Seismic delineation of the southern margin of the middle Devonian prairie evaporite in the Elk Point Basin, south-central Saskatchewan

Abstract

The present study focuses on delineation of the southern edge of the middle Devonian Prairie Evaporite (PE) in south-central Saskatchewan. The purpose of this work was to improve the accuracy and resolution of subsurface mapping by including additional information from well logs and seismic data not included in the previous studies.Approximately 330 km of 2-D seismic data were integrated with horizon picks from 1334 well logs to improve the delineation of the southern margin of the PE. Thirteen seismic lines were re-processed with an emphasis on high-frequency imaging. The resulting seismic sections show marked improvement in the accuracy and resolution of mapping of the PE salt edges, with the estimated depth resolution improved to ~15 m. Seismic data indicate that salt dissolution structures were created by multistage processes. Salt collapses were identified within the body of the Prairie Evaporite and off-salt.Well log data were combined with seismic results and gridded to create an updated map of the Prairie Evaporite. Different gridding methods provided different interpolations of the data set, particularly where the salt layer is thin near its margin. Incorporation of seismic interpretations resulted in 2-9 km changes in the positions of the salt edges derived earlier from well log and limited seismic interpretations. Therefore, integration of the seismic and well log data should increase the accuracy of the positions of the salt edge.In order to evaluate the effects of the basin fill on regional gravity signatures and to determine whether the effect of the salt edge could be observed in gravity data, two gravity profiles crossing the salt collapse margin and the Trans-Hudson Orogen and the Wyoming Structural Province were analysed. Regional-scale gravity modeling showed that the transition from the Trans-Hudson Orogen to Wyoming Province was marked by deep-seated structures within the basemen. Detailed gravity modeling of a shorter profile well-constrained by seismic data showed that the salt collapses contribute ~0.4 mgal to the total anomaly of about 4 mgal. Although a direct observation of salt edge by gravity appears hardly feasible, performing high-resolution gravity survey with station interval ~100 m might still be useful to constrain the overburden and thereby help detect salt collapses.