Palaeohydrodynamics of fluids in the Brent Group (Oseberg Field, Norwegian North Sea) from chemical and isotopic compositions of formation waters

Generally, the history of past sub-surface fluid movements is difficult to reconstruct. However, the composition of oil-field waters characterizes the origins and mixing processes that allow such a reconstruction. We have investigated present-day formation waters from Brent Group sedimentary rocks of the Oseberg Field in order to assess both their geochemical variations, and their origin(s). Water samples (sampled at the separator) produced from immediately above the oil–water contact and from the aquifer (water-saturated zone below the oil–water contact) were taken from 11 wells across the field. In addition, 3 trace water samples were extracted from oil produced from higher up in the oil column. The water samples were analysed for their chemical components and isotopic compositions. Conservative tracers such as Cl, Br, δD, and δ 18O were used to evaluate the origin of the waters. All formation waters can be characterised as Na–Cl-brines. The separator samples are of aquifer origin, indicating that aquifer water, drawn up by the pressure reduction near the well, is produced from the lower few tens of metres of the oil-zone. By defining plausible endmembers, the waters can be described as mixtures of seawater (60–90%), meteoric water (10–30%), evaporated seawater (primary brines) (3–5%), and possibly waters which have dissolved evaporites (secondary brines). Alternatively, using multidimensional scaling, the waters can be described as mixtures of only 3 endmembers without presupposing their compositions. In fact, they are seawater, very dilute brine, and a secondary brine (confirming the power of this approach). Meteoric water was introduced into the reservoir during the end-Brent and early-Cretaceous periods of emergence and erosion, and partially replaced the marine pore fluids. Lateral chemical variations across the Oseberg Field are extremely small. The waters from closer to the erosion surfaces show slightly stronger meteoric water isotopic signatures. The primary and secondary brines are believed to come from Permian and Triassic evaporitic rocks in the deeply buried Viking Graben to the west, and to have been modified by water–rock interactions along their migration path. These primary basinal brines have not been detected in the oil–zone waters, suggesting that the brines entered the reservoir after the main phase of oil-migration. There are indications that these external fluids were introduced into the reservoir along faults. Present-day aquifer