Saskatchewan Geological Survey 1 Summary of Investigations 2007, Volume 1 An Enigmatic Interval Rich in Sand-size Quartz Particles in the Mississippian of Southeastern Saskatchewan: A Possible Solution to Its Origin Based on Sequence Stratigraphic Principles D.M. Kent 1 Kent, D.M. (2007): An enigmatic interval rich in sand-size quartz particles in the Mississippian of southeastern Saskatchewan: a possible solution to its origin based on sequence stratigraphic principles; in Summary of Investigations 2007, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2007-4.1, CD-ROM, Paper A-5, 16p. Abstract The Kisbey interval includes beds of quartz sandstone, interbedded quartz sandstone and dolomicrite, and interbedded quartz sandstone, anhydrite, and allochemic grainstones. The quartz sand accumulations range in thickness from <1 to 30 m. In places, two thick quartz sand bodies separated by a thin carbonate may total 50 m in thickness. The quartz sand grains are commonly very fine to fine grained, angular to subangular, and well sorted. These, along with small amounts of feldspar and microcrystalline quartz, are supported by a matrix of 5 to 20% interstitial dolomicrite. The sedimentary structures in some of the quartz sandstones along with the ichnofauna and ubiquitous interstitial dolomicrite together suggest that these deposits are a result of marine sedimentation. Not all the rocks of the Kisbey interval, however, are marine in origin. In the extreme eastern part of the study area, sabkha deposits are present as thin sandstones associated with purplish red and moderate reddish brown argillaceous dolostone, dolomitic mudrock, and nodular and vertically elongate anhydrite. In addition, at least one cored interval is interpreted to represent a coastal dune quartz sand deposit. The model for the deposition of the Kisbey interval presented here is based on a sequence stratigraphic approach and includes both lowstand and transgressive sequences. Initial uplift and weathering of the Alida bedrock during a humid climate period resulted in the formation of karst features. Concomitant with this was the creation of a lowstand carbonate factory along the newly formed coastline. A change to arid conditions led to eolian transport of quartz sands into the marine environment causing vertical and lateral interlayering of quartz sand and carbonates. When the transgressive sequence subsequently commenced, the quartz-sand accumulations were reworked and distributed over a broader area by bottom currents and longshore drift. Prior to the flooding of the weathered Alida bedrock, sabkha and salina conditions developed along the coastline so that supratidal evaporite minerals formed in the interstices of the weathered rock, covered quartz sand, and supplied abundant anhydrite to infill karsted caverns and the pores of the sands along the shoreline. This model is of economic importance because it permits prediction of the areal distribution of facies which have low susceptibility to diagenesis and plugging by anhydrite. The model predicts that the anhydrite plugging of sands and carbonates occurred in the nearshore environment which was most influenced by the sabkha-like highstand at the end of Kisbey deposition. The reservoir capabilities of the sandstone are best developed close to the sub- Mesozoic subcrop, suggesting that diagenetic fluids associated with the subcrop may have enhanced the reservoir quality of these sandstones. Keywords: Mississippian, Kisbey, quartz sandstone, allochemic grainstones, dolomicrite, lowstand, transgression, coastal dunes, exposed bedrock, sheet sands, reservoir, diagenesis. 1. Introduction In some areas in the northeastern Williston Basin, the Mississippian Mission Canyon succession contains quartz sand accumulations as either a single sand body or several vertically stacked sand bodies separated by carbonate layers. The original stratigraphic nomenclature for this interval was developed in the Kisbey area of southeastern Saskatchewan where only one sand body, designated as the “Kisbey Sandstone” (Fuller, 1956), occurs. The discovery of multiple sandy beds over a much larger area has resulted in problems in stratigraphic correlation and nomenclature. This paper proposes a modified stratigraphic nomenclature that is based on the application of sequence stratigraphic principles to the interpretation of the timing of deposition of the various quartz-bearing beds and the settings in which they were deposited. This information will facilitate exploration for potential hydrocarbon 1 D.M. Kent Consulting Geologist Ltd., 86 Metcalfe Road, Regina, SK S4V 0H8.
Transcript
Saskatchewan Geological Survey 1 Summary of Investigations 2007, Volume 1
An Enigmatic Interval Rich in Sand-size Quartz Particles in the Mississippian of Southeastern Saskatchewan: A Possible Solution to
Its Origin Based on Sequence Stratigraphic Principles
D.M. Kent 1
Kent, D.M. (2007): An enigmatic interval rich in sand-size quartz particles in the Mississippian of southeastern Saskatchewan: a possible solution to its origin based on sequence stratigraphic principles; in Summary of Investigations 2007, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2007-4.1, CD-ROM, Paper A-5, 16p.
Abstract The Kisbey interval includes beds of quartz sandstone, interbedded quartz sandstone and dolomicrite, and interbedded quartz sandstone, anhydrite, and allochemic grainstones. The quartz sand accumulations range in thickness from <1 to 30 m. In places, two thick quartz sand bodies separated by a thin carbonate may total 50 m in thickness. The quartz sand grains are commonly very fine to fine grained, angular to subangular, and well sorted. These, along with small amounts of feldspar and microcrystalline quartz, are supported by a matrix of 5 to 20% interstitial dolomicrite. The sedimentary structures in some of the quartz sandstones along with the ichnofauna and ubiquitous interstitial dolomicrite together suggest that these deposits are a result of marine sedimentation. Not all the rocks of the Kisbey interval, however, are marine in origin. In the extreme eastern part of the study area, sabkha deposits are present as thin sandstones associated with purplish red and moderate reddish brown argillaceous dolostone, dolomitic mudrock, and nodular and vertically elongate anhydrite. In addition, at least one cored interval is interpreted to represent a coastal dune quartz sand deposit.
