EGGENKAMP, H. G. M., University of Utrecht, Netherlands, and M. L. COLEMAN, BP Research Centre, Sunbury-on-Thames, England, and University of Reading, England

ABSTRACT: Extreme (delta){37}Cl Variations in Formation Water and its Possible Relation to the Migration from Source to Trap

Stable chlorine isotopes are very conservative tracers, their values almost unchanged by chemical reactions making them excellent indicators of physical processes. Despite availability of methods for measurement for a number of years, only small variations have been described. We have measured recently the largest ever ranges of values of (delta){37}Cl recorded for waters. In Forties Field (North Sea) and Westland (Netherlands) there is a positive correlation between Cl content and (delta){37}Cl, while Paris basin (France) shows a negative correlation. In all cases, (delta){37}Cl ranges from near zero to negative values (-4.3 o/oo, -1.8 o/oo, and -1.9 o/oo for Forties, Westland, and Paris basin, respectively).

Forties data are interpreted as mixtures between saline formation water, resulting from dissolution of deep Zechstein evaporite, and a less saline component. Diffusion from shale (probably oil source rock) caused negative values in the dilute brine, but the exact process cannot be defined yet. Aqueous fluids would follow the same migration paths as petroleum. Although in a different environment, the Westland samples resulted from a similar process.

Waters in the Paris basin Upper Keuper sandstone reservoir facies again are mixtures but on basin-scale dimensions. The chloride source for both components is underlying Keuper halite, separated from the reservoir by shale. Basin-margin fluids are dilute brines, originally meteoric water, which probably accessed the salt via basin-margin faults. The concentrated brine of the basin center probably was overpressured, and its negative values resulted either from ultrafiltration or diffusion during cross-formational flow.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.