The element chlorine is a very conservative one. This means that it is not significantly involved in biological and chemical processes, which may effect its isotopic composition. Because of this conservative behaviour, the variations in chlorine stable isotope ratios (37Cl/35Cl) in nature are rather limited and the fractionation is caused mainly by physical processes such as diffusion and ion-filtration. Variations in the chlorine isotopic ratio are reported as d37Cl in per mil relative to the ratio in ocean water, which is constant and is called "Standard Mean Ocean Chloride" (SMOC).
In a large set of formation water and source rock samples from the North Sea and adjacent areas the chlorine isotopic composition have been determined. Big differences were recognised between and within fields, and also between different types of the source of the chlorine. Unfortunately not all types of chloride were measured in single fields, but the large number of measurements, and the striking differences between types of chloride give faith that these differences will be found within single fields.
Effectively three types of samples were measured. Chloride from mud rocks (mainly from the Kimmeridge) was extracted by adding water to the mud rock and shaking it for at least a day. The mud rock then was disaggregated and it was assumed that all chloride went into solution. Two basic types of formation water were measured. Water coproduced with oil, which appears to be identical with that ultra-centrifuged from cores ("free" formation water) and water that was extracted from dry oil ("water-in-oil"). "Free" formation water could be measured directly. "Water-in-oil" was extracted using a method developed recently in our laboratory.
d37Cl in the mud rocks were generally close to 0”. In the shallowest samples, which had a burial depth that was never deeper than 500 meters d37Cl is positive (up to 0.8”), while in deeper samples slightly negative values are found (down to -0.5”). In source rocks from one specific field more negative values (down to -2.5”) were found.
Large variations, especially within fields are found for the "free" formation water. In two fields the measured variation is over 4”. The more negative values (down to -5.2”) are found in samples taken from the oil-zone, while higher values are found in the aquifer zone. In one field a near perfect mixing line between two end-members is found. It is suggested that the water from the oil-zone comes from the source rock and that it is fractionated substantially during the expulsion from the rock by ion-filtration processes. The aquifer water is expected to be sea or meteoric water (with or without evaporite dissolution) and has an isotopic composition close to 0”.
The origin of the "water-in-oil" is not clear yet. The Cl isotopic composition is generally constant or near constant within fields, and may show fractionated values. Two possibilities can be considered seriously. It might be that this water represents the original (fossil) formation water that was present before oil invaded the reservoir. If this be the case then this can give valuable information about the
original composition of formation water and chemistry of basins prior to the filling with oil. A second, more speculative possibility is that this water was originally entrained with the oil when it was expelled from the mud rock. This may indicate that two phases of water may be present in a reservoir; one heavily fractionated and a non or only slightly fractionated phase. The non fractionated water might be emulsified in the oil in very small droplets, which are so small that they are not affected by the fractionating effect of ion filtration and pass the mud without fractionation.
It might be clear that, despite the large amount of
chlorine isotope work already done on formation waters it is still not completely clear which processes define the differences in d37Cl between different samples. Especially the existence of the two types of water that are considered to be from the oil-zone is not easy to explain. It is important that it is found what may be the reason for this as it can teach us something about the distribution of water in the oil-zone of a reservoir. It might be that the negative "free" water is mainly found as a water film around grains, while the "water-in-oil" is found in fine emulsion in the oil. We hope to solve these problems in the next few years.