Abstract. 1. Introduction

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1 Sedimentary Facies Distribution of the Upper Devonian Lower Mississippian Bakken Formation, Williston Basin, Southeastern Saskatchewan: Implications for Understanding Reservoir Geometry, Paleogeography, and Depositional History Solange Angulo and Luis Buatois Angulo, S. and Buatois, L. (): Sedimentary facies distribution of the Upper Devonian Lower Mississippian Bakken Formation, Williston Basin, southeastern Saskatchewan: implications for understanding reservoir geometry, paleogeography, and depositional history; in Summary of Investigations, Volume, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. -., Paper A-, 8p. Abstract The Upper Devonian to Lower Mississippian Bakken Formation of southeastern Saskatchewan comprises three members: Lower and Upper organic-rich shale members; and a calcareous/dolomitic, sandy to silty Middle Member. Integration of sedimentological and ichnological data reveals not only open marine deposits, but also a brackish, marginal-marine interval. Accordingly, the Bakken Formation has been subdivided into three intervals based on depositional facies associations. In ascending order, these intervals include: ) open-marine, ) brackish marginal-marine, and ) open-marine. Detailed isopach maps of facies are presented in this study. Previously published isopach maps have included the individual Bakken members or, in the most detailed case, the units and subunits defined by earlier authors. Detailed facies isopach maps provide the higher resolution needed to better understand the distribution of sedimentary facies that reflect subtle variations in depositional conditions. Isopach maps from the open-marine facies show a relatively clear pattern of distribution, while isopach maps from the brackish marginal-marine facies reveal a more heterogeneous nature and localized distribution. Open-marine facies from the lower interval are distributed in a series of northwest-trending facies belts with distal deposits reaching their maximum thickness in the southwest and more proximal deposits becoming increasingly thicker toward the northeast. This trend suggests that a shoreline existed northeast of the study area. Isopach maps of the brackish marginal-marine facies, however, reflect an irregularly embayed shoreline that resulted from a fall and subsequent rise of relative sea level. The resolution achieved with core-based facies maps calibrated to geophysical logs provides an enhanced understanding of the distribution of sedimentary facies and the detailed geometry of sandstone bodies which facilitates the interpretation of local paleogeography and depositional evolution of the Bakken Formation. Keywords: Bakken Formation, Late Devonian, Early Mississippian, Williston Basin, Western Canada Sedimentary Basin, sedimentology, facies distribution, isopach maps, sedimentary facies, Saskatchewan.. Introduction The Upper Devonian Lower Mississippian Bakken Formation of the Williston Basin occurs within the subsurface of Manitoba, Montana, orth Dakota, and Saskatchewan (Christopher, 96; Meissner, 978; LeFever, 99; Martiniuk, 99; Smith and Bustin, ; Kreis and Costa, 5; Kreis et al., 5, 6; Halabura et al., 7; Angulo et al., 8; Angulo and Buatois, 9). Recently, the Bakken Formation has become one of the most popular exploration targets in Saskatchewan, Montana, and orth Dakota. The Bakken Formation is subdivided into three members (Figure ). The Lower and Upper members are homogeneous and consist of black shale, while the Middle Member is much more heterogeneous and comprises calcareous muddy siltstone, dolomitic sandy siltstone, dolomitic silty sandstone, sandstone, calcareous sandstone, and interlaminated mudstone and sandstone. Several recent studies have been conducted on the Bakken in southeastern Saskatchewan (Kreis and Costa, 5; Kreis et al., 5; Kohlruss and ickel, 9). These studies have been primarily based on geophysical well-log correlations, and isopach maps that were constructed for the individual Bakken members or, in the most detailed case, for the units/subunits that were defined by previous authors. These maps are useful in addressing regional trends, but because different facies (involving various sedimentary processes) are commonly grouped into a single mapping unit as a result of similar log responses, a much higher resolution is needed to better understand the distribution of sedimentary facies that reflect subtle variations in depositional conditions. Department of Geological Sciences, University of Saskatchewan, Science Place, Saskatoon, SK S7 5E. Saskatchewan Geological Survey Summary of Investigations, Volume

