Seismic Delineation of the Prairie Evaporite Dissolution Edge in South-central Saskatchewan H. Hamid 1, I.B. Morozov 1, and L.K. Kreis Hamid, H., Morozov, I.B., and Kreis, L.K. (2005): Seismic delineation of the Prairie Evaporite dissolution edge in south-central Saskatchewan; in Summary of Investigations 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-8, 11p. Abstract Approximately 330 km of 2-D seismic data were integrated with well log information to improve the delineation of the southern margin of the Middle Devonian Prairie Evaporite in Saskatchewan. Thirteen seismic lines were re- processed with an emphasis on enhancing high-frequency imaging. The resulting seismic sections show marked improvement in the accuracy and quality of subsurface mapping of the Prairie Evaporite salt edges. Seismic data indicate that salt dissolution structures were created by multistage processes. Thickening of overlying strata related to salt dissolution was observed within both salt-free areas and areas of preserved Prairie Evaporite. 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, especially where the salt layer is thin near its margin. Comparisons with seismic interpretations show that interpolation of well data alone using different interpolation techniques can result in shifts in the delineated position of the salt edges of about 2 to 9 km. Therefore, integration of the seismic and well log data should increase the accuracy of delineating the salt edge. An attempt was also made to determine whether the effect of the salt edge could be observed in gravity data. An approximately 33 km long gravity profile close to the seismic line NOR-83314 across a known salt collapse was extracted from the national gravity data base. A 2-D model was designed based on the interpretation of seismic line NOR-83314 and well log data. Gravity modeling shows that the salt collapse contributes ~0.4 mgal to the total anomaly (4 mgal). We suggest that performing a high-resolution gravity survey with a station interval of about 100 m might still be useful to constrain the overburden and help detect salt collapses. Keywords: collapse structures, Devonian, gravity, Prairie Evaporite, salt, salt dissolution, seismic, subsurface mapping, well log, Williston Basin. 1. Introduction Deposits of the Elk Point and Manitoba Groups contain the largest amount of salt in the Western Canada Sedimentary Basin with an approximate volume of salt of about one million cubic kilometres (Zharkov, 1988). Most of the salt is contained within four formations: 1) Lotsberg, 2) Cold Lake, 3) Prairie Evaporite, and 4) Dawson Bay (Meijer Drees, 1986). The Middle Devonian Prairie Evaporite is the most widespread of these deposits and underlies much of southern Saskatchewan (Holter, 1969). Baillie (1953) introduced the term “Prairie Evaporite” for the evaporites found between the Winnipegosis and the Second Red Bed of the Dawson Bay Formation. Subsurface dissolution of the Prairie Evaporite has been reported for over 50 years (Sloss and Laird, 1947; Baillie, 1953; Bishop, 1954). In parts of the study area, located south and south-southeast of Regina (Figure 1), the Prairie Evaporite has been dissolved and removed either partially or completely by groundwater (Holter, 1969). Salt appears to have been dissolved from either the top or bottom of the formation and sometimes from both (Gendzwill and Martin, 1996). The Prairie Evaporite Formation is among the key economic targets in southeastern Saskatchewan. Three main potash members have been recognized within the upper part of the Prairie Evaporite: Esterhazy, Belle Plaine, and Patience Lake (Holter, 1969). Knowledge of the locations and dimensions of salt collapse structures is essential for avoiding mining into hazardous zones and for evaluating hydrocarbon exploration potential. Salt dissolution and the resulting subsidence are important hydrocarbon trapping mechanisms in western Canada (Edmunds, 1980). In the study area, collapse structures may be multistage and located off-salt where all salt has been removed (e.g., the Hummingbird Structure) or on-salt within the Prairie Evaporite itself, such as the Kisbey Structure (Figure 1; 1 University of Saskatchewan, Department of Geological Sciences, 114 Science Place, Saskatoon, SK S7N 5E2; E-mail: haitham.hamid @usask.ca. Saskatchewan Geological Survey 1 Summary of Investigations 2005, Volume 1 Halabura, 1998). Accurate Regina mapping of the salt margin Sawatzky et al. (1960) contributes to the understanding 15 of the process of salt dissolution, Holter (1969) basement control, and their impact on hydrocarbon Sask. Dept. of Mineral Resources (1961) production. Structures created