| The Lower Paleozoic carbonate rocks in Gucheng area of Tarim Basin are mainly developed in limited
platform, semi limited platform, and open platform. There is significant silicification in some parts, which is mainly
developed in the Cambrian, Ordovician Penglaiba Formation, Yingshan Member 3 and 4, and Yijianfang Formation.
Based on thin sections, there are two forms of petrological features under the microscope: cryptocrystalline siliceous
replacement and filling of pores, and authigenic quartz filling of pores. Experimental analyses including homogeneous
temperature of fluid inclusions, major and trace elements, and silicon and oxygen isotope show that the homogenization
temperature of fluid inclusions in quartz is high, ranging from 121 ℃ to 206 ℃, mainly ranging from 150 ℃ to 175 ℃;
siliceous rock has the characteristics of high Fe, Mn and low Al;δ30Si of siliceous rock is 0.1‰-3.6‰, δ18O is 12.7‰-
27.9‰, and the calculated δ18O value of siliceous metasomatic fluid is 8.23‰-12.65‰, which is equivalent to mediumacid
magmatic water. It is concluded that the silicon in the Lower Paleozoic carbonate rocks in Gucheng area may be
derived from the medium-acid magmatic hydrothermal fluid. Most of the early cryptocrystalline siliceous materials filled
pores or metasomatic protolith in a large area, which should be the products of high temperature and high salinity fluid.
This silicification has a destructive effect on the development of reservoir space at this stage. The late authigenic quartz
often represents the product of acidic fluid background. This hydrothermal solution can cause significant dissolution of
carbonate minerals, forming dissolution pores and fractures. As the concentration of siliceous fluid decreases, the siliceous
fluid begins to crystallize slowly and gradually into quartz in the areas with sufficient pore space. At this stage,
silicification plays a certain role in reservoir construction. The early silicification mainly occurred in the Cambrian,
Ordovician Penglaiba Formation, Yingshan Member 4 and Member 3, while the late silicification mainly occurred in the
Cambrian and Yingshan Member 3 with reservoir improvement. |