2 BAKKE MEMBERS STRATIGRAPHIC SUBDIVISIO Thrasher (985) Christopher (96) Karma and Parslow (989) Units/Subunits LeFever et al. (99) Units Units/Subunits Subunits Smith and Bustin (996) FACIES This paper DEPOSITIOAL SETTIGS This paper Upper Member B C C Mb SMm,SMh, B Open Marine Middle Member B B B B B B B Sw, Sl Sf, St, Sr, Lo 8A/B/C, 9, 6, 7 Brackish Marginal Marine B B Sw, Sl, 5 A A A SMm, SMh, A Open Marine Lower Member Mb Figure Lithostratigraphic subdivisions of the Bakken Formation (modified from LeFever et al., 99; Angulo et al., 8; Angulo and Buatois, 9), and depositional settings as interpreted in this paper. Previous studies have interpreted open marine conditions for the entire Bakken Formation. However, integration of ichnological and sedimentological data from 6 cored wells (Figure, Table ) indicates that deposition of the Bakken Formation in southeastern Saskatchewan occurred in two paleoenvironmental settings: open marine and brackish-water marginal marine. Accordingly we have subdivided the Bakken into a lower open-marine interval, a middle brackish-water marginal marine interval, and an upper open-marine interval which, in all, comprise sedimentary facies (Figure, Table ) (Angulo et al., 8; Angulo and Buatois, 9). The facies maps produced from this core-based study can be integrated with geophysical log data to provide a much higher resolution for understanding the lateral facies relationships, geometry of sandstone bodies, paleogeography, and the depositional history of the Bakken Formation in southeastern Saskatchewan. SASKATCHEWA BAKKE FORMATIO T7 T8 T9 R5 R R R9 R7 R5 R R R9 R7 R T T T T T5 T RW Figure Location of study area within Saskatchewan. umbered points represent locations of studied core. Saskatchewan Geological Survey Summary of Investigations, Volume

3 Table List of 6 cored wells examined for this study; well numbers on the table correspond to well locations shown on Figure. Well umber Well ID Well umber Well ID /-9--5W/ /6-5-6-W/ /-6--8W/ /-9-6-6W/ /5---9W/ /-5-6-6W/ /---W/ 5 /--6-9W/ 5 /-5--6W/ 6 /5--6-W/ 6 /---6W/ 7 /--6-5W/ 7 /---W/ 8 /-5-7-5W/ 8 /---6W/ 9 / W/ 9 /---9W/ /--7-W/ /6---9W/ /--7-W/ /-5--W/ 9/7--7-W/ /9-5--6W/ /-6-7-W/ /5---W/ /6-9-7-W/ /7---W/ 5 /6--7-5W/ 5 /-8--W/ 6 / W/ 6 /8---W/ 7 /5--7-W/ 7 9/---9W/ 8 /--7-W/ 8 /8---W/ 9 /--7-W/ 9 /6---W/ 5 / W/ /9-8--W/ 5 / W/ /--5-9W/ 5 /7--8-W/ /5--5-W/ 5 /--8-W/ /9--5-W/ 5 /8--8-W/ /--5-7W/ 55 /--9-W/ 5 /6--5-W/ 56 /--9-7W/ 6 / W/ 57 /6-6--W/ 7 /-8-6-5W/ 58 /5-5--8W/ 8 /6-8-6-W/ 59 /---W/ 9 /7-8-6-W/ 6 /-5--5W/ /-6-6-W/ 6 /6---9W/ /5--6-W/ 6 /5---W/. Methodology This study is based on sedimentological core descriptions and interpretations. Eighty-one well cores were described in Saskatchewan, representing a total length of 9 m. Sixty-two cores, with a total length of 5 m, are located within the study area in southeastern Saskatchewan (Figure ; Table ). Cores of the Bakken Middle Member were slabbed for detailed examination. Isopach maps for the lower open-marine, middle brackish-water marginal marine, and upper open-marine intervals are presented in this paper, as well as detailed maps for sedimentary facies identified in the area. The mapping area covers approximately 9 9 km in southeast Saskatchewan and includes Townships through, Ranges to 5 West of the Second Meridian (Figure ). Cores were analyzed for identification of sedimentary facies and the nature of the bounding surfaces (Angulo and Buatois, 9). Definition of facies was based on lithology, physical sedimentary structures, bioturbation index, and trace fossil content. Estimation of bioturbation index (BI) follows the scheme of Taylor and Goldring (99). In this scheme, BI= is characterized by no bioturbation (%). BI= ( to %) is for sparse bioturbation with few discrete traces. BI= (5 to %) is represented by low bioturbation in sediment that still has preserved sedimentary structures. BI= ( to 6%) describes an ichnofabric with discrete trace fossils, moderate bioturbation, and stilldistinguishable bedding boundaries. BI= (6 to 9%) is represented by intense bioturbation, high trace-fossil density, common overlap of trace fossils, and primary sedimentary structures are mostly erased. BI=5 (9 to 99%) is characterized by sediment with completely disturbed bedding and intense bioturbation. BI=6 (%) is for completely bioturbated and reworked sediment, related to repeated overprinting of trace fossils. Contacts between Facies, A,, and 5 are often gradational and the changes from one facies to the other are so subtle that it is often difficult to locate facies boundaries (Angulo and Buatois, 9), which introduces a degree of subjectivity when measuring facies thickness. Many of the Bakken facies are confined to specific intervals. For example, Facies is present only in the Lower and the Upper members, while Facies, A,, and 5, only occur in the basal part of the Middle Member, and invariably Saskatchewan Geological Survey Summary of Investigations, Volume

4 Table Sedimentary facies defined in the Bakken formation and their sedimentological and ichnological characteristics. Facies Lithology Sedimentary Structures Black shale, pyrite, and rare fragments of shells are locally present. Massive, locally parallel lamination and injection cracks. Bioturbation Index Ichnofossils ; locally at the top Chondrites and Thalassinoides isp. Zoophycos isp. occurs in outcrops of the Exshaw Formation. Sedimentary Environment Facies in Angulo et al. (8) Shelf Greenish grey, burrow-mottled siltstone, commonly calcareous, with fragments of shells and crinoids. Massive with burrow mottled texture 5 to 6 Phycosiphon incertum, burrow mottlings. Lower Offshore A Light grey or greenish grey, burrow-mottled, sandy siltstone to silty very fine-grained sandstone, commonly calcareous, pyritic, locally with shell remains and discontinuous thin laminae of shale. Massive. Discrete beds are absent or extremely rare, but sandier and siltier zones are detected through the interval. Very rarely microhummocky and very thin parallel lamination occur in the sandier intervals. 5 Dominant ichnotaxa: Phycosiphon incertum and ereites missouriensis. Subordinate ichnotaxa: Asterosoma isp., Techichnus rectus, and Planolites montanus. Rare elements: Rosselia isp. Upper Offshore A B Interbedded dark grey, highly bioturbated siltstone and light grey, very fine-grained sandstone. Interbedded light grey, massive, very fine-grained sandstone and siltstone. Deposits are generally slightly to moderately calcareous. 5 Interbedded massive light grey, very fine-grained sandstone with muddy partings (< mm) and thinly laminated very fine-grained sandstone. 6 Light brownish grey, fine-grained sandstone, well sorted, calcareous, locally with oolites and pyrite. 7 Light grey, very fine-grained sandstone, well sorted, with mud drapes. 8A Light to dark grey, beige and locally light red, commonly pyritic, in places slightly calcareous, very fine-grained sandstone. 8B Light to dark grey very finegrained sandstone, shale laminae are common; locally mud clasts occur (<5 mm). 8C Light, very fine-grained sandstone with common shale laminae. 9 Dark grey, very thinly interlaminated, very fine-grained sandstone and muddy siltstone; locally calcareous. Microhummocky crossstratification and very thin parallel lamination occur in the sandstones. In some cases, wave ripples occur on top of microhummocky beds. Bed boundaries are diffuse. Locally continuous shale laminae occur. Massive with common intervals of wavy or parallel lamination. Locally parallel and wavy lamination. Continuous shale laminae occur. Erosive-based high-angle planar cross-stratified, some intervals are massive or present parallel lamination/low-angle crossstratification. Flaser bedded, with wave and current ripples, climbing ripples and mudstone drapes ( mm to 8 cm thick) are also common. Wavy lamination; mudstone drapes; microfaults occur rarely. Burrow mottled texture, irregular shale laminations, common soft deformation structures and rare microfaults occur. Mudstone drapes are common (< mm) and occur rhythmically. Locally inclined heterolithic stratification is also present. Very rarely roots are present. Parallel lamination; locally current ripple cross-lamination and mudstone drapes. Very rarely roots are present. Highly variable: in the siltstones 6; in the sandstones to Dominant elements in the siltstone: Phycosiphon incertum and ereites missouriensis. Dominant elements in sandstone: Teichichnus rectus. Rare elements: Planolites montanus, Siphonichnus eccaensis. to 5 Dominant elements: ereites missouriensis and Planolites montanus. Subordinate ichnotaxa: Phycosiphon incertum and Asterosoma isp. Rare elements: Rosselia isp. Highly variable: in the massive intervals to 5; in the laminated intervals to Dominant elements: Planolites montanus. Subordinate: ereites missouriensis, Phycosiphon incertum, and Asterosoma isp. Upper Offshore B Offshore Transition Distal Delta Front 5 Barrier bar 6 Wave- dominated tidal flats to Planolites montanus and burrow mottlings. to Dominant elements: Planolites montanus, Phycosiphon incertum. Rare elements: ereites missouriensis and Teichichnus rectus. 7 Bay 7 Bay 7 to Planolites montanus Tidal flats 7 to Planolites montanus Bay 8 Very thinly interlaminated, dark grey, mudstone and light grey, very fine-grained silty sandstone. Horizontal thin parallel lamination. Locally mudstone drapes. Syneresis cracks are commonly present. Sandstone lens and wave ripples occur. to Dominant ichnotaxa: Planolites montanus and Teichichnus rectus. Rare elements: Rosselia isp., Thalassinoides isp., and Siphonichnus eccaensis. Bay 9 Sharp-based and poorly sorted coquina with sandy matrix. Massive with burrow mottled texture. to Burrow mottlings High-energy ravinement during drowning of the bay Saskatchewan Geological Survey Summary of Investigations, Volume