by SE-84506 salt collapse may influence fluid Sa lt edge interpreted migration, reservoir by this study SE-84524 enhancement, and oil SE-84529 10 NOR-93WN3 entrapment. Knowledge of their Kreis et al. (2003) distribution and nature therefore PCR-11CBY-5W - Weyburn Kisbey structure aids hydrocarbon exploration. CBY-7W NOR-83314 The southern edge of salt beds S-793 was defined from earlier 5 composite seismic anomaly maps - 9-NS summarized by Holter (1969). R-19 The precise location of the Prairie Evaporite edge is Hummingbird uncertain as it is irregular and Structure CBS-8 SE-850844 1 complex. The Prairie Evaporite 20 15 10 5 ranges in thickness from 0 to 220 m in the central part of the Figure 1 - Study area in south-central Saskatchewan. The seismic lines used in this Williston Basin. In this study, study are shown in blue; those in red were analyzed in the International Energy which extends the results of Agency Weyburn CO2 Monitoring and Storage Project (Hajnal, pers. comm., 2003). Labelled green and black lines indicate the positions of the Prairie Evaporite edge Hamid et al. (2004), additional from previous studies, and the brown line shows the edge interpreted in the present seismic and well-log data are study. The brown polygons in southern and northeastern areas indicate local salt used to improve the accuracy of dissolution. Note the differences in location of the salt-dissolution edge between subsurface mapping of the previous mapping attempts and that of the present study. Numbers along the edges of southern edge of the Prairie the plot indicate townships and ranges. Evaporite Formation. The potential association of salt dissolution edges and collapse structures with basement faulting is also investigated. 2. Objectives The primary objective of this study was to improve delineation of the dissolution edge along the southeastern edge of the Prairie Evaporite Formation (Figure 1). In its utilization and interpretation of seismic and well datasets, the project ties in with regional 2-D seismic studies (Hajnal, pers. comm., 2003) and subsurface geological mapping (Kreis et al., 2003) conducted as part of the IEA Weyburn CO2 Monitoring and Storage Project. Additional objectives were correlation of areas of salt dissolution with underlying structural features such as basement highs, Winnipegosis mounds, and faults, and to document their potential inter-relationships. Specifically, we aimed to: 1) use the available 2-D seismic data acquired by the oil industry to improve delineation of the southeastern margin of the Prairie Evaporite Formation south of Regina; 2) investigate and develop processing and interpretation techniques to help identify thin salt beds and salt collapses near the dissolution edge and seismically evaluate the underlying strata, with particular attention paid to the Precambrian basement, in an attempt to recognize structural features which may have influenced the location of the present-day salt edge; 3) evaluate the effects of different mapping (spatial interpolation) techniques at locations of salt edges; and 4) investigate the potential use of gravity inversion for delineation of salt edges. 3. Seismic Data The approximately 330 km of seismic reflection lines used in this study were surveyed between 1979 and 1984, and were donated to us by Encana Corporation, Petro-Canada, Olympic Seismic Ltd., and Kary Data Consultants Ltd. Thirteen seismic lines were shot using different recording systems and a variety of dynamite and air-gun sources. Saskatchewan Geological Survey 2 Summary of Investigations 2005, Volume 1 Spread lengths extended mostly from 1.5 to 3 km. Station intervals ranged from 25 to 67 m, and shot intervals from 75 to 200 m. Record lengths were 3 s at 2 ms sampling intervals. a) Seismic-data Processing Field data were completely re-processed from digital raw field records using PROMAX software (®Landmark Graphics). In the study area, the depth of the Prairie Evaporite ranges from 1600 to 2500 m, and the thickness from 0 to 220 m. At these depths, the higher frequencies are significantly attenuated because of the absorption and scattering effects of the earth, and are also contaminated by noise. Therefore, the primary goal of re-processing seismic data was to recover sufficiently high seismic frequencies to identify thin beds and map the salt dissolution edge of the Prairie Evaporite. In order to increase the resolution and consistency of seismic imaging, all data were re-processed in a uniform manner, with an emphasis on high-frequency enhancement. The processing steps are shown in Table 1. The processing steps can be subdivided into five groups: 1) pre-processing includes SEG-Y