A MAGNUS-OPUS - HELIUM, NEON, AND ARGON ISOTOPES IN A NORTH-SEA OIL-FIELD

This study of the Magnus oilfield, located in the East Shetland Basin, northern North Sea, represents the most detailed investigation of nob le gas isotope systematics in a liquid hydrocarbon reservoir yet under taken. Samples from nine producing wells across this Middle Jurassic f ield were taken and the helium, neon, and argon isotopic ratios and ab undances in the oil were determined. Both the helium and the neon isot ope systematics require a contribution from a mantle source. If the ma ntle endmember is modeled using mid-ocean ridge (MOR) values, 2.3-4.5% of the He-4 and 4.3-6.2% of the Ne-21 is mantle-derived. The remainde r of the He-4 and 9.0-12.0% of the Ne-21 is crustal-radiogenic and the remaining Ne-21 is atmosphere-derived. The resolved mantle-derived He /Ne ratio is quite distinct from upper-mantle values estimated from MO R samples, but indistinguishable from values resolved in other regions of continental extension. This result provides strong evidence that t he subcontinental lithospheric mantle not only has a different noble g as inventory to the convecting mantle under MORs, but also has a near uniform composition; The quantity of radiogenic noble gas associated w ith the Magnus oil/groundwater system can only be accounted for by pro duction predominantly from outside the volume of the Magnus Sandstone aquifer/reservoir drainage area and the associated Kimmeridge Clay sou rce rock formation and together with the mantle-derived noble gases, p rovides strong evidence for cross-formational communication with deepe r regions of the crust. The Ne-20 and Ar-36 must have been input into the oil phase by interaction with an air-equilibrated groundwater. Nob le 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 Ne-20 and Ar-36 concentrations in the central and southern Magnus samples. The volume of groundwater which has equilibrated with the Ma gnus oil is indistinguishable from the static volume of water estimate d to be in the down-dip Magnus aquifer/reservoir drainage volume. This suggests that the Magnus oil has obtained complete equilibrium with t he groundwater in the reservoir drainage volume, probably during secon dary migration, and further suggests that concurrent cementation of th e Magnus sandstone aquifer has occurred with little or no large-scale movement of air-equilibrated groundwater through the aquifer system. H igher concentrations of Ne-20 and Ar-36 in the N. Magnus oil cannot be accounted for by equilibration with a seawater-derived groundwater. T his is qualitatively consistent with the earlier and more mature oil i n this section equilibrating with freshwater, which is known to have b een trapped in the crest of the reservoir structure.