Siliceous genesis and reservoir significance in Ordovician carbonate rocks in southwestern Tarim Basin

The Ordovician carbonate rocks in the southwestern region of the Tarim Basin are characterized by the development of siliceous rocks, with diverse occurrences and facies types, but the research level is relatively low. This article takes the drilling in the Yubei area of southwestern Tarim and the outcrop of siliceous rocks in the Ordovician in the field of northern Tarim as the research object. Through petrological and geochemical analysis, the occurrence types of siliceous rocks in the Ordovician carbonate rocks, the types and sources of silica-rich fluids, and the activity periods are studied. The research results indicate that the siliceous rocks in the Ordovician carbonates of the Yubei area can be divided into three types based on their occurrence: sedimentary, filling, and metasomatic. Sedimentary siliceous rocks are mainly composed of a low-order A-type opal and CT-type opal, while filling and metasomatic siliceous rocks are mainly composed of relatively a high-order CT-type opal and quartz. The geochemical properties of siliceous rocks with different occurrences and phase types indicate their homology. The increase in temperature during the Late Permian Period led to an increase in the solubility differences between different phases of siliceous rocks and weakened the dynamic barrier of siliceous transformation, thereby promoting the activation and dissolution of low-order phase siliceous rocks in sedimentary siliceous rocks to form silica-rich fluids, which then filled spaces or replaced minerals as higher-order siliceous rocks. The spatial distribution of siliceous rocks with different occurrences and phases was controlled by the spatial configuration relationship of fractures and dissolution pores. The activity of silicon-rich fluids has changed the spatial configuration of reservoir space. In the Yubei area, silica-rich fluids originate from the activation of sedimentary siliceous rocks in the formation itself. The original uniformly distributed A-type opal was activated, migrated, and aggregated into quasi-syngenetic dissolution pores and fractures. On the one hand, this leads to partial or complete filling of the early reservoir space. On the other hand, the space displaced by sedimentary siliceous rocks activated from the original sedimentary layers is also dispersed, with poor connectivity.