The model for the deposition of the Kisbey interval presented here is based on a sequence stratigraphic approach and includes both lowstand and transgressive sequences. Initial uplift and weathering of the Alida bedrock during a humid climate period resulted in the formation of karst features. Concomitant with this was the creation of a lowstand carbonate factory along the newly formed coastline. A change to arid conditions led to eolian transport of quartz sands into the marine environment causing vertical and lateral interlayering of quartz sand and carbonates. When the transgressive sequence subsequently commenced, the quartz-sand accumulations were reworked and distributed over a broader area by bottom currents and longshore drift. Prior to the flooding of the weathered Alida bedrock, sabkha and salina conditions developed along the coastline so that supratidal evaporite minerals formed in the interstices of the weathered rock, covered quartz sand, and supplied abundant anhydrite to infill karsted caverns and the pores of the sands along the shoreline.
This model is of economic importance because it permits prediction of the areal distribution of facies which have low susceptibility to diagenesis and plugging by anhydrite. The model predicts that the anhydrite plugging of sands and carbonates occurred in the nearshore environment which was most influenced by the sabkha-like highstand at the end of Kisbey deposition. The reservoir capabilities of the sandstone are best developed close to the sub-Mesozoic subcrop, suggesting that diagenetic fluids associated with the subcrop may have enhanced the reservoir quality of these sandstones.
1. Introduction In some areas in the northeastern Williston Basin, the Mississippian Mission Canyon succession contains quartz sand accumulations as either a single sand body or several vertically stacked sand bodies separated by carbonate layers. The original stratigraphic nomenclature for this interval was developed in the Kisbey area of southeastern Saskatchewan where only one sand body, designated as the “Kisbey Sandstone” (Fuller, 1956), occurs. The discovery of multiple sandy beds over a much larger area has resulted in problems in stratigraphic correlation and nomenclature. This paper proposes a modified stratigraphic nomenclature that is based on the application of sequence stratigraphic principles to the interpretation of the timing of deposition of the various quartz-bearing beds and the settings in which they were deposited. This information will facilitate exploration for potential hydrocarbon
1 D.M. Kent Consulting Geologist Ltd., 86 Metcalfe Road, Regina, SK S4V 0H8.
Saskatchewan Geological Survey 2 Summary of Investigations 2007, Volume 1
occurrences within these and adjacent strata in the study area (Figure 1). Unless otherwise indicated, lithological descriptions presented in this paper are based on the author’s observations made during his involvement in two regional multidisciplinary geoscience studies, the IEA Weyburn CO2 Monitoring and Storage Project and the Targeted Geoscience Initiative Phase 2 Williston Basin Architecture and Hydrocarbon Potential Project.
2. Stratigraphic Nomenclature Fuller (1956) identified an individual sand body in the Kisbey area of southeastern Saskatchewan and named it the Kisbey Sandstone. Edie (1958) recognized two separate sand bodies and identified them as 1st Kisbey and 2nd Kisbey and correlated the former from the Weyburn area to Gainsborough in the extreme southeastern corner of the province. Fuzesy (1960) realized that there were additional quartz sand occurrences at various stratigraphic levels, but followed Edie’s regional correlation and used the 1st Kisbey to separate the underlying carbonate rocks assigned to the Alida Beds from the superjacent carbonates of the Frobisher Beds. Employing Edie’s 1st Kisbey as the
Figure 1 - Location map showing the outline of the Williston Basin (grey shading) and the area of study (green shading).
Lake Athabasca
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Saskatchewan Geological Survey 3 Summary of Investigations 2007, Volume 1
marker separating Frobisher Beds from Alida Beds has led to the inclusion of underlying sand-bearing bodies into the Alida, and of overlying sand-bearing bodies into the Frobisher with general confusion about which body is the Kisbey. Legault (1999) proposed a solution by employing K1, K2, and K3 marker-bed terminology that had been applied to quartz sand bodies in the North Dakota portion of the Williston Basin with the K2 marker bed being the equivalent of the 1st Kisbey (Carlson and LeFever, 1987). Correlation of the different marker beds is, however, difficult in many areas, so predicting the reservoir potential of these strata is challenging.
It is proposed here that all beds containing quartz sand or silt be included in a unit named the “Kisbey interval”. In wells in which quartz grains have been identified in core and/or from the PE curve on the compensated neutron lithology density log, the base of this unit is defined as the base of the lowermost sandstone or carbonate bed containing sand- or silt-sized quartz grains, and the top of the unit is defined as the top of the uppermost sandstone or carbonate bed in which sand- or silt-sized quartz grains are present (Figures 2a and 3). These lower and upper beds are also commonly characterized by a higher gamma-ray response than the underlying Alida Beds and the overlying Frobisher (Figures 2a and 3). Contacts with the Alida and Frobisher Beds may be either disconformable or conformable.
A higher gamma-ray response also characterizes the Kisbey interval in wells in which no quartz grains have been identified (e.g., 16-7-4-33W1, Figure 3). In these cases, the base of the Kisbey is defined by the base of the lowermost higher gamma-ray response and the top of the unit by the top of the uppermost higher gamma-ray response . Because of the limitations of interpreting the presence of quartz grains within a carbonate unit on geophysical logs, the thickness of the Kisbey interval identified in core may differ somewhat from that interpreted from geophysical logs.
Figure 4 shows the area in which the Kisbey interval can be recognized on geophysical logs and in core. The quartz-bearing Kisbey interval is well documented through cores and neutron-density log interpretation in the area from Rge 31W1 to Rge 5W2. The presence of quartz sandstone in the 12 to 20 m thick areas shown in Figure 4 has been interpreted by the writer from the PE factor on neutron-density logs. In addition to observations made by Perras (1990) and Howard (2000), the writer has identified quartz-bearing beds from core and neutron-density logs in wells along the erosional edge as far west as Rge 15W2. To the west of Rge 15W2, the Kisbey interval is composed primarily of argillaceous dolostone.