5 in the same stratigraphic order. Exceptions occur in the middle part of the Middle Member where some of the facies are interstratified with others and therefore reoccur vertically. For these facies, the isopach maps reflect the net thicknesses, rather than discrete occurrences. Two isopach maps were constructed for Facies ; one for each of the black shales comprising the Lower and Upper members. Due to a lack of cores penetrating Facies, the isopach maps of these two members are based on geophysical log data. For all other facies in the Middle Member, isopach maps were constructed entirely with core data. Isopach maps of the different facies were created using only reliable data. Thickness data were not included in cases where a core was too fractured or had been extensively sampled, where facies identification was uncertain, or where the entire facies unit (including base and top) was not cored.. Results Integration of ichnological data with conventional sedimentological analysis reveals two paleoenvironmental settings: open marine and brackish-water marginal marine. Accordingly, the Bakken Formation can be subdivided into three intervals: ) a lower open-marine interval that includes the Lower Member (Figure ); ) a middle brackish marginal-marine interval (Figure ); and ) an upper open-marine interval (Figure 5). a) Basal Open-marine Interval The basal open-marine interval comprises the black shale of the Lower Member and the lower portion of the Middle Member. In ascending order, the Lower Member black shale (Facies ), deposited in a shelf environment, is overlain by lower-offshore, calcareous muddy siltstone (Facies ), followed by upper-offshore, dolomitic, sandy siltstone (Subfacies A), and finally by offshore-transition, dolomitic, silty, very fine-grained sandstone (Facies ) (Angulo et al., 8; Angulo and Buatois, 9) (Table ). Distal delta front deposits were identified locally at the top of the basal open-marine unit (Facies 5). With the exception of the sharp, but conformable contact between the shelf and the lower-offshore deposits (Facies /Facies ), facies contacts in this lower interval are gradational. These deposits are regarded as open marine because, for the most part, they represent sedimentation in fully marine environments developed under conditions of normal-marine salinity as opposed to those of the overlying marginal-marine interval, which is characterized by persistent brackish-water conditions. In any case, the widespread occurrence of anoxic black shale indicates the presence of a sill that created some restriction at a regional scale, leading to stratification of the water mass (Ettensohn and Barron, 98) Figure Isopach map of the basal open-marine interval. et thickness of Facies,, A,, and 5 combined. Saskatchewan Geological Survey 5 Summary of Investigations, Volume

6 Figure Isopach map of the middle brackish marginal-marine interval. et thickness of Facies 6, 7, 8A to 8C, 9, and combined R5 R R R R R R9 R8 R7 R6 R5 R R R R. R R9 R8 R7 R6 R5 R RW Figure 5 Isopach map of the upper open-marine interval. et thickness of Facies, B, and combined. Saskatchewan Geological Survey 6 Summary of Investigations, Volume

7 Except for the black shale of the Lower Member, where the lack of bioturbation suggests anoxic to dysoxic conditions, basal open-marine deposits are characterized by a high bioturbation index ( to 6). The trace fossil assemblage indicates a distal Cruziana ichnofacies, in which dominant elements are ereites missouriensis, and Phycosiphon incertum, subordinate elements are Planolites montanus, Asterosoma isp., Teichichnus rectus, and rare elements include Chondrites isp., Rosselia isp., and Thalassinoides isp. (Angulo and Buatois, 9). Although Facies 5 contains the same trace-fossil association as Facies, the bioturbation index in Facies 5 is highly variable due to intercalation of beds with high and low bioturbation indexes (e.g., indices to 5 intercalated with to ), suggesting some sporadic brackish-water influence due to deltaic influence. The low preservation of storm deposits in the lower-offshore, upper-offshore, and offshore-transition deposits of the basal open-marine interval is explained by homogenization due to a high-degree of biogenic reworking. With the exception of Facies and 5, isopach maps of the basal open-marine facies (Figures 6, 7, 8, 9, and ) reveal a wide distribution covering the entire study area. Comparison of isopach maps of Facies, Subfacies A and Facies (Figures 7, 8, and 9) indicates a clear trend in the distribution and maximum thickness of these facies from southwest to northeast. The maximum thickness (. m) of the more distal lower-offshore deposits (Facies ) occurs in the southwest and south-central regions (Figure 7), while the thickest upper-offshore deposits (. m; Subfacies A) are in the central and south-central portion of the eastern half of the study area (Figure 8), and offshoretransition deposits (Facies ) are thickest (. m) in the central and eastern areas (Figure 9). Facies 5 is restricted to two locations in the west-central and the southeast parts of the study area where it reaches maximum thicknesses of and.8 m respectively (Figure ). Facies and 5 are considered to record the shallowest deposits (offshore transition and distal delta front deposits, respectively) of the progradational succession formed during the basal highstand systems tract. In contrast, comparison of the isopach maps of the basal Facies and (Figures 6 and 7) indicate a shift in the area of maximum thickness from northeast to southwest. This pattern reflects the regional extent and transgressive nature of Facies. The isopach map of net thickness of basal open-marine facies (Figure ) is most strongly influenced by the distribution of Facies. R5 R R R R R R9 R R R R9 R8 R7 R6 R5 R RW R8 R7 R6 R5 R R Figure 6 Isopach map of Facies (Lower Member). Facies is present across the entire study area. It ranges in thickness from. m in the northeast to a maximum of. m in the north-central region Saskatchewan Geological Survey 7 Summary of Investigations, Volume

