The noble gas isotope record of hydrocarbon field formation time scales

Noble gases may be considered as the most prominent tracers of natural fluids, including hydrocarbons. The atmosphere is the only source of 20Ne, 36Ar, 84Kr, 130Xe in subsurface environments, and their concentrations in pore waters after recharge are known from the solubility data. This allows modeling of noble gas partitioning between coexisting gas, oil and water phases in the course of hydrocarbon formation, migration, and storage. Radiogenic isotopes, 4He*, 21Ne*, 40Ar*, 136Xe*, 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 4He*/20Ne, 21Ne*/20Ne, 4He*/40ArAIR 40Ar*/40ArAIR 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 21Ne*/20Ne, 40Ar*/40ArAIR 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.</p

The noble gas isotope record of hydrocarbon field formation time scales

Noble gases may be considered as the most prominent tracers of natural fluids, including hydrocarbons. The atmosphere is the only source of 20Ne, 36Ar, 84Kr, 130Xe in subsurface environments, and their concentrations in pore waters after recharge are known from the solubility data. This allows modeling of noble gas partitioning between coexisting gas, oil and water phases in the course of hydrocarbon formation, migration, and storage. Radiogenic isotopes, 4He*, 21Ne*, 40Ar*, 136Xe*, 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 4He*/20Ne, 21Ne*/20Ne, 4He*/40ArAIR 40Ar*/40ArAIR 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 21Ne*/20Ne, 40Ar*/40ArAIR 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.</p

Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics

International audienceUnderstanding the growth rate of the continental crust through time is a fundamental issue in Earth sciences1–8.The isotopic signatures of noble gases in the silicate Earth (mantle, crust) and in the atmosphere afford exceptional insight into the evolution through time of these geochemical reservoirs9. However, no data for the compositions of these reservoirs exists for the distant past, and temporal exchange rates between Earth’s interior and its surface are severely under-constrainedowing to a lack of samples preserving the original signature of the atmosphere at the time of their formation. Here, we report the analysis of argon in Archaean (3.5-billion-year-old) hydrothermal quartz. Noble gases are hosted in primary fluid inclusions containing a mixture of Archaean freshwater and hydrothermal fluid. Our analysis reveals Archaean atmospheric argon with a 40Ar/36Ar value of 143624, lower than the present-day value of 298.6 (for which 40Ar has been produced by the radioactive decay of the potassium isotope 40K, with a half-life of 1.25 billion years;36Ar is primordial in origin). This ratio is consistent with an early development of the felsic crust, which might have had an important role in climate variability during the first half of Earth’s history