaluated, and its formation mechanism is discussed.The primarily rock types of Xujiahe formation are lithic sandstone, lithic quartzarenite and quartzarenite, but lacking feldspar sandstone. Middle grain size, locking feldspar and riching quartz, high composition maturity and poor structural maturity are the characteristics of Xujiahe formation reservoir sandstone. Reservoir sandstone has much authigenic mineral, and its average content achieves 9%.Including authigenic carbonate mineral, authigenic quartzite and clay mineral.The authigenic carbonate mineral mainly consists of calcite, ankerite and siderite. The are hosted in different resevoir space,for example,intergranular dissolution porosity, mineral intercrystal porosity, feldspar dissolution porosity, et al.. Total carbonate cement are almost same from longitudinal distribution. However, calcite appered mainly in T3x4 , ankerite appered mainly in T3x2.Reservoir space are mainly secondary dissolution pores, including lithic dissolution porosity, intergranular dissolution porosity, mineral intercrystal porosity, feldspar dissolution porosity, the primary porosity is poor. The Xujiahe formation coal stratum produces acidic fluid and it dissolve the component of aluminosilicate mineral. On the one hand, the acidic fluid increased sandstone secondary porosity, on the other hand, the material resources of th authigenic carbonate mineral come from dissolution especial feldspar in Xinchang area.