Palaeohydrodynamics of fluids in the Brent Group (Oseberg Field, NorwegianNorth 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 inv estigated present-day formation waters from Brent Group sedimentary rocks o f the Oseberg Field in order to assess both their geochemical variations, a nd their origin(s). Water samples (sampled at the separator) produced from immediately above the oil-water contact and from the aquifer (water-saturat ed zone below the oil-water contact) were taken from 11 wells across the fi eld. In addition, 3 trace water samples were extracted from oil produced fr om higher up in the oil column. The water samples were analysed for their c hemical components and isotopic compositions. Conservative tracers such as Cl, Br, deltaD, and delta O-18 were used to evaluate the origin of the wate rs. All formation waters can be characterised as Na-Cl-brines. The separato r 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%), ev aporated seawater (primary brines) (3-5%), and possibly waters which have d issolved evaporites (secondary brines). Alternatively, using multidimension al scaling, the waters can be described as mixtures of only 3 endmembers wi thout presupposing their compositions. In fact, they are seawater, very dil ute brine, and a secondary brine (confirming the power of this approach). M eteoric water was introduced into the reservoir during the end-Brent and ea rly-Cretaceous periods of emergence and erosion, and partially replaced the marine pore fluids. Lateral chemical Variations across the Oseberg Field a re extremely small. The waters from closer to the erosion surfaces show sli ghtly stronger meteoric water isotopic signatures. The primary and secondar y 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 b rines have not been detected in the oil-zone waters, suggesting that the br ines 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 waters are mixtures of waters from differ ent origins and hardly vary at a field-scale. They are different in composi tion to the water trapped in the present oil-zone. One of the oil-zone samp les is a very dilute brine. It is thought to represent a simple mixture of seawater and meteoric water. Due to oil-emplacement, this geochemical signa ture was preserved in the waters trapped within the oil-zone. Another oil-z one water shows a very similar chemical signature to the aquifer waters, bu t the chlorine isotopic signature is similar to that of the dilute oil-zone water. This water is interpreted to represent a palaeo-aquifer water. That is, it was within the aquifer zone in the past, but was trapped by subsequ ent emplacement of more oil. These Vertical differences can be explained by two features: (i) emergence of the Brent Group sedimentary rocks in the Ea rly Cretaceous allowed ingress of meteoric water; (ii) subsequent rapid bur ial of Viking Graben rocks caused migration of petroleum and aqueous fluids into the adjacent, less deeply buried Oseberg Field. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.