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Saskatchewan Geological Survey 1 Summary of Investigations 2004, Volume 1

Progress in Seismic Delineation of the Southern Margin of the Middle Devonian Prairie Evaporite Formation in the Elk Point

Basin, South-central Saskatchewan

H. Hamid 1, I.B. Morozov 1, and L.K. Kreis

Hamid, H., Morozov, I.B., and Kreis, L.K. (2004): Progress in seismic delineation of the southern margin of the Middle Devonian Prairie Evaporite Formation in the Elk Point Basin, south-central Saskatchewan; in Summary of Investigations 2004, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.1, CD-ROM, Paper A-5, 9p.

Abstract Salt dissolution and collapse structures of the Middle Devonian Prairie Evaporite Formation are among the most prominent features of this lithostratigraphic unit in Saskatchewan. Knowledge of the distribution of the collapse structures is critical for avoiding mining into hazardous zones and for evaluating hydrocarbon exploration potential.

The present study focuses on the southern edge of the Prairie Evaporite defined by seismic anomaly maps published by Holter (1969) and subsurface geological mapping based on well data. The edge is complex in shape with uncertainties in its location ranging from one to five kilometres. The purpose of this study is to help increase the accuracy and quality of subsurface mapping by including additional information from well logs and re-processing of old and new seismic data.

Achieving sufficiently high seismic frequencies is the key factor in detection of thin beds and salt dissolution edges of the Prairie Evaporite Formation. To enhance the high frequencies, post-stack spectral whitening was applied followed by F-X deconvolution, resulting in marked improvements in the resolution of thin beds and salt dissolution edges.

In order to evaluate the effects of the basin fill on regional gravity signatures, an approximately 350 km long east-west gravity profile crossing the Trans-Hudson Orogen and the Wyoming Structural Province was extracted and modeled from an interpolated 2-D gravity grid. The transition between the Trans-Hudson Orogen and Wyoming Province was marked by a significant decrease in gravity that was most marked near the eastern margin of a salt dissolution feature. The regional gravity anomalies are dominated by deep-seated structures in the basement, and the contribution of the salt collapse features (2 to 3 mgal) are weak compared to the total anomaly of about 35 mgal. However, information from high-resolution gravity surveys over salt collapses, in conjunction with seismic data, can be useful for detection of salt-dissolution edges.

Keywords: collapse structures, Devonian, gravity, Prairie Evaporite, salt, salt dissolution, seismic, subsurface mapping, well log, Williston Basin.

1. Introduction This research focuses on using presently available 2-D and 3-D seismic and well log data to help improve mapping of the southern edge of the Prairie Evaporite Formation in Saskatchewan. The study area is located south and south-southeast of Regina (Figure 1).

The Middle Devonian Elk Point Group contains the largest volume of salt deposits preserved in the Western Canada Sedimentary Basin. These deposits extend from the northern contiguous United States of America northward for more than 1900 km (1200 miles) to Canada’s Northwest Territories (DeMille et al., 1964). In the study area, the most widely developed of such deposits is the Prairie Evaporite Formation, which is present through much of the Williston Basin region. Its thickness ranges from 0 to about 220 m. One of the most important features related to the Prairie Evaporite is salt collapse as its spatial relationship with deep oil production has been proven in many places, (e.g., the Hummingbird Structure, Figure 1). Structures created by salt collapse may influence fluid migration, oil entrapment, and reservoir enhancement. Accurate mapping of salt collapse is thus necessary for understanding

1 University of Saskatchewan, Department of Geological Sciences, 114 Science Place, Saskatoon, SK S7N 5E2; E-mail: [email protected]


processes such as possible basement control that are associated with salt dissolution, and their impact on hydrocarbon production and potash mining.

From the perspective of potash mining, understanding of salt dissolution – both its processes and its effects – is principally important for hazard mitigation. Salt collapse structures are capable of producing great amounts of water, and may also be associated with water infiltration through fault zones that cut overlying strata. Encountering collapses has resulted in loss of mines due to flooding (Gendzwill and Martin, 1996). Dissolution and removal, either partial or complete, of evaporite beds from any of the Paleozoic salt-bearing units within the Williston Basin may result in fracturing, collapse, and subsidence of overlying strata (Halabura, 1998). Salt dissolution structures in the basin were first recognized by Bishop (1954) and Baillie (1955). They may be multistage (in which case they commonly create complex structural patterns), and/or be located off-salt, and/or affect overlying sediments, (e.g., the Hummingbird Structure, Figure 1). They may alternatively be on-salt (i.e., within the area underlain by the Prairie Evaporite, as at Kisbey, see Figure 1).

Within the study area, salt removal started soon after deposition of the Prairie Evaporite and continues to the present day (Holter, 1969; McTavish and Vigrass, 1987). Two types of salt collapse have been identified, one resulting from salt dissolution that is spatially related to underlying porous and permeable Winnipegosis mounds (Gendzwill, 1978), the other from dissolution related to periodic movement along faults rooted in the Precambrian basement (Holter, 1969; McTavish and Vigrass, 1987). Salt collapse can appear as localized features, as series of dissolution lenses, as channel-like structures, or as dissolution edges. The shape of salt dissolution edges or collapse structures may provide clues as to how they formed (Lefever and Lefever, 1995).

Figure 1 - Study area in south-central Saskatchewan. The seismic lines of this study are shown in blue, and red indicates the seismic lines analyzed by a University of Saskatchewan research group led by Dr. Z. Hajnal as part of the International Energy Agency Weyburn CO2 Monitoring and Storage Project. Labelled contours indicate the positions of the Prairie Evaporite edge interpreted from previous studies. Note the differences in the existing mapping of the southern margin of the Prairie Evaporite Formation (shown in green, from Holter, 1969, and the Saskatchewan Department of Mineral Resources, 1961). Numbers along the edges of the plot indicate townships and ranges. Black dots indicate the preliminary interpreted positions of the salt collapse from seismic lines CBY-5W and NOR-83314.


Various models explaining mechanisms for Prairie Evaporite salt dissolution have been discussed in the literature (e.g., Holter, 1969). All models require that unsaturated fluids come into contact with the salt section and are able to transport saline fluids away from the dissolution site either through regional- or local-scale fluid movements. Many require faulting or fracturing in the Precambrian basement (Sawatsky et al. 1960; Christopher, 1961) as the result of tectonic movements to permit communication with the salt section. Another model invokes dissolution of the salt section by low salinity fluids in areas where it overlies porous and permeable facies of the subjacent Winnipegosis Formations (Gendzwill, 1978). DeMille et al. (1964) considered a model that included both tectonic movements and fluid flow through underlying permeable Winnipegosis strata. Seismic and map data from the present study commonly show the presence of both faulting and Winnipegosis build-ups in areas of salt dissolution.

2. Objectives This project focuses on helping to delineate accurately the dissolution edge along the Prairie Evaporite Formation edge south-southeast of Regina. 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. Our objectives are to correlate areas of salt dissolution with underlying structural features such as basement highs, Winnipegosis mounds and faults, and to document their potential relationships. Specifically, we aim to:

1) use the available 2-D and 3-D seismic data acquired by industry to improve delineation of the Prairie Evaporite Formation edge south-southeast 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 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 on determination of the positions of salt edges; and

4) investigate the usefulness of gravity for delineation of salt edges. Selected preliminary results of this on-going effort follow below.

3. Seismic Imaging Seismic observations are critical for the present study because they provide detailed and continuous coverage for correlation with surface and subsurface geological mapping and well logs. The seismic data used in this project were acquired in 1979 and 1984 and donated by Encana Corporation, Petro-Canada, Simonson, Olympic Seismic Ltd., and Kary Data Consultants Ltd. Thirteen seismic lines had been acquired using different recording systems and a variety of dynamite and air-gun sources. Spread length extended mostly from 1.5 to 3 km, station intervals range from 25 to 67 m, and shot intervals from 125 to 134 m.

Field data were received by the University of Saskatchewan on magnetic tape in SEG-B and SEG-Y formats and completely re-processed using PROMAX software (Landmark Graphics). The general processing procedure is shown in Table 1.

Table 1 - Seismic data processing steps.

Process Purpose

SEG-Y-INPUT Reading the data Geometry Loading the geographic location and elevation of the shot points and receivers Trace Editing Removing bad traces, reversing polarity, and muting Refraction Static Time correction F-K filter Attenuating the ground roll Deconvolution Compressing input pulse, attenuating reverberation NMO correction Horizontal time correction Residual static correction Removing the remaining small travel-time variation caused by inaccurate static correction Radon filter Suppression of multiple reflections Stacking the data Increasing signal-to-noise ratio


In order to achieve good vertical and horizontal resolution of thin structures near salt edges, seismic imaging resolution is critical. This limit on the recoverable frequencies leads to a minimum thickness resolution which is generally estimated as ¼ λ, where λ is the seismic wavelength (Sheriff and Geldart, 1995). In order to improve the resolution, several techniques for enhancement of high-frequency energy were tried. The resulting optimal procedure included a combination of trace equalization (Automatic Gain Control) and F-X deconvolution with time-variant spectral whitening.

An example from seismic line CBY-5W (by Olympic Seismic Ltd., Figure 2) shows that reflection amplitudes near the target depth drop by ~50 dB from 10 Hz to 80 Hz because of the strong absorption and attenuation within the thick sedimentary cover. Without compensation for this attenuation, the dominant frequencies in the final stack are close to 30 Hz, and consequently the ¼ λ vertical resolution (with velocities of about 4475 m/s) is ~37 m (Figure 2).

Salt dissolution causes subsidence of overlying strata, which can be clearly seen in the western part of the seismic section shown in Figure 2. Discontinuities in the seismic reflection events show that several faults exist in the area. Fault offsets are, however, difficult to measure from this section, and internal thin beds of the Prairie Evaporite and the salt edge are not accurately displayed. The internal thin beds of the Prairie Evaporite appear as inter-mingling events between 1260 and 1330 ms.

To recover seismic frequencies that are sufficiently high to allow detection of the faint and compact signatures of the Prairie Evaporite, post-stack spectral whitening followed by F-X deconvolution were applied. Figure 3 shows the migrated stack after using these procedures. In terms of the dominant frequency, this leads to an improvement from ~30 Hz to ~50 Hz at the formation depth, thereby improving the estimated depth resolution to ~23 m. The resulting stacked section (Figure 3) shows a marked improvement in the detail and continuity of the image, and generally recognizable features include: 1) internal thin beds within the Prairie Evaporite; 2) a more accurately delineated salt edge; and 3) clearer fault displacements that can be measured more accurately from the seismic events.

Stratigraphic details suggest several deep and shallow faults with basement uplift, with some deep faults rooted in the basement and extending to the surface. In the upper part of the seismic sections, shallow faults that penetrate into the Devonian strata have been identified. As Holter (1969) suggested, such basement uplifts and faults could allow water to circulate through the salt and control salt dissolution. Our data (Figure 4) also suggest that the locations and the origins of the salt dissolution areas could be associated with basement faults.

In the study area, a close spatial relationship is noticed between the basement features, Precambrian tectonic boundaries, and salt collapses. Line CBY-5W shows the Prairie Evaporite Formation decrease in thickness from approximately 110 m in the east to zero near the

Figure 2 - Final stacked section from line CBY-5W (by Olympic Seismic Ltd.) without spectral whitening applied. Note the slumping of the strata caused by salt dissolution in the western part of the section. Also note that the depth resolution of the image is low. P.E., Prairie Evaporite.

Figure 3 - Final stacked section from line CBY-5W with F-X deconvolution and post-stack spectral whitening applied. The internal thin beds of the Prairie Evaporite are more visible and are thinning toward the west. Note the interpreted salt collapse that perturbed the strata beneath the Bakken Formation. P.E., Prairie Evaporite.


western end. Note the depression in the reflecting horizons that could be caused by salt dissolution (Figure 4). Reflections between the Prairie Evaporite and Bakken formations show depression, whereas above the Bakken layers between ~1180 and ~980 ms thicken, and reflections shallower than 980 ms are relatively flat. Dissolution apparently took place during the Late Devonian. A more accurate estimate of thickening will be made from log data as the next step of this project. In contrast, seismic line NOR-83314 (Figure 5) shows a similar depression in the Prairie Evaporite that affects all the reflecting horizons above it, with no thickened layers present. This indicates

that salt collapse occurred more recently than that shown in line CBY-5W.

4. Sub-surface Map Interpolation Subsurface mapping of lithological units and their geographic limits is a complex procedure involving integration of point data such as formation tops interpreted from geophysical well logs with the available seismic data. Together, these data improve the accuracy of the map. The precision of the interpolation is dependent upon the method used and the data available, and is particularly critical where the data coverage is low and where we have to rely on data extrapolation, such as the salt edges of this study. We examined this variability by using well picks and maps of Middle Devonian strata in the IEA Weyburn CO2 Monitoring and Storage Project area (Kreis et al., 2003). Isopach maps of the Prairie Evaporite formation generated in Surfer, Matlab and Generic Mapping Tools (GMT) products were compared to evaluate the dependence of isopach contours on the interpolation techniques. After several experiments, GMT was chosen as the preferred option as it has two significant advantages:

1) the interpolation methods are published (Smith and Wessel, 1990), the code is open-source, and thus the details of algorithms can be understood and adjusted if necessary; and

2) GMT programs offer a choice of “spline tension” parameter T, with tighter splines resulting in smoother maps.

