Fluid inclusions and in situ U-Pb dating constraints on hydrocarbon accumulation in the Ordovician strata of the Ordos Basin, China

Abstract With the growing focus on deep petroleum resources, determining the timing of hydrocarbon accumulation in ancient carbonate strata, such as the Ordovician strata of China's Ordos Basin, becomes crucial, especially given the ongoing debates regarding the basin's Paleozoic tectonic history. Analyses of carbonate cement phases and in situ U-Pb dating of primary inclusions reveal that the Ordovician strata experienced deep burial, reaching ~140 °C during the Late Silurian Period. This indicates that the lower Paleozoic source rocks entered the hydrocarbon generation stage by the Caledonian Orogeny. Organic matter maturity profiles and solid hydrocarbon inclusions, preserving early Paleozoic thermal information, suggest that significant uplift of the basin (with a denudation thickness of 2.6–4.0 km) occurred during the Late Silurian to Early Carboniferous. This uplift halted hydrocarbon generation in the Ordovician strata, and freshwater leaching took place in the Early Permian. Subsequent Mesozoic subsidence and sedimentation led to increased temperatures and salinity within the Ordovician strata. This triggered large-scale gas generation and oil cracking, indicating secondary hydrocarbon activities. Early hydrocarbon charges may have positively influenced deep carbonate reservoirs. Gas accumulation in the Late Triassic, based on the thermal evolution of the source rocks, was likely sourced from the overlying Carboniferous–Permian source rocks. These findings enhance our understanding of the hydrocarbon accumulation history in the Ordos Basin and provide valuable insights for future deep petroleum exploration in ancient carbonate strata.