The noble gas isotope record of hydrocarbon field formation time scales

Abstract Noble gases may be considered as the most prominent tracers of natural fluids, including hydrocarbons. The atmosphere is the only source of 20 Ne, 36 Ar, 84 Kr, 130 Xe in subsurface environments, and their concentrations in pore waters after recharge are known from the solubility data. This allows modelling of noble gas partitioning between coexisting gas, oil and water phases in the course of hydrocarbon formation, migration, and storage. Radiogenic isotopes, 4 He*, 21 Ne*, 40 Ar*, 136 Xe*, after being released from source rocks, are mixed with air-derived noble gases already present in the pore space. Concentrations of radiogenic species in the pore space of “typical” hydrocarbon fields are generally so high, that they can hardly be accumulated in situ and thus indicate noble gas transfer from ground waters. The time bearing ratios 4 He*/ 20 Ne, 21 Ne*/ 20 Ne, 4 He*/ 40 Ar AIR 40 Ar*/ 40 Ar AIR in hydrocarbon fields are thus proportional to the time interval between the ground water recharge and noble gases partitioning into the hydrocarbon phase(s), the ‘recharge – partition interval’. The largest available data set allows the recharge-partition intervals to be constrained for a large number of hydrocarbon fields, situated in different tectonic settings (ancient plates, young plates, mobile belts). These intervals increase systematically with the ages of hydrocarbon source and trap lithologies and are comparable with these ages. This important feature, valid in general for different hydrocarbon fields, implies: (i) local sources of radiogenic noble gas isotopes in ground waters; (ii) relatively recent formation of hydrocarbon fields and (iii) their short formation time scales. In some cases the duration of formation of a hydrocarbon field can be constrained. For example, nearly constant 21 Ne*/ 20 Ne, 40 Ar*/ 40 Ar AIR ratios, measured in samples from the Magnus oil field (North Sea), give an accumulation time scale ≈ 10 Ma. It should be emphasized that the above noble gas isotope ratios give the time estimates, which are independent of geological reconstructions. Sometimes the noble gas inventory in a hydrocarbon field and ground waters allows characterization of the source rock volume, involved in formation of the field; generally this volume exceeds that of the hydrocarbon field rocks by orders of magnitude.