Noble gases in crude oils from the Paris Basin, France: Implications for the origin of fluids and constraints on oil-water-gas interactions

Abstract

In order to investigate the potential of noble gases to trace the dynamics of oil reservoirs, we have analysed the abundance and isotopic composition of all noble gases (He, Ne, Ar, Kr, and Xe) in crude oils from the Paris Basin, France, using a new extraction and purification procedure. The main oil reservoirs are presently located in the Jurassic (Dogger) limestone and in the Triassic (Keuper) sandstone, but hydrocarbons originated from a common source rock formation located in the interbedded Liassic sequence. Despite this common origin, the abundance and isotopic ratios of the noble gases differ between the Dogger and the Keuper. The isotopic compositions of Kr and Xe are indistinguishable from that of air. 3He/ 4He ratios, higher than those predicted from radiogenic production in the sediments or in the crust, are attributed to the occurrence of mantle-derived 3He in the basin. Each sedimentary sequence is characterised by well defined and homogeneous 21Ne/ 22Ne and 40Ar/ 36Ar ratios, which average 0.0306 ± 0.0008 and 312 ± 10 for the Dogger and 0.0367 ± 0.0012 and 664 ± 30 for the Keuper, respectively. The main source of radiogenic noble gases appears to be the continental crust underlying the basin, with possible regional contributions of noble gas isotopes produced in the sediments. The helium and argon isotopic ratios of the Dogger oils are very similar to those observed in geothermal waters flowing in the Dogger aquifer throughout the basin, demonstrating that noble gases in oils derive from associated groundwaters. Oil reservoirs in the Paris Basin therefore accumulate noble gases from wide regions of the continental crust through cross-formational flow of groundwaters and subsequent partitioning into oil. This observation implies that noble gases cannot be directly used to date oils, but can provide time constraints if (1) water/oil interactions are quantified and (2) the residence time as well as the noble gas characteristics of associated groundwaters are known. Oil-water-gas partitioning processes are well recorded in the fractionation of noble gas elemental abundance. Two distinct processes have been identified: (1) accumulation of atmosphere-derived (ANG) and radiogenic noble gases both initially dissolved in groundwaters, resulting in a positive correlation between absolute amount of ANG and the extent of heavy noble gas fractionation and (2) subsequent fractional degassing, resulting in a negative correlation between ANG abundance and heavy noble gas fractionation. Degassing is particularly evident for the Keuper oils and might have occurred in the reservoirs following hydrodynamic gas stripping. The ANG abundance in the Dogger reservoirs requires that about one order of magnitude more water than presently observed has interacted with the oil. Given current estimates of the residence time for groundwaters in the Dogger aquifer, the duration of oil-water interaction is in qualitative agreement with a Palaeocene-Oligocene age for the major episode of secondary oil migration in the Paris Basin. High xenon contents in Keuper reservoirs suggest that they have experienced hydrodynamic interactions between flowing waters and oils for long time, and that the Trias might have reached hydrostatic condition only recently.