Age assignment of dolomite in palaeo-reservoirsof the Qiangtang Basin: New evidence from palaeontology and carbonate in situ U-Pb dating

Abstract

The dolostone in the south Qiangtang depression has been proven to be a favourable reservoir rock of the famous Longeni-Angdaerco palaeo-reservoir, being of great significance for oil and gas exploration in the Qiangtang basin. However, its geochronology and diagenetic process are still controversial. In particular, the previous interpretation that oil-bearing dolostone belongs to the Middle Jurassic Buqu Formation is challenged by recent works. Based on previous macro-palaeontological studies, we carried out the latest micro-palaeontological identification and carbonate in-situ U–Pb isotope dating analysis to re-assign the depositional age of the dolomite in the palaeo-reservoirs. We also establish a high-resolution diagenetic time framework of dolomite combined with microscopic identification, scanning electron microscope, and rare earth elements analysis. In this study, age-diagnostic foraminifers, which include Lituolipora termieri, Siphovalvulina colomi, Siphovalvulina variabilis, Siphovalvulina gibraltarensis, Duotaxis metula, and Variostoma sp., were recovered from the overlying and underlying limestone sequences of the studied Xiaogaxiaona palaeo-reservoir section. These foraminiferal assemblages suggest a Late Triassic to Early Jurassic age of the carbonate sequences. Nine U–Pb ages of different carbonate components were obtained. Contrary to previous dolomitization models, U–Pb dating suggests a Late Triassic metasomatic dolomitization event (216.2 ± 6.1 Ma to 210.1 ± 9.4 Ma), which was likely driven by seawater fluids. This event was probably followed by calcite vein cementation (U–Pb ages of 203 ± 13 Ma). Another Early Jurassic dolomitization event (185.5 ± 3.8 Ma to 182.7 ± 5.8 Ma) perhaps involved the mixed-zone model. Finally, saddle dolomite cement yielded Early and Middle Jurassic ages of 181 ± 18 Ma and 161 ± 23 Ma and crystallized in pre-existing vugs, representing two hydrothermal fluid events. This study suggests that the palaeo-reservoir carbonate successions were probably deposited in the Late Triassic rather than the previous suggestion of the Middle Jurassic. The high-resolution time framework of the dolostone in the palaeo-reservoir reveals a possible link between dolomitization and hydrocarbon accumulation in basin-scale fluid-flow events in the Qiangtang Basin.