3. Lithological Composition of Underlying and Overlying Strata The lithological character of the underlying Alida Beds varies with geographic location. In the area between the Manitoba border and Rge 2W2, the Alida Beds are dominated by three lithologies, a thin basal coated-grain unit, a medial partially dolomitized skeletal micrite, and an upper coated-grain interval (Kent et al., 1988; Vigrass et al., 1994). The coated grains include ooids, pisoids, and oncolites ranging from grainstones to wackestones. In the upper interval, they appear to be concentrated in linear bodies that have southwest axial trends. Where the coated-grain grainstones to wackestones subcrop at the sub-Mesozoic unconformity, they commonly form positive paleotopographic features (Kent et al., 1988; Vigrass et al., 1994). Where no drill cores are available, the presence of the coated-grain build-ups can be interpreted from their gamma-ray log signatures, which are recessed and relatively subdued (e.g., 5-11-3-34W1, 12-27-1-34W1, and 14-1-3-33W1 on cross-sections B-B' and C-C', Figures 5 and 6). These intervals are mostly 15 to 20 m thick on gamma-ray logs. The lower coated-grain layer rarely exceeds 3 m in thickness. The dotted lines on cross-sections A-A' (Figure 3), B-B' (Figure 5), and C-C' (Figure 6) outline the medial micritic rocks. They are texturally dominated by wackestones, but, in places, have sufficient proportions of allochems to reach the packstone textural category. The skeletal components include abundant crinoid columnals, as well as brachiopod valve fragments, rugose horn corals, and the tabulate coral, Syringopora. Thin sections show that ramose bryozoan fragments, kamaenid algae, and foraminifera are also present (Kent et al., 1988; Mundy and Roulson, 1998). The proportion of dolomite rhombs in the matrix as reported by Mundy and Roulson (1998) for the Pheasant Rump Pool is between 6 and 32% and is rarely sufficiently high to suppress vigorous efflorescence when 10% hydrochloric acid is applied to the surface of a specimen of core. Mundy and Roulson (1998) suggest that an additional 26 to 38% of the matrix may originally have been dolomite, but may have been dedolomitized. The micritic rocks can be recognized on gamma-ray logs by an increase of about 20 API units above that of the normal carbonate signature. These rocks were likely deposited in a distal ramp setting just below fair-weather wave base. West of Rge 2W2, drill cores from the Alida Beds are rare, and the rocks appear to be predominantly lime mudstones and wackestones, suggesting deposition in a more distal ramp setting.
The lowermost unit of the Frobisher Beds overlying the Kisbey interval is most commonly composed of coated grains in textural proportions from wackestone to grainstone as demonstrated in lithologs of wells 12-13-4-34W1 (Figure 7), 6-28-3-31W1 (Figure 8), and 9-29-3-31W1 (Figure 9). The coated grains are ooids, pisoids, and oncolites; but, in places, gastropod shells, brachiopod valve fragments, and crinoid columnals are interspersed with the non-skeletal allochems. An exception to this normal succession occurs in the Silverton–East Silverton area where Kent (2004) has shown that the uppermost quartz sandstone bed of the Kisbey interval is overlain by silty
Saskatchewan Geological Survey 4 Summary of Investigations 2007, Volume 1
Figure 2 - Reference well Provident Hastings E 131/6-2-4-33W1. a) Stratigraphic nomenclature used in this paper is shown together with gamma-ray and photoelectric log curves; also shown is the cored interval described in Figure 2b. b) Columnar lithological log of the cored portion of Kisbey interval shown in Figure 2a; the scale at the top of the lithologic column, ranging from mudstone at the left to grainstone at the right, is based on a modification of the Dunham carbonate rock classification (i.e., the same proportions of interstitial filler to sand-size particles used in the Dunham classification are applied to the quartz particle-bearing rocks such that a rock with 95% quartz particles and 5% interstitial filler is shown as equivalent to a grainstone); scales to the right of the lithologic column are crystal sizes in micrometres and particle sizes based on a standard grain-size chart.
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Pseudo - ooids with quartz grain nuclei. Sharp convex downward contact, emphasized by solution seamfLvfU-fU
vfU-fU
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fU-mL
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Faint horizontal layering
Clasts - well rounded dolomicrite.Quartz grains subangular to subrounded, frosted, and vitreous. Calcareous.
Quartz grains subangular to subrounded, frosted and vitreous. Calcareous.
Quartz grains subangular to subrounded, frosted and vitreous
Sharp, flat contact, 20-40 mm dolomicrite clasts above contact.Flat, elongate, and irregularly shaped dolomicrite clast, 5-20 mm.
Very pale pink staining, frosted, and vitreous quartz grains. Sharp irregular lower contact, abundantelongate dolomicrite clasts in basal layer immediately above contact.
M
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Horizontal to slightly dipping poorly defined layering,subangular to subrounded, frosted andvitreous quartz grains, also about 10% peloids. Recognizable lower contact.
Foreset crossbedding apparent dips 12-20 ,o o
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Swaley crossbedding. 5-25 mm discoid dolomicrite clasts.
Dolomitized peloids with finely disseminated organic material imparting low-angle crossbedded appearanceFDolomicrite matrix with poorly defined layering, limonitic staining with streaks ofpurplish red. Scattered clasts of dolomicrite 10-35 mm long.Dolomicrite - purplish red, laminated to thinly bedded
Silt-fine sand
Silt-fine sand
Dolomicrite matrix, purplish-red patterns.VfL-fL
Dolomicrite matrix, pale pinkish red staining in part, -type burrowsThalassinoidesSilt-vfine sand
Dolomicrite matrix, pale yellowish grey, , , and Zoophycus Paleophycus Planolites Glossifungites
vfineLayering in lower 0.1 m emphasized by organic material
Layering recognizable through alternations in coated-grain sizes
Dolomicrite matrix clasts and thin beds. Glossifungites borings in some dolomicrite layers. Light purplish red staining, particularly peloids and rims of skeletal particles.