8 .... Figure 7 Isopach map of Facies. Facies covers the entire study area. The isopach map of Facies shows obvious thickening towards the south-southwest of the analyzed area. Its thickness ranges from cm in the northeast, to. m in the southwest and south-central regions R5 R R R R R R9 R8 R7 R6 R5 R R R R R R9 R8 R7 R6 R5 R RW Figure 8 Isopach map of Subfacies A. Subfacies A was identified everywhere in the study area except for the extreme northeast corner. This facies thickens progressively toward the south and reaches a maximum thickness of. m in the central-southeast area. Saskatchewan Geological Survey 8 Summary of Investigations, Volume

9 km Figure 9 Isopach map of Facies. Facies also shows fairly regional distribution; however, it is not present across the southern portion of the study area and locally disappears along the margins of the mapping area. A maximum thickness of. m is reached in the central northeast region of the study area. km Figure Isopach map of Facies 5. Facies 5 is restricted to two locations in the west-central and the southeast parts of the study area where it reaches maximum thicknesses of and.8 m, respectively. Saskatchewan Geological Survey 9 Summary of Investigations, Volume

10 b) Middle Brackish Marginal-marine Interval The middle brackish marginal-marine interval comprises the middle part of the Middle Member. Sedimentary facies in this interval show much more lateral and vertical variation than the underlying and overlying open-marine intervals. The marginal-marine facies association includes: high-angle, cross-stratified sandstone (Facies 6); flaserbedded sandstone (Facies 7); wavy-laminated sandstone (Subfacies 8A, 8B, and 8C); thinly interlaminated siltstone, mudstone and very fine-grained sandstone (Facies 9); and thinly interlaminated very fine-grained sandstone and mudstone (Facies ) (Table ). The brackish-water, marginal-marine deposits are characterized by sparse to no bioturbation, low ichnodiversity, and an impoverished Cruziana ichnofacies. Dominant elements are Planolites montanus and Teichichnus rectus, while subordinate elements are Rosselia isp., Thalassinoides isp., and Siphonichnus eccaensis. Syneresis cracks are common, which is typical of environments affected by salinity fluctuations (MacEachern and Pemberton, 99). Mud drapes, flaser bedding, and rhythmic interlamination of mudstone, siltstone and very fine-grained sandstone in this interval suggest tidal influence. The local presence of root traces suggests the existence of waterlogged paleosols, but no evidence of persistent subaerial exposure has been observed. Isopach maps of the brackish-water marginal-marine interval reveal the complexity and heterogeneous distribution of sedimentary facies. Some of the facies of this interval are restricted exclusively to certain areas, while others are widely distributed. There is no clear trend in the distribution and thickness of facies, and generally the lateral facies relationships and thickness variations do not reflect any particular trend or pattern (Figures to 7) km...8 Figure Isopach map of Facies 6. Facies 6 occurs in the central region of the analyzed area, where it forms a southeastnorthwest trending belt. It thickness varies from in the north, east, and west portions of the map to 5. m in the centralsoutheast region. Saskatchewan Geological Survey Summary of Investigations, Volume

11 Figure Isopach map of Facies 7. Facies 7 is restricted to the western portion of the study area. Its thickness ranges from to m R5 R R R R R R9 R8 R7 R6 R5 R R R R R R9 R8 R7 R6 R5 R RW Figure Isopach map of Subfacies 8A. Subfacies 8A occurs in the southwest, south-central, and central area of the map. Its thickness ranges from to.6 m. Saskatchewan Geological Survey Summary of Investigations, Volume

