Hydrocarbon Trapping in the Mississippian Midale and Frobisher Beds, Southeastern Saskatchewan: Looking Away from the Subcrop Edge

A.R. Nimegeers and E. Nickel

Nimegeers, A.R. and Nickel, E. (2005): Hydrocarbon trapping in the Mississippian Midale and Frobisher beds, southeastern Saskatchewan: Looking away from the subcrop edge; in Summary of Investigations 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-13, 16p.

Abstract Oil exploration in the Mississippian strata of southeastern Saskatchewan has traditionally targeted hydrocarbons trapped subjacent to the sub-Mesozoic unconformity surface and areas overlying structural highs or “bumps”. The potential for stratigraphic trapping related to facies changes and reservoir pinch-out is sometimes overlooked, even in areas where these trapping mechanisms may be important to production. This paper identifies and describes several stratigraphic trends related to reservoir development in the Midale and Frobisher beds of southeastern Saskatchewan (Tp 3, Rge 6W2 to Tp 4, Rge 13W2, and Tp 5, Rge 12W2).

Abrupt facies and thickness changes are observed in the Midale and Frobisher beds along an east-west–trending linear region that contains the West Kingsford, Macoun, Elswick, and Bromhead oil fields in southeastern Saskatchewan. Stacked, coated-grain banks (>60 m thick) in the Frobisher developed aggradationally along this trend over areas with topographic relief. Off-bank areas typically dominated by muddy wackestones and mudstones are up to 15 m thinner than adjacent bank areas. Oolitic and bioclastic shoals in the Midale Beds were deposited in high-energy settings near the crests of underlying Frobisher banks. In the West Kingsford and Macoun areas, oolitic shoals of the Lower Midale (Vuggy) grade laterally into lagoonal limestones that contain abundant nodular anhydrite (S2 anhydrite). In the Elswick and Bromhead areas, bