A Magnus opus: Helium, neon, and argon isotopes in a North Sea oilfield

Abstract

This study of the Magnus oilfield, located in the East Shetland Basin, northern North Sea, represents the most detailed investigation of noble gas isotope systematics in a liquid hydrocarbon reservoir yet undertaken. Samples from nine producing wells across this Middle Jurassic field were taken and the helium, neon, and argon isotopic ratios and abundances in the oil were determined. Both the helium and the neon isotope systematics require a contribution from a mantle source. If the mantle endmember is modeled using mid-ocean ridge (MOR) values, 2.3–4.5% of the 4He and 4.3–6.2% of the 21Ne is mantle-derived. The remainder of the 4He and 9.0–12.0% of the 21Ne is crustal-radiogenic and the remaining 21Ne is atmosphere-derived. The resolved mantle-derived He/Ne ratio is quite distinct from upper-mantle values estimated from MOR samples, but indistinguishable from values resolved in other regions of continental extension. This result provides strong evidence that the subcontinental lithospheric mantle not only has a different noble gas inventory to the convecting mantle under MORs, but also has a near uniform composition. The quantity of radiogenic noble gas associated with the Magnus oil/groundwater system can only be accounted for by production predominantly from outside the volume of the Magnus Sandstone aquifer/reservoir drainage area and the associated Kimmeridge Clay source rock formation and together with the mantle-derived noble gases, provides strong evidence for cross-formational communication with deeper regions of the crust. The 20Ne and 36Ar must have been input into the oil phase by interaction with an air-equilibrated groundwater. Noble gas partitioning between a seawater-derived groundwater and the oil phase at the average Magnus Sandstone aquifer temperature requires a subsurface seawater/oil volume ratio of 110 (±40) to account for both the 20Ne and 36Ar concentrations in the central and southern Magnus samples. The volume of groundwater which has equilibrated with the Magnus oil is indistinguishable from the static volume of water estimated to be in the down-dip Magnus aquifer/reservoir drainage volume. This suggests that the Magnus oil has obtained complete equilibrium with the groundwater in the reservoir drainage volume, probably during secondary migration, and further suggests that concurrent cementation of the Magnus sandstone aquifer has occurred with little or no large-scale movement of air-equilibrated groundwater through the aquifer system. Higher concentrations of 20Ne and 36Ar in the N. Magnus oil cannot be accounted for by equilibration with a seawater-derived groundwater. This is qualitatively consistent with the earlier and more mature oil in this section equilibrating with freshwater, which is known to have been trapped in the crest of the reservoir structure.