A Magnus opus: Helium, neon, and argon isotopes in a North Sea oilfield
- Univ. of Cambridge (United Kingdom)
- BP Exploration, Middlesex (United Kingdom)
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 mantle source. If the mantle endmember is modeled using mid-ocean ridge (MOR) values, 2.3-4.5% of the {sup 4}He and 4.3-6.2% of the {sup 21}Ne is mantle-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 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 {sup 20}Ne and {sup 36}Ar 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. 68 refs., 10 figs., 8 tabs.
- OSTI ID:
- 443900
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 60, Issue 5; Other Information: PBD: Mar 1996
- Country of Publication:
- United States
- Language:
- English
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