Chlorine Isotope Fractionation During Aqueous Fluid Transport

Abstract

Some water co-produced with oil has chemical and isotopic compositions resulting from its migration from mudrocks. In some fields chemical compositions of produced waters may change systematically with time after a well comes on stream and as the water-cut (proportion of water in the total produced fluid) increases. In Forties field (Central North Sea) the variation resulted from an increased proportion of aquifer mixed with oil zone water (Thurlow and Coleman, 1997). As oil is produced from the reservoir, reduction of pressure, localised to the zone around the perforations at the well bottom, draws up a cone of aquifer water and increases watercut as it mixes with oil-zone water. In Forties oilzone water is only 40% of the salinity of the aquifer. We developed a method for separating trace water from oil phase which has no visible co-produced water. It is oil-zone water and, in many fields, its composition is noticeably different from aquifer composition (Thurlow and Coleman, 1997). Here we explore the reasons why many oil-zone waters are characterised by unusual, extremely-fractionated chlorine isotope values. Chlorine stable isotope geochemistry. Chlorine and bromine are relatively un-reactive at the temperatures expected in most sedimentary environments. Thus, they are considered as conservative tracers of solute movement and mixing processes. C1 is measured relative to ocean chloride, which shows no variation within analytical error (approx 0.05%o 1 ~). Chlorine isotope compositions were measured using standard techniques: precipitation as AgC1, conversion to CH3C1, and stable isotope ratio mass spectrometry (Eggenkamp, 1994). Unlike other light stable isotopic elements, chlorine shows relatively little natural variation. Evaporites of all ages show the same limited range of values (~37C1, approx. 0_+0.5%0), so marine C1 has not varied during the Phanerozoic. Measurement of C1 isotope compositions of oilfield samples showed that many have extremely fractionated values very different from seawater. Postgraduate Research Institute For Sedimentology, The University of Reading, Reading RG6 6AB UK