12 Figure Isopach map of Subfacies 8B. Subfacies 8B is restricted to the southwest, south-central, and northeast region of the map area and it ranges from to m thick Figure 5 Isopach map of Subfacies 8C. Subfacies 8C is restricted to the southwestern portion of the study area. It thickness ranges from to. m. Saskatchewan Geological Survey Summary of Investigations, Volume

13 R5 R R R R R R9 R8 R6 R5 R R R R R R9 R8 R7 R6 R5 R RW R Figure 6 Isopach map of Facies 9. Facies 9 is widespread across the study area. Its thickness ranges from in the west, north-central and east-central region of the map to.7 m in the south-central, central, and northeast portion of the analyzed area Figure 7 Isopach map of Facies. With the exception of two localized areas (northeast and central area of the map), Facies covers the entire study area. Its thickness varies from to. m. The isopach map of Facies shows a general thickening toward the west. Saskatchewan Geological Survey Summary of Investigations, Volume

14 c) Upper Open-marine Interval The upper open-marine interval comprises a transgressive lag (Facies ), and upper-offshore, interbedded siltstone, and microhummocky cross-stratified sandstone (Subfacies B) from the upper part of the Middle Member; and the shelf black shale (Facies ) of the Upper Member. Where the transgressive lag is present, the basal contact with the underlying brackish marginal-marine interval is sharp, otherwise it is gradational. The contacts between Facies, Subfacies B, and Facies are generally sharp, but conformable. Upper open-marine deposits are characterized by a variable bioturbation index and a distal Cruziana ichnofacies, with a similar trace-fossil assemblage to the basal open-marine interval. Dominant elements are ereites missouriensis and Phycosiphon incertum, and subordinate elements are Planolites montanus and Teichichnus rectus. Rare elements are Siphonichnus eccaensis. o bioturbation was observed in the black shale (Facies ) of the Upper Member. With the exception of Facies, which is confined to the southeast portion of the study area (Figure 8), isopach maps from the upper open-marine interval show a wide distribution covering the entire study area (Figures 9 and ) Figure 8 Isopach map of Facies. Facies is mainly restricted to the southeast portion of the map. Its thickness ranges from to cm. Saskatchewan Geological Survey Summary of Investigations, Volume

15 Figure 9 Isopach map of Subfacies B. Subfacies B occurs across the entire study area and its thickness ranges from m in the south and west to. m in a thickened zone toward the northeast Figure Isopach map of Facies (Upper Member). Facies of the upper black shale is also distributed in the whole study area. Its thickness ranges from. to.6 m. Saskatchewan Geological Survey 5 Summary of Investigations, Volume