Low angle (5 -10 ) crossbeddingo o
vfU-fL
VfU-vUSharp sloping lower contact, 6 cm of relief. Abundant solution seamsimmediately above contact over 17 cm interval. Solution seam emphasizing contact.
Dolomicrite matrix. Sharp lower contact emphasized by solution seam.Solution seams dispersed through interval
Slightly friable91.8
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SymbolsBrachiopodBryozoaCrinoid
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Horn CoralOoidsPisoidsPeloids
OncolitesIntraclastsGlossifungitesPlanolites
ThalassinoidesZoophycusQuartz Sand/Silt
CrossbeddingDisseminated
Organic MaterialM
Staining
Dark Oil Staining
b) 131/6-2-4-33W1/00
12 - 20 ,oo
Medium OilGastropod
Unidentified Laminae MetasomaticBoring Anhydrite
Saskatchewan Geological Survey 5 Summary of Investigations 2007, Volume 1
Fig
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Saskatchewan Geological Survey 6 Summary of Investigations 2007, Volume 1
Fig
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sim
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opac
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Saskatchewan Geological Survey 7 Summary of Investigations 2007, Volume 1
Figure 5 - Cross-section B-B'. Gamma-ray curves are shown for all wells; PE curves are included for the wells for which they are available. Datum is top of the top of the lower Frobisher. The dotted lines enclose section comprising the medial micritic rocks of the Alida Beds. The location of the cross section is shown in Figure 4.
and sandy dolomitic, moderate reddish brown to purplish red mudrocks that directly underlie the Hastings Evaporite.
4. Lithological Characteristics of the Kisbey Sandstones In hand specimen, the quartz arenites are composed of sand-size particles that include abundant subangular, clear quartz grains, and 1 to5% subrounded frosted grains as well as a trace to 2% dark grey to black particles identified as microcrystalline quartz. Thin sections show that feldspar may make up a portion of the sand-size component. Variable amounts of feldspar have been reported by different authors, ranging from 25% orthoclase and <5 to 8% microcline (Fuller, 1956; Rehman, 1987; Kent, 2004). Thin sections also indicate that the rocks are largely interstitial-filler supported in the proportion of 80 to 95% sand-size grains to 5 to 20% interstitial filler. The interstitial filler is dominantly 5 to 15 µm planar-e and planar-s dolomite, but non-planar dolomite also occurs with the planar-s. The 5 to 15 µm planar-e dolomite is commonly found where there is a higher ratio of sand-size particles to interstitial filler. The planar-s and non-planar dolomites are prominent where the proportion of interstitial filler is greatest. There are also 40 to 60 µm planar-e dolomite crystals dispersed through the planar-s and non-planar interstitial filler. In places, quartz sandstone layers are in contact with and grade into dolomitic rocks in which the proportion of sand-size particles is 50% or less. The quartz sandstone layers may also be interbedded with dolomicrites that commonly, but not invariably, contain trace fossils such as Planolites, Chondrites, Palaeophycus, Thalassinoides, and Zoophycus as well as resting traces as seen in the core from 6-2-4-33W1 well (Figure 2). In addition to the dolomicritic rocks, some quartz intervals are interbedded with grainstones, packstones, and wackestones bearing ooids and/or skeletal debris. This is particularly evident in the southeast portion of Tp 4 Rge 33W1, where the author examined several cores that had variable proportions of quartz sandstone and interbedded allochemic carbonates suggesting that there was considerable interfingering of the two lithological types. This
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GR GRPE PE PE PE
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(C-C’) (C-C’) (C) (A-A’)U
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Predominantly Carbonate
Predominantly Sandstone
Alida Shoals
Legend
Saskatchewan Geological Survey 8 Summary of Investigations 2007, Volume 1
Figure 6 - Cross-section C-C'. Gamma-ray curves are shown for all wells; PE curves are included for the wells for which they are available. Datum is top of the top of the lower Frobisher. The dotted lines enclose section comprising the medial micritic rocks of the Alida Beds. The location of the cross section is shown in Figure 4.
interfingering relationship can also be deduced from the lithological columnar sections in cores from 6-28-3-31W1 and 9-29-3-31W1(Figures 8 and 9) These two cored intervals are from wells within 800 m of one another. An important characteristic about the thick sand bodies (>30 m) is that their vertical continuity is commonly interrupted by a dolostone layer of variable thickness as illustrated in cross-section B–B' (Figure 5). Many of the quartz sand intervals have no recognizable sedimentary structures, particularly, those bodies that are greater than 30 m thick. Others have faint to well defined horizontal bedding or 5° to 20° planar foreset crossbedding. Some quartz sand occurrences also act as host rock for isolated and coalescing nodular anhydrite as well as infill between vertically elongate anhydrite (e.g., 12-13-4-34W1, Figure 7). These latter occurrences are commonly associated with moderate reddish brown to purplish red argillaceous dolostone and dolomitic mudrock interpreted as weathered residues.
