VARIATIONS OF THE δ81BR AND δ37CL STABLE ISOTOPE SIGNATURE FOR MISSISSIPPIAN TO CAMBRIAN FORMATION BRINES OF THE WILLISTON BASIN

Understanding fluid transport in deep basinal aquifers is important because it plays a key role in many geologic processes including diagenesis, hydrocarbon migration and ore deposit formation. Over the last decade, renewed interest in deep hydrocarbon exploration within the basin has provided opportunities to sample deep formation waters that were not previously accessible. The Williston Basin is a roughly circular sedimentary basin covering approximately 518,000 km² in the interior of the North American Continent.

Brine samples were collected from and analyzed from the central part of the Williston Basin from Mississippian to Cambrian formations. The formations range in age between 330 million years (Mississippian) and 500 million years (Cambrian). Two new methodologies have been developed for chlorine and bromine stable isotope analyses using Continuous Flow Isotope ratio Mass Spectrometry (CF-IRMS). These new methodologies are being applied to determine the δ⁸¹Br and δ³⁷Cl signatures of the formation waters. Chemical composition and traditional isotopes (²H, ¹⁸O) were also determined.

The formation brines examined in this study are predominantly either Na-Cl or Ca-Cl type brines. These brines are characterized by high TDS values that range between 100 and 500 g/L. δ⁸¹Br values obtained for these formation waters ranged between -1.50‰ and +2.83‰ relative to SMOB. The δ³⁷Cl values ranged between -0.74‰ and +0.47‰ relative to SMOC. The Upper Ordovician, Yeoman, formation waters showed the most depleted values for both δ⁸¹Br and δ³⁷Cl, while the Upper Devonian, Bakken, formation waters showed the most enriched values for both isotopes. The comparison of the δ⁸¹Br versus time provides a curve that suggests a systematic temporal variation of seawaterδ⁸¹Br signature. The comparison between the obtained δ³⁷Cl values versus time also illustrates a similar trend. The temporal variations for the δ⁸¹Br values are more pronounced in comparison to the variations in the δ³⁷Cl values.