Diagenesis and timing of hydrocarbon fluid charge in the Eocene reservoir of the SW Qaidam Basin, western China

The sandstone in the Paleogene (E₃¹) is an important reservoir in the SW Qaidam Basin, where complex tectonic activities in the Paleogene complicated the understanding of hydrocarbon accumulation process. Herein, detailed reservoir diagenesis investigation was carried out using optical microscopy, cathodoluminescence and SEM observations, while the timing of hydrocarbon charge was determined using an integrated fluid inclusion technology including petrography, fluorescence spectroscopy, FT-IR spectroscopy, and microthermometry analyses. The established diagenetic sequence includes early calcite and illite/chlorite, feldspar dissolution, quartz overgrowth/kaolinite, feldspar overgrowth/sparry calcite, ankerite, and anhydrite. Three types of hydrocarbon inclusions were distinguished, which are yellow-fluorescent oil inclusions, bluefluorescent oil inclusions, and non-fluorescent gas inclusions, representing two episodes of oil charge and readjustment in the reservoirs. The first episode of oil charge occurred at ~25 Ma and is represented by the yellow-fluorescent oil inclusions trapped prior to quartz overgrowth. The second episode of oil readjustment occurred at 5‒0 Ma, which is marked by the blue-fluorescent fluid inclusions occurring later then the formation of quartz overgrowth. The produced crude oil has similar thermal maturity to that of the yellow-fluorescent inclusion oil, implying the first episode of oil charge (~25 Ma) is the main contributor to the present-day oil production.