To assess the sensitivity of interpretation to the choice of T, several values of spline tension parameter T were used (Figure 6). The resulting maps show marked differences, especially near the edge of the salt where its thickness is small. GMT maps are similar yet differ in detail from the results of Matlab and Surfer. These differences, and particularly their implications for imaging salt collapses, will be evaluated in future work. The map with parameter T=0 was chosen as the preferred option because it shows data trends with minimal “bull’s eye” artefacts around isolated data points. The positions of the salt edge using the different interpolation techniques are shown in Figure 3. Note the uncertainties of about 1 to 5 km between the seismic interpretation and different mapping techniques.

5. Gravity Basement rocks of the Williston Basin area belong to three major tectonic regions, which are, from east to west: the Archean Superior Province, the Trans-

Figure 4 - Salt-dissolution induced subsidence that did not affect strata shallower than about 980 ms (reflection line CBY-5W). P.E., Prairie Evaporite.

Figure 5 - Effect of a salt collapse on seismic events (Line NOR-83314). Note that, compared to the collapse feature shown in Figure 4, this collapse structure must have formed more recently as it disturbs all of the overlying strata. P.E., Prairie Evaporite.


Hudson Orogen (a Proterozoic collision belt), and the Wyoming Province. In the present study, an attempt is being made to explore basement features and to investigate the nature of the tectonic boundary between the Trans-Hudson Orogen and the Wyoming Province using gravity data available from the Geological Survey of Canada. From the interpolated 2-D gravity grid, an ~350 km east-west profile across the Williston Basin was extracted (Figures 7 and 8). It traverses a large segment of the northeastern Williston Basin, and crosses a known salt-collapse feature and the Wyoming–Trans-Hudson boundary. From borehole data, basement depths along the profile range from 2000 to 2300 m.

The Bouguer gravity profile (Figure 8) shows that the gravity field decreases from about -20 mgal in the east to -55 mgal in the west, an approximately 35 mgal change over its total length. Although at this scale the salt collapse contributes very little (2 to 3 mgal) to the amplitude of the Bouguer anomaly, gravity modeling is still important from two perspectives: 1) to gain understanding of the regional architecture of the basement of the Williston Basin and its relation to salt deposition and dissolution, and 2) to evaluate whether additional, very high-resolution gravity studies could be useful in identification and delineation of salt collapses.

The gravity map across the study area is characterized by two groups of anomalies:

1) a broad, high-gravity anomaly in the east (over the Trans-Hudson Orogen), and a low-gravity anomaly occupying the western half of the study area (Wyoming Province); generally the

density within the basement decreases from the east (2.82 to 3.03 g/cm3 within the Trans-Hudson Orogen) to the west (2.73 g/cm3 for the granitic complex of the Wyoming Craton) (Leclair et al., 1993, 1994); and

2) localized, low- and high-amplitude anomalies (labelled L and H, respectively, in Figure 8) are superimposed on the first group; their amplitudes do not exceed 10 mgal and they have widths of ~15 to 16 km. One anomaly of this kind is in the Trans-Hudson Orogen, and another is in the Wyoming Province.

Figure 6 - Interpolated isopach maps of the Prairie Evaporite Formation using GMT programs (Smith and Wessel, 1990) with two values for spline tension parameter T, as labelled. T typically ranges from 0 to 1, with larger values corresponding to smoother interpolations. Note the differences between maps using different interpolation parameters. Coordinates are UTM in kilometres, black dots indicate the wells used for mapping.


Preliminary 2-D modeling of the gravity data was performed using the GM-SYS software. A polygonal gravity model was designed based on known lithologies, deep boreholes, densities, previous studies and nearby seismic work. The accuracy of this modeling depends on assumptions such as: 1) the density of the Williston Basin sedimentary rocks (2.61 g/cm3) and 2) the density of basement rocks. However, it is unlikely that variations of these properties affect the first-order observations outlined below.

a) Trans-Hudson Anomaly This anomaly has a width of about 200 km and extends across several lithotectonic units, represented by polygons 2 through 5 in our gravity model (Figure 8). The approximate locations of these features were taken from the magnetic map of Saskatchewan (Miles et al., 1997).

The easternmost polygon represents the granulitic Pikwitonei Gneiss unit, which is marked by high gravity corresponding to density of 2.93 g/cm3. Polygons 2, 3, 4, and 5 in our model are assumed to be the extension of the Flin Flon Domain (characterized by low metamorphic grade, mafic volcanic and granitoid rocks with density ranging between 2.8 and 2.93 g/cm3). The contact between Pikwitonei and Flin Flon lithotectonic units (polygons 1 and 2) dips west, and the regional gravity field was found to decrease at a rate of 0.3 mgal/km toward the west. Bodies 3 and 5 are separated by a narrow gravity low about 15 km wide labelled L in Figure 8 (top). This anomaly is associated with a low-density polygon 4 that could be related to the extension of the

Figure 7 - Bouguer anomaly map of Saskatchewan obtained using GMT (minimum-curvature spline with T=0) interpolation of gravity readings from the Geological Survey of Canada database. Dark blue and red lines show the edge of the Prairie Evaporite interpreted by Holter (1969) and Kreis et al. (2003), respectively (Figure 1). The seismic lines of this study are shown in yellow. A-A’ is the line of the gravity cross-section extracted from the data and modeled in Figure 8. The major tectonic domains are indicated. Coordinates are UTM in kilometres.

Figure 8 - Gravity model along the cross-section A-A’ in Figure 7. Note that the regional gravity anomalies require deep-rooted (~6 to 7 km) density (D) variations within the basement. L and H indicate the localized gravity low and high, respectively, referred to in the text.


Tabbernor Fault Zone, which is marked by a steep magnetic gradient in the centre of the Trans-Hudson Orogen (Miles et al., 1997).

b) Wyoming Anomaly The crustal transition from the Trans-Hudson Orogen to the Wyoming Province is marked by a sharply decreasing gravity field (0.6 mgal/km) that is spatially coincident with the eastern margin of the Hummingbird Trough, a salt-dissolution area. The Wyoming anomaly is characterized by the presence of a complex anomaly represented by polygon 6 in our model (Figure 8). This block may correspond to a granitic intrusion with density of 2.73 g/cm3 located beneath the Hummingbird Trough (Figures 1 and 7). The positive anomaly west of this possible intrusion represents basement rocks with a density of 2.85 g/cm3 positioned beneath the Roncott Anticlinorium (polygon 7). Farther west, the gravity field is represented by alternating gravity lows (polygons 8 and 10) and highs (polygon 9) with small amplitudes (~5 mgal) and widths of ~16 km. These features are generally associated with the granitic complex of the Wyoming Province.

Gravity modeling indicates that the observed regional gravity pattern is dominantly affected by deep-seated structures (about 6 to 7 km deep). Gravity signatures of salt collapse structures are significantly weaker and therefore almost irresolvable at this scale. However, performing high-resolution gravity surveys with station intervals ~100 m or less apart might still be useful for constraining the overburden and help detect salt collapses.

6. Conclusions 1) Spatial interpolation of well picks used in subsurface mapping results in uncertainties of the positions of salt

edges of from 1 to 5 km. 2) Preliminary interpretations of the positions of the Prairie Evaporite edge in the seismic lines processed to date

generally agree with the salt edge mapped by Holter (1969). 3) Seismic sections, even those recorded in the 1970s, contribute critical information to help accurately locate salt

dissolution edges. 4) High-frequency enhancement seismic data processing techniques help improve the resolution and result in

better quality images of salt edges and thin beds. 5) In the seismic sections analyzed to date, indications of a Late Devonian and a more recent salt collapse event

have been observed. 6) Gravity anomalies within the region are mostly related to lateral variations between deep-seated structures

within the basement. High-resolution gravity surveying with station spacing of ~100 m, in combination with seismic imaging, are likely to be useful in detecting salt collapses in study area.

7. Acknowledgments This research was made possible through a grant from the Saskatchewan Industry and Resources to the University of Saskatchewan. We thank Drs. Z. Hajnal and D. Gendzwill for many valuable discussions, advice, and critical reviews of this manuscript. Dr. Hajnal’s support was also critical in obtaining the seismic data and formulating the initial goals of the project. We are grateful to J. Closson and Dr. B. Pandit for their help with data processing, and to S. Sule for his advice on interpretation. Careful editing and comments by Dr. C. Gilboy have greatly improved the manuscript. This work was facilitated by software grants from Landmark Graphics Corporation, Schlumberger Limited, and Hampson-Russell Limited. GMT programs (Wessel and Smith, 1995) were used in preparation of some of the illustrations.

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