Deeply buried (>7000 m) Middle Cambrian (Miaolingian) carbonate rocks in the northern Tarim Basin, China, represent a highly promising target for resource exploration. Their reservoir quality is governed by complex diagenetic processes, including multiple stages of dolomitization and subsequent dedolomitization. This study presents an integrated analysis using petrology, stable isotopes, trace elements, fluid inclusion microthermometry, and in situ U–Pb dating from seven wells. We systematically interpret the complex diagenetic history and fluid evolution of the Miaolingian carbonates. Results reveal a complete paragenetic sequence from near-surface to deep-burial diagenetic realms. Early diagenesis was dominated by marine-derived fluids in near-surface to shallow-burial realms, where supratidal and seepage–reflux dolomitization led to the formation of early dolomicrite (Md1) and grainy/microbial dolomite (Md2), along with the precipitation of dogtooth/fibrous cements (Cd1). With increasing burial depth, normal seawater influenced by geothermal circulation formed very fine- to fine-crystalline dolomite (Md3) and associated granular cements (Cd2). In the intermediate- to deep-burial realm, the diagenetic system was dominated by basinal fluids. The fabric of early dolomites was destroyed under elevated temperatures (∼110 °C) to form fine- to medium-crystalline dolomite (Md4). This was followed by cementation of euhedral (Cd3) and blocky (Cd4) dolomite cements. Late-stage diagenesis was tightly coupled with regional tectonic activity. Saddle dolomite (Sd) precipitated from high-temperature (∼139°C), high-salinity (∼22 wt.%) tectonic–hydrothermal fluids, and some Md4 may have been altered by these fluids. U–Pb dating of early calcite cements (Cc-Ⅰ) yields an age of 459.1 ± 3.3 Ma, constraining the initial episode of the middle Caledonian Orogeny. Fluid inclusions within these cements have high temperatures (110.2–125.2 °C) and exhibit seawater-like rare earth elements and yttrium (REY) patterns, indicating a record of buried formation water. Subsequently, episodes II and III of the middle Caledonian Orogeny induced fluid mixing between Ca2+-rich formation water and meteoric water in the intermediate burial realm, causing localized dedolomitization that formed matrix calcite (Mc) and late-stage calcite cements (Cc-Ⅱ), characterized by lower homogenization temperature (Th) values (∼90–110 °C). Integration of diagenetic phases with reservoir quality reveals contrasting porosity evolution. Seepage–reflux and seawater dolomitization were the constructive processes, while supratidal, burial and hydrothermal dolomitization, and dedolomitization were predominantly destructive, occluding pore space and deteriorating reservoir quality.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)