The hydrocarbon generation process of the deeply buried Cambrian Yuertusi Formation in the Tabei uplift, Tarim Basin, northwestern China: Constraints from calcite veins hosting oil inclusions in the source rock

Abstract In the northern Tarim Basin (northwestern China), Cambrian source rocks are widely distributed, especially within the Lower Cambrian Yuertusi Formation (Є1y). Understanding the hydrocarbon generation process of the Yuertusi Formation source rocks is essential for reconstructing the evolutionary history of the ultra-deep petroleum system in the Tarim Basin. We employed a novel method of reconstructing hydrocarbon generation history by determining the episodes and absolute ages of hydrocarbon generation and expulsion in source rock via the integration of the fluorescence of oil inclusions, inclusion homogenization temperatures, and in situ U-Pb dating of calcite veins containing hydrocarbon inclusions. In the QT1 well area of the Tabei uplift belt, our reconstructed hydrocarbon generation history indicates that during the Late Hercynian (ca. 288–270 Ma), the source rocks generated oil inclusions with yellow-green fluorescence color (λmax [wavelength at maximal light absorbance] ranging from 490 nm to 545 nm, a maturity of ~0.8%). In the Himalayan period (ca. 10 Ma), oil inclusions were generated with blue-white fluorescence color (λmax ranging from 430 nm to 490 nm, a maturity level of ~1.2%). In the central part of the Tabei uplift belt, in the Late Hercynian (ca. 270–260 Ma), oil inclusions with blue-white fluorescence color (λmax ranging from 430 nm to 490 nm, a maturity of ~1.2%) were generated (reaching their hydrocarbon generation peak). Combining basin modeling and constraints from fluid inclusions indicates that the source rocks mainly generated oils at the Late Hercynian period in the Tabei area.