5. Geometry and Depositional Environments of Kisbey Facies and Relative Timing of Their Deposition
Howard (2000) concluded that the various sand bodies of the Mississippian in southeastern Saskatchewan were isolated occurrences deposited in one of three depositional environments: 1) wave-influenced sand shoals, 2) storm-
GR
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C
PEPE
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erFr
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her B
eds
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a B
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bey
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ton
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Predominantly Carbonate
Predominantly Sandstone
Alida Shoals
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(B-B’) (B)(B-B’)
Saskatchewan Geological Survey 9 Summary of Investigations 2007, Volume 1
Figure 7 - Lithological columnar section of a cored interval from 12-13-4-34W1 (see Figure 2b for symbol legend and for explanation of scale at the top of the column). The core photograph shows the lithological components of the Kisbey interval which is here interpreted as a typical sabkha deposit formed as part of the transgressive sequence.
transported sand accumulations in open-marine environments, and 3) tidal channel deposits. Perras (1990) also proposed tidal channel and subtidal sand wave origins for the quartz sandstones in the Lost Horse Hill area. Throughout the area of Figure 4, however, the Kisbey interval varies significantly in its lithological composition. For example, where the mapped interval is less than 6 m thick in the western portion of the study area, it is essentially made up of argillaceous dolostone that is silty in places. Legault (1999) interpreted the argillaceous carbonates to contain wind-winnowed terrigenous sediment. By contrast, rocks of the same thickness in the southeast include thin sand occurrences invariably associated with purplish red and moderate reddish brown argillaceous dolostone and dolomitic mudrock as well as nodular and vertically elongate anhydrite (e.g., 12-13-4-34W1, Figure 7, and 16-5-4-32W1 on A-A', Figure 3). Where the interval is 6 to 12 m thick, in the region enclosed within the dashed line on Figure 4, quartz sand bodies are common. They are generally dolomitic sandstones with thin beds of dolomicrite or dolomitic sandstones with interbeds of allochemic carbonates. Where the Kisbey interval is 20 to 50 m thick, the isolated sand bodies described by Howard (2000) are present. The thickest of these seldom have any recognizable sedimentary structures, but are commonly separated into two quartz sand intervals by a layer of dolostone. Whereas most of these thicker sand bodies are interpreted to have been deposited in paleo-topographic depressions, Kent (2004) described a cored interval in the Silverton–East Silverton area that features a thick quartz sand accumulation overlying an exposure surface. The 20° dips on foreset crossbedding and the red colouration indicate that this sand accumulation may have originated as a coastal sand dune.
Top
Bottom
vf m cvc
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96.6
97.9
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00.3
00.500.6
00.9
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Lattice-like anhydrite, replacing carbonate.
Mosaic anhydriteIsolated nodular anhydrite in purplish red argillaceous dolomite grading to pale yellowish greydolomite becomes sandy toward base
Grainstone - >2 mm diameter pores, vuggy (enhanced interparticle and oomoldic)
Grainstone - >2 mm diameter pores, vuggy (enhanced interparticle and oomoldic)
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ad
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Saskatchewan Geological Survey 10 Summary of Investigations 2007, Volume 1
Figure 8 - Columnar lithological log of 6-28-3-31W1 (see Figure 2b for symbol legend and for explanation of scale at the top of the column).
Mds
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t
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Dolomicritic matrix, bimodal sand size distribution, vfU-fU, ml-mU, subangular to subrounded,vitreous to frosted, quartz grains, rare chert.
Bimodal distribution of allochem particle sizes - mL-mU and cU-granule size. Medium size ooidsdominate and act as matrix to 5-10% pisoids and oncolites.
Red beds with angular fragments of Mississippian carbonate.
Sandy dolomicrite, vfU-fL quartz grains, subangular to subrounded, virteous to rare frosted.
Dolomicritic matrix, bimodal size distribution, vfU-fL, 4% mL, subangular to subrounded, vitreous to frosted quartz grains, trace of chert.Bimodal size distribution - vfU-fu, mL-mU, vitreous and frosted, subangular to subrounded, trace of chert
78.778.8
79.7
80.180.4
89.2
90.590.7
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Figure 9 - Columnar lithological log of 9-29-3-31W; gamma-ray log illustrated in Figure 3 (see Figure 2b for symbol legend and for explanation of scale at the top of the column).
Mds
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kpks
t
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VfU-fU, subangular, clear vitreous quartz grains, rare mL, subrounded, frosted grains; ripple-drift crossbedding; discoid clasts of argillaceous dolomicrite near base.
rare rose;
Coated-grain grainstone and packstone, >2 mm moldic and enhanced interparticle vugs, in upper partvugs occluded by anhydrite. Thin calcrete crusts.
Oolitic grainstone with rare pisoids, mU - cL
Oolitic grainstone with rare pisoids, mU - cL
Poorly preserved coated grains, weathered with pale yellowish green claystone streaks and iron oxidestaining
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Other than the interpreted coastal sand dune deposit of the Silverton–East Silverton area, the quartz sand bodies were deposited in marine waters. The evidence for this includes thin, finely crystalline dolostone interbeds, many of which contain marine ichnofossils, as well as the gradations from quartz sand-rich layers to sandy dolostone, but the most significant evidence is the dominance of finely crystalline dolomite interstitial filler in even the richest quartzose interval. The dolomite was probably originally lime mud that was dolomitized during the early stages of burial.