16 . Discussion The interpretation of facies isopach maps is constrained by the fact that they represent the distribution of diachronic lithofacies. However, these maps not only provide useful information about the geometry of the sedimentary deposits, but also valuable information to understand the spatial and temporal evolution of the different processes and sedimentary sub-environments. The basal open-marine interval was deposited during transgressive and highstand systems tracts. The lower part of the Bakken Lower Member is interpreted to be transgressive, while the upper section of the Lower Member and the basal portion of the Middle Member record a progradational succession. This highstand system tract was characterized by seaward-advancing clastic wedges which gradually filled the accommodation space that was created during the previous transgression in a shallow epicontinental sea. Highstand deposits show a progradational trend from shelf environments to lower-offshore, upper-offshore, and finally to offshore-transition environments. Locally, in the west and southeast of the study area, two distal deltaic lobes are interpreted. The most proximal deposits of the highstand systems tract (shoreface and proximal delta front) were likely present in the northeast of the study area, but were subsequently removed due to erosion resulting from a fall in relative sea level or transgressive erosion. Open-marine facies from the lower interval are distributed in a series of northwest-trending facies belts with distal deposits reaching their maximum thickness in the southwest and more proximal deposits becoming increasingly thicker toward the northeast. This trend suggests that a shoreline existed northeast of the study area. Brackish marginal-marine facies are interpreted to have been deposited during early transgression following a relative sea-level fall. These facies make up the basal section of a transgressive systems tract that covered the highstand open-marine facies throughout the study area. Fluvial lowstand deposits are not recognized in the study area due to either by-pass during the lowstand or erosion during transgressive ravinement. The basal contact of the brackish marginal-marine interval is generally sharp, with the exception of areas where Facies 5 occurs and the contact appears gradational. The contact is typically characterized by a drastic drop in bioturbation index and ichnodiversity in the overlying brackish-water marginal-marine deposits. This contact is interpreted as a coplanar surface, which comprises a sequence boundary and a transgressive surface involving a hiatus (Angulo et al., 8). The Devonian-Mississippian boundary may occur at this contact (Angulo et al., 8, Angulo and Buatois, 9). Sandberg et al. () reported a eustatic sea-level fall across the Devonian-Mississippian boundary associated with the Devonian Southern Hemisphere glaciation. Isopach maps of the brackish marginal-marine facies reflect an irregularly embayed shoreline that resulted from a fall and subsequent rise of relative sea level. The heterogeneous nature of the distribution of the brackish marginal-marine facies suggests the complex facies mosaic of an irregularly embayed shoreline during this interval of Bakken deposition. The Plymouth embayment in Massachusetts, U.S.A. provides a possible modern analogue. As the transgression proceeded, full marine conditions were re-established across the study area and open-marine facies were deposited over the underlying brackish-water, marginal-marine deposits formed during the early phase of the transgression. In the southeast region of the study area, a transgressive lag was deposited on top of the brackish-water marginal-marine interval. It consists of abundant shell fragments (Facies ) and represents a marine ravinement surface (Angulo et al., 8). The lag is overlain by upper-offshore deposits (Subfacies B) consisting of highly bioturbated siltstone and non-bioturbated, or sparsely bioturbated, micro-hummocky, cross-laminated sandstone. Where the lag is not present, heterolithic deposits from the brackish-water interval (Facies ) grade into these upper-offshore deposits. Finally, as sea level continued to rise, the entire area was covered by shelfal black shale of the Upper Member. The overall wide distribution of deposits forming the upper open-marine interval is consistent with a transgressive scenario. Anomalously thickened zones of some of the facies can be explained due to localized salt dissolution and collapse of the underlying Middle Devonian Prairie Evaporite Formation. Cugnet (8) reported two areas in southeast Saskatchewan (Tatagwa, and southeast of Estevan) which experienced collapse of the Prairie Evaporite, affecting the thickness of the Bakken Formation. A better understanding of the salt dissolution of the Prairie Evaporite Formation can provide valuable information for a more accurate interpretation of the Bakken facies isopach maps. 5. Conclusions Facies isopach maps from the lower and upper open-marine intervals reflect a wide and homogeneous distribution in the study area. Comparison of isopach maps from Facies (Figure 7), Subfacies A (Figure 8), and Facies (Figure 9) reveals a clear southwest-northeast trend, in which more distal facies (lower offshore) appear in the southwest region of the study area, while more proximal facies (offshore transition) occur in the northeast portion of the map. In contrast, isopach maps from the brackish marginal-marine facies show a much more localized and variable distribution, reflecting the complexity of the embayment. The isopach maps of this study further support the depositional model that both open marine and restricted brackish-water environments existed during Bakken time. The resolution achieved with maps based on core data calibrated with geophysical logs has a significant impact for Saskatchewan Geological Survey 6 Summary of Investigations, Volume

