Origin of Chert in the Ordovician Carbonates in the Western Tarim Basin and Their Impact on Reservoir Quality: Constraints From Petrology and Geochemistry

ABSTRACT To investigate the formation of chert and silicified carbonates and their impact on reservoir quality in deep‐buried strata, we conducted a detailed petrographic and geochemical study of the Lower Ordovician strata in the western Tarim Basin using both outcrop and drill core samples. Our analyses revealed various forms of chert, including nodules, brecciated cherts, replacive silica, and silica cements, indicating formation during late diagenesis. Petrographic observations show that replacive silica is commonly associated with coarse euhedral dolomite, suggesting an association with deep burial dolomitization. Significant dissolution vugs are observed in silicified carbonates, with visual porosity estimates reaching 15%–20%—significantly higher than the 2%–5% porosity typical of early diagenetic karstification. Geochemical results show Sm/Nd ratios of 0.11–0.32, similar to crustal materials, and chondrite‐normalised REE patterns display right‐dipping trends. The δ 30 Si values range from 0.3‰ to 3.5‰, consistent with silicified precursor cherts and indicative of a magma‐derived silicon‐rich hydrothermal fluid origin. These fluids migrated along deep faults and fractures, facilitating silicification and extensive dissolution that considerably enhanced reservoir quality. This study provides diagnostic criteria and a genetic model for effective prediction of high‐quality reservoirs in deep hydrothermal silicification systems, highlighting the importance of fault‐controlled fluid activity for hydrocarbon exploration in deeply buried carbonates.