Due to the difference in thickness of some of the sand accumulations, the geometry of the quartz sand bodies of this mixed carbonate-siliciclastic system is not easily compared to the geometry of other such systems in the Holocene or in the rock record. For example, another North American Mississippian mixed carbonate-siliciclastic setting in southwestern Kansas (Handford, 1988) has quartz sand bodies that are almost an order of magnitude thinner than those of the Mississippian in the Williston Basin (1 to 15 m in contrast to 20 to 50 m). Other papers describing individual sand bodies that are 30 m or more thick and that are part of siliciclastic-carbonate successions show them as covering a much broader shelf area than those of the Kisbey interval (Handford and Franka, 1991; Shew, 1991). Halabura (2005) infers that these thick quartz sandstones in southeastern Saskatchewan are valley-fill deposits in incised valleys. This implies that sea level dropped at least 50 m prior to valley formation and subsequent infilling by quartz sand sediment. According to Handford and Loucks (1993), incised valleys of this magnitude could be formed in subaerially exposed carbonate rocks if sufficient time elapsed before the next transgressive event. Time may not, however, be an important factor when meteoric water solution processes are involved in the creation of incised valleys. Handford and Loucks (1993) indicate that as much as 55 m of incision can occur in 110,000 years provided that meteoric precipitation takes place at a sufficiently high rate. Evidence that solution may have played a role in the creation of incised relief on the exposed Alida surface is equivocal. Kent et al. (1998) described probable roof breakdown blocks and dripstone or flowstone deposits representative of a paleocave in a 9 m thick interval from core taken from 9-31-2-30W1 in extreme southeastern Saskatchewan. The top of the paleocave lies about 9 m below the base of a 1 m thick quartz sandstone attributed to the Kisbey interval. In this occurrence of possible karst, the voids between collapse blocks are, however, infilled by anhydrite rather than quartz sandstone. None of the cores with thick quartz sandstone successions have carbonate rock intervals that could be interpreted as having an allochthonous source. The cores do not, therefore, have the characteristics needed to identify roof breakdown blocks (Kent et al., 1998), nor is dipping of the internal layering in the carbonates present to suggest vertical displacement as described by Johnson and Simo (2002).
The gamma-ray log cross-section A-A' (Figure 3) illustrates the distribution of sand bodies along a line from Tp 3, Rge 32W1 to Tp 5, Rge 33W1. It shows that areas of positive relief are covered by a weathered interval of pale greenish grey argillaceous dolostone or by a layer of nodular anhydrite in either a quartz sand or reddish brown argillaceous dolostone host. These are adjacent to depressions filled by a succession of interbedded quartz sandstones and carbonates or thin sandstone bodies. Cross-sections B-B' and C-C' (Figures 5 and 6) also show that the skeletal micritic unit of the Alida Beds, as identified by its log signature, is not truncated by a sandstone-bearing interval. This suggests that, where incision of the Alida took place, it was limited to the upper coated-grain layer except in 6-12-4-33W1, section A-A'. On the other hand, it may also indicate that coated-grain shoals and quartz sand accumulations are contemporaneous. Gamma-ray log cross-section B-B' is perpendicular to A-A'. It gives the impression that the thickest quartz sand occurrence perhaps developed in the distal portion of a deepened ramp (Figures 8 and 9). The sand grains may have been transported in depressions between oolitic shoals (Figure 8), and with sea-level rise, the quartz sands possibly spread out from the thick accumulations to create sheets of sand 10 to 20 m thick.
6. Sequence Stratigraphy of the Mississippian Quartz Sandstones Both Perras (1990) and Howard (2000) interpret the various sand bodies in the Mississippian succession as facies of the carbonate rocks with which they are associated. Consideration of the existence of possible facies interfaces is appropriate where carbonates and siliciclastics are recognizably interbedded such as in the southeastern corner of Tp 4 Rge 33W1, but some quartz sand bodies and associated carbonates are clearly in unconformable relationships, indicating that they belong to different depositional episodes. A suitable scenario for the depositional episodes of the quartz sands may be determined by applying sequence stratigraphic principles. The combination of quartz sand bodies and intervening carbonates can be interpreted as representative of two sequence stratigraphic systems tracts, a low stand and a transgressive, the latter passing upward through continuous sedimentation into a high stand represented by the overlying carbonates and evaporites of the Frobisher Beds.
In this scenario, the low stand was initiated through a drop in sea level related either to eustacy or, more likely, to upwarping of the eastern and northeastern margin of the Williston Basin, resulting in partial exposure of carbonate rocks belonging to the Alida Beds, specifically those rocks in close proximity to the present day Manitoba-Saskatchewan border. The source and method of transport of the siliciclastics from the hinterland onto the newly exposed carbonate bedrock and across it to the newly created coastline are not clear. The remnant singular thin sand bodies that lie on the exposed Alida bedrock possibly were thinly spread sheet sands that, in places, became the
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hosts for displacive anhydrite. In addition, some of the sheet sands may have been formed into coastal dunes (Kent, 2004) that migrated to the shoreline of the newly exposed bedrock where the sand was dumped into the marine waters and formed beach, shoreface, and offshore-bar deposits. In addition, the tidal channels between still flooded, pre-uplift coated-grain shoals may have acted as pathways for the seaward migration of quartz sand. Similar conditions exist in the Holocene setting of the Umm Said Sabkha of the Qatar Peninsula (Shinn, 1973).
A drop in sea level, either due to eustacy or forced regression, does not necessarily terminate carbonate sedimentation – it can simply result in a lowstand displacement of the carbonate factory. If the carbonate factory is on a shallow sloping ramp, the site of carbonate deposition may be as large as the area exposed by the sea level drop, or it may be more restricted if the sea floor was steepened by a hinged tectonic uplift (Handford and Loucks, 1993). In this proposed scenario, it would be possible for quartz sand bodies to be forming in a low-stand marine setting at the same time that carbonate sedimentation was taking place. Thick and narrow accumulations of point-sourced quartz sands are known to co-exist with carbonate sediments in the Holocene setting of the north shelf of Puerto Rico (Pilkey et al., 1988), although these quartz sands have a fine siliciclastic component that subdues carbonate production. With regard to the Kisbey interval, however, if the transported sands were wind blown across carbonate bedrock, much of the fine component would probably have been winnowed away and an influx of clean quartz sand would not necessarily have reduced rates of carbonate accumulation. According to the model developed by Pilkey et al. (1988), the Puerto Rican sand accumulations can be 40 m or more in thickness. A process similar to that taking place on the Puerto Rican shelf could account for the abnormally thick quartz sandstones noted in cross-sections B-B' (Figure 5) and C-C' (Figure 6) and present at other locations (Figure 4). In addition, the side-by-side accumulation of carbonate sediment and quartz sand could account for some of the interbedding of quartz sand and carbonate seen in the cores from 6-2-4-33W1, 6-28-3-31W1, and 9-29-3-31W1 (Figures 2b, 8, and 9).