17 understanding the distribution of sedimentary facies and the detailed geometry of sandstone bodies, allowing a clearer interpretation of local paleogeography and depositional evolution history of the Bakken Formation. 6. Acknowledgments Financial support for this project is provided by the Saskatchewan Ministry of Energy and Resources and the University of Saskatchewan. Additional funds have been provided by Shell Canada, the AAPG, and the IAS. We would like to thank Andrew imegeers and John Lake for reviewing this paper, and the Geological Subsurface Laboratory staff for their assistance in core slabbing and displaying material. Special thanks to Andrew imegeers, Melinda Yurkowski, Daniel Kohlruss, Erik ickel, Fran Haidl, Chris Gilboy, and Kim Kreis for all their support and help. 7. References Angulo, S. and Buatois, L. (9): Sedimentological and ichnological aspects of a sandy low-energy coast: Upper Devonian Lower Mississippian Bakken Formation, Williston Basin, southeastern Saskatchewan; in Summary of Investigations 9, Volume, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 9-., Paper A-5, 7p. Angulo, S., Buatois, L., and Halabura S. (8): Paleoenvironmental and sequence-stratigraphic reinterpretation of the Upper Devonian Lower Mississippian Bakken Formation of subsurface Saskatchewan integrating sedimentological and ichnological data; in Summary of Investigations 8, Volume, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 8-., CD-ROM, Paper A-, p. Christopher, J.E. (96): Transitional Devonian-Mississippian Formations of Southern Saskatchewan; Sask. Dept. Miner. Resour., Rep. 66, p. Cugnet, J. (8): Effects of Prairie Evaporite salt collapse on the Bakken Formation, southeast Saskatchewan; unpubl. B.Sc. thesis, Univ. of Regina, Regina, 5p. Ettensohn, F.R. and Barron, L.S. (98): Depositional model for the Devonian-Mississippian black-shale sequence of orth America: a tectono-climatic approach; Technical Information Center, Morgantown, U.S. Dept. of Energy, DOE/METC/-, 8p. Halabura, S., Buatois, L., Angulo, S., and Piché, L. (7): From source to trap: a review of the Bakken petroleum system, Upper Devonian Mississippian, southeast Saskatchewan; in Summary of Investigations 7, Volume, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 7-., CD-ROM, Paper A-, 8p. Karma, R. and Parslow, G. R. (989): Sedimentology and geochemistry of the Bakken Formation (Devonian- Mississippian) in southern Saskatchewan; in Summary of Investigations 989, Saskatchewan Geological Survey. Sask. Energy Mines, Misc. Rep. 89-, p-7. Kohlruss, D. and ickel, E. (9): Facies analysis of the Upper Devonian Lower Mississippian Bakken Formation, southeastern Saskatchewan; in Summary of Investigations 9, Volume, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 9-., Paper A-6, p. Kreis, L.K. and Costa, A. (5): Hydrocarbon potential of the Bakken and Torquay formations, southeastern Saskatchewan; in Summary of Investigations 5, Volume, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 5-., CD-ROM, Paper A-, p. Kreis, L.K., Costa, A., and Osadetz, K.G. (5): ew Perspectives on the hydrocarbon potential of Bakken and Torquay formations, southeastern Saskatchewan; URL < accessed 8 January 7. (6): Hydrocarbon potential of Bakken and Torquay formations, southeastern Saskatchewan; in Gilboy, C.F. and Whittaker, S.G. (eds.), Saskatchewan and orthern Plains Oil & Gas Symposium 6, Sask. Geol. Soc., Spec. Publ. o. 9, p8-7. LeFever, J.A. (99): History of oil production from the Bakken Formation, orth Dakota; in Hansen, W.B. (ed.), Geology and Horizontal Drilling of the Bakken Formation, Montana Geological Society, 99 Guidebook, p- 7. Saskatchewan Geological Survey 7 Summary of Investigations, Volume

18 LeFever, J.A., Martiniuk, C.D., Dancsok, E.F.R., and Mahnic, P.A. (99): Petroleum potential of the middle Member, Bakken Formation, Williston Basin; in Christopher, J.E., and Haidl, F. (eds.), Sixth International Williston Basin Symposium, Saskatchewan Geological Society, Spec. Publ. o., p7-9. MacEachern, J.A. and Pemberton, G.S. (99): Ichnological aspects of incised-valley fill systems from the Viking Formation of the Western Canada Sedimentary Basin, Alberta, Canada; in Dalrymple, R., Boyd, R., and Zaitlin, B.A. (eds.), Incised Valley Systems: Origin and Sedimentary Sequences, Soc. Econ. Paleont. Mineral., Spec. Publ. o. 5, p9-57. Martiniuk, C.D. (99): Regional geology and petroleum potential of the Bakken Formation, southwestern Manitoba; in Hansen, W.B. (ed.), Geology and Horizontal Drilling of the Bakken Formation, Montana Geological Society, 99 Guidebook, p-68. Meissner, F. (978): Petroleum geology of the Bakken Shales Formation, Williston Basin orth Dakota and Montana; in Proceedings of 978 Williston Basin Symposium, Montana Geological Society, Billings, p7-7. Sandberg, C.A., Morrow, J.R., and Ziegler, W. (): Late Devonian sea-level changes, catastrophic events and mass extinctions; in Koeberl, C. and MacLeod, K.G. (eds.), Catastrophic Events and Mass Extinctions: Impacts and Beyond, Geol. Soc. Amer., Spec. Pap. 56, p7-87. Smith, M.G. and Bustin, R.M. (996): Lithofacies and paleoenvironments of the Upper Devonian and Lower Mississippian Bakken Formation, Williston Basin; Bull. Can. Soc. Petrol. Geol., v, p (): Late Devonian and Early Mississippian Bakken and Exshaw black shale source rocks, Western Canada Sedimentary Basin: a sequence stratigraphic interpretation; Amer. Assoc. Petrol. Geol., Bull., v8, p9-96. Taylor, A. and Goldring, R. (99): Description and analysis of bioturbation and ichnofabric, Geol. Soc. Lon. J., v5, p-8. Thrasher, L. (985): Macrofossils and biostratigraphy of the Bakken Formation (Devonian and Mississippian) in western orth Dakota; unpubl. M.Sc. Thesis, Univ. orth Dakota, Grand Forks, 9p. Saskatchewan Geological Survey 8 Summary of Investigations, Volume