Integrated pyrobitumen Re-Os and U-Pb geochronology to date hydrocarbon generation and accumulation in the Ediacaran Dengying gas field, central Sichuan Basin, Southwest China

The Qiongzhusi-Dengying petroleum system is recognized as the oldest giant gas reservoir in the central Sichuan Basin, Southwest China, attracting widespread attention since its discovery. This study integrates petrology, fluid inclusion analysis, in situ laser ablation–inductively coupled plasma–mass spectrometry dolomite U-Pb dating, and pyrobitumen Re-Os geochronology to reconstruct a long-lived petroleum system in the Ediacaran Dengying Formation. Dolomites of the Dengying Formation in the study area primarily developed dissolution pores and vugs through selective meteoric fresh-water dissolution and epigenetic karstification. Mineral filling sequences in vugs were categorized as fibrous dolomite (I) → micritic dolomite (II) → isopachous crust fine dolomite (III, 509–502 Ma) → type I bitumen → fine- to medium-crystalline dolomite (IV, 453 ± 52 Ma) → saddle dolomite + fluorite + sphalerite (V, 275–240 Ma) → type II bitumen + quartz. Hydrocarbon inclusions and chronological data indicate a three-phase hydrocarbon evolution: (1) middle Cambrian to Late Ordovician low-maturity hydrocarbon charging (510–450 Ma); (2) late Permian high-maturity hydrocarbon accumulation (275–240 Ma); and (3) post–Late Jurassic gas reservoir formation through thermal cracking. Furthermore, the pyrobitumen Re-Os ages of 333 ± 24 Ma and 140.4 ± 2.3 Ma, respectively, indicate (1) significant alteration of the Dengying Formation paleo–oil reservoirs during the early Carboniferous Caledonian orogeny, and (2) thermal cracking of paleo–oil reservoirs to gas since the Late Jurassic. The initial 187Os/188Os ratios suggest that the hydrocarbon source for the Dengying Formation in the study area is likely the lower Cambrian Qiongzhusi Formation. The multimineralogical and multichronological framework established herein provides an effective methodology for constraining hydrocarbon accumulation processes in deep to ultradeep superimposed basins.