Initiation of the transgressive systems tract is recognized in the core from the 6-2-4-33W1 (Figure 2b) through the presence of a coated-grain unit followed by marine quartz sands; the thickness of carbonate and quartz sandstone successions is well illustrated in 6-2-4-33W1. During the early stages of the transgression, displacive anhydrite may have formed in the interstices of the thin sheet sands, as is happening on the Trucial Coast today during the present post-glacial sea-level rise (Kirkham, 1998). Elsewhere, coastal dune sands that migrated on the exposed Alida bedrock surface were stabilized by anhydrite cement precipitated from calcium sulphate–rich groundwater rising through the dune sands by capillary evaporation (Kirkham, 1998). The sand body in the 8-6-3-32W1 core is thought to have formed in this manner (Kent, 2004).
Figure 10 is a schematic depiction of the phases of growth of the quartz sandstones and carbonate beds that make up the Kisbey interval. Profile A represents sedimentation prior to the drop in sea level. Profile B illustrates the drop in sea level and the establishment of a lowstand carbonate factory, at which time exposed Alida bedrock was weathering, giving rise to karst features, particularly solution caverns; these features suggest that climatic conditions were probably humid at this time. The onset of arid conditions resulted in migration of quartz sand by aeolian processes such that dunes developed at the lowstand coastline and quartz sand was deposited on the inner and outer marine shelf. Profile C shows a transgression with back stepping of the shoreline and submergence of solution features which were reshaped into embayments filled by quartz sands and carbonate allochemic sediments. This kind of setting could account for the apparent narrow accumulation in the southeastern corner of Tp 4 Rge 33W1. The profile also shows locations for the formation of sabkha evaporites such as those shown in the 12-13-4-34W1 well (Figure 7).
7. Hydrocarbon Reservoirs in the Sequence Stratigraphic Framework Predicting the occurrence of hydrocarbon reservoirs in the quartz sand deposits is dependent on understanding the mechanism by which reservoir porosity is created. Perras (1990) suggested that the original interstitial filler was lime mud that was dolomitized, and that the porosity was enhanced by later removal of remnant micrite. The porosity-formation process may be more complex than that, as indicated from the thin-section photomicrograph (Figure 11a) and scanning electron micrograph (Figure 11b) which show that dolomite rhombs with intercrystal porosity are associated with quartz grains that are clustered in a grain-supported framework. This suggests that dolomitizing fluids took advantage of primary porosity and permeability associated with the grain-supported framework and dolomite crystals were able to grow in these areas without interference. It is possible that removal of the original micrite matrix as suggested by Perras (1990) may have been, in fact, effected by magnesium-rich fluids significantly depleted in calcium that used the micrite with which it came in contact to produce the dolomite.
It appears as though the thickest sand bodies do not make the best reservoir rocks, nor do those that were in a nearshore setting and were susceptible to early calcium sulphate occlusion of porosity. The best potential reservoir rock is located where the intercrystal porosity of the dolomite interstitial filler in the quartz sandstone could most readily be enhanced. This appears to have occurred at the sub-Mesozoic unconformity. The best exploration targets are the thicker sand bodies proximal to the sub-Mesozoic subcrop.
Saskatchewan Geological Survey 14 Summary of Investigations 2007, Volume 1
Figure 10 - A schematic representation of the stages of development of the Kisbey interval. Profile A) prior to the drop in sea level; Profile B) following the drop in sea level and the establishment of a lowstand carbonate factory; and Profile C) after a transgression with back stepping of the shoreline and submergence of solution features which were reshaped into embayments filled by quartz sands and carbonate allochemic sediments.
Figure 11 - a) Plane-polarized light photomicrograph illustrating removal of interstitial filler by solution (note finely crystalline rhombic dolomite lining the walls of the solution pores); the air bubble in the centre of the field is 0.10 mm across; 13-13-8-6W2, 1190.5 m below KB. b) Scanning electron micrograph showing clusters of dolomite rhombs in the interstices between quartz grains that are in grain-to-grain contact; note porosity varies from intercrystalline to solution-enhanced interparticle; porosity for this specimen is 22% and permeability is 70 mD (from Kent et al., 1988).
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a) b)
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8. Conclusions Incorporating quartz sandstone layers and carbonates into one unit, the Kisbey interval, and subdividing the interval into low-stand and transgressive sequences, have made it possible to establish a better understanding of the depositional history of these rocks. The chronology of development of the Kisbey interval is graphically demonstrated in Figure 10 and is interpreted as follows: 1) uplift of the northeastern flank of the Williston Basin; 2) weathering of the exposed Alida bedrock, creating both terra rosa and karst; 3) establishment of a lowstand carbonate factory; 4) change of climatic conditions from humid to arid; 5) migration of dune sands and deposition of sands into the carbonate factory; 6) transgression accompanied by backstepping of shorelines and conversion of drowned karst caverns into embayments filled by quartz sand and interbedded carbonates; 7) formation of sabkha evaporites in thin sheet sands and red beds and stabilization of coastal dunes; and 8) high-stand sedimentation of lower Frobisher carbonates.
9. Acknowledgments I thank John Lake of Lake Geological Services; Dr. Hairuo Qing of the Department of Geology, University of Regina; and the editors, Chris Gilboy and Fran Haidl, for their comments and suggestions which have led to the improvement of this paper.
10. References Carlson, C.G. and Lefever, J.A. (1987): The Madison, a nomenclatural review with a look to the future; in Carlson,
C.G. and Christopher, J.E. (eds.), Fifth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 9, p77-82.
Edie, R.W. (1958): Mississippian sedimentation and oil fields in southeastern Saskatchewan; AAPG Bull., v42, p94-126.
Fuller, J.G.C.M. (1956): Mississippian Rocks and Oilfields of Southeastern Saskatchewan; Sask. Dep. Miner. Resour., Rep. 19, 72p.
Fuzesy, L.M. (1960): Correlation and Subcrops of the Mississippian Strata in Southeastern and South-Central Saskatchewan; Sask. Dep. Miner. Resour., Rep. 51, 63p.
Halabura, S.P. (2005): Application of sequence stratigraphy to the Mississippian of southeast Saskatchewan: fact or fantasy?; Proceedings of Thirteenth Williston Basin Petroleum Conference, April 24 to 26, Regina, Sask. Industry Resources/N. Dak. Geol. Surv., Tuesday Session 5, p4-28.
Handford, C.R. (1988): Review of carbonate sand-belt deposition of ooid grainstones and application to Mississippian reservoirs, Damme Field, southwestern Kansas; AAPG Bull., v72, p1184-1199.
Handford, C.R. and Franka, B.J. (1991): Mississippian carbonate-siliciclastic eolianites in southwestern Kansas; in Lamando, A.J. and Harris, P.M. (eds.), Mixed Carbonate – Siliciclastic Sequences, Soc. Econ. Geol., Core Workshop No. 5, p205-243.
Handford, C.R. and Loucks, R.G. (1993): Carbonate depositional sequences and systems tracts – responses of carbonate platforms to relative sea level changes; in Loucks, R.G. and Sarg, J.F. (eds.), Carbonate Sequence Stratigraphy, AAPG Mem. 57, p3-41.
Howard, P. (2000): Mississippian sedimentology, depositional and diagenetic control on the Kisbey Sandstone petroleum reservoir development, Williston Basin; unpubl. M.Sc. thesis, Univ. British Columbia, Vancouver, 250p.
Johnson, C.L. and Simo, J.A. (2002): Sedimentology and sequence stratigraphy of a Lower Ordovician mixed siliciclastic-carbonate system, Shakopee Formation, Fox River valley of east-central Wisconsin; Wisc. Geol. Nat. Hist. Surv., Wisc. Geosci., v17, 13p.
Kent, D.M. (2004): Quartz sandstone bodies in the Mississippian Frobisher, Kisbey, and Alida Beds of the Silverton–East Silverton area, southeastern Saskatchewan: remnants of an arid coastal setting; in Summary of Investigations 2004, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.1, CD-ROM, Paper A-8, 8p.
Saskatchewan Geological Survey 16 Summary of Investigations 2007, Volume 1
Kent, D.M., Haidl, F.M., and MacEachern, J.A. (1988): Mississippian oil fields in the northern Williston Basin; in Goolsby, S.M. and Longman, M.W. (eds.), Occurrence and Petrophysical Properties of Carbonate Reservoirs in the Rocky Mountain Region, Rky. Mtn. Assoc. Geol., Denver, p381-417.
Kent, D.M., Lake, J.H., and Ware, M.J. (1998) Some karst features in Mississippian carbonate rocks of southern Saskatchewan: origin, geometry and implications to petroleum exploration; in Christopher, J.E., Gilboy, C.F., Paterson, D.F., and Bend, S.L. (eds.), Eighth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 13, p72-85.
Kirkham, A. (1998): A Quaternary proximal foreland ramp and its continental fringe, Arabian Gulf, U.A.E.; in Wright, V.P. and Burchette T.P. (eds.), Carbonate Ramps, Geol. Soc. Lon., Spec. Publ. 149, p15-41.
Legault, A.R. (1999): A depositional model for the Frobisher-Alida K-2 and K-3 marker beds, Williston Basin, southeastern Saskatchewan; in Summary of Investigations 1999, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.1, p27-32.
Mundy, D.J.C. and Roulson, P.E. (1998): Diagenesis and porosity development of a subcropped Mississippian carbonate reservoir, an example from the Alida Beds of the Pheasant Rump Pool, southeast Saskatchewan; in Christopher, J.E., Gilboy, C.F., Paterson, D.F., and Bend, S.L. (eds.), Eighth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 13, p86-102.
Perras, G.L. (1990): Sedimentological and reservoir characteristics of the Frobisher-Alida Beds Lost Horse Hill Field, southeastern Saskatchewan; unpubl. M.Sc. thesis, Univ. Regina, Regina, 214p.
Pilkey, O.H., Bush, D.M., and Rodriguez, R.W. (1988): Carbonate-terrigenous sedimentation on the north Puerto Rico shelf; in Doyle, L.J. and Roberts, H.H. (eds.), Carbonate–Clastic Transitions, Elsevier Science Publishers, Amsterdam, p231-250.
Rehman, J. (1987): Depositional history and diagenesis of the Kisbey Sandstone and related carbonate rocks – Alida Beds Pool, southeastern Saskatchewan; unpubl. B.Sc. honours thesis, Univ. Regina, Regina, 133p.
Shew, R.D. (1991): Upward-shoaling sequence of mixed siliciclastics and carbonates from the Jurassic Smackover Formation of central Mississippi; in Lamando, A.J. and Harris, P.M. (eds.), Mixed Carbonate – Siliciclastic Sequences, Soc. Econ. Geol., Core Workshop No. 5, p135-167.
Shinn, E.A. (1973): Sedimentary accretion along the leeward, SE coast of Qatar Peninsula, Persian Gulf; in Purser, B.H. (ed.), The Persian Gulf: Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea, Springer-Verlag, Berlin, p199-209.
Vigrass, L., Vigrass, R., Stalwick, K., and Kristoff, B. (1994): Horizontal wells in western Canada: a preliminary study on the effects of geology and drilling/completion practices on well productivity and reserves, U.S.-Canada Co-operative Agreement on Tar Sand and Heavy Oil; Sask. Energy Mines, Misc. Rep. 94-2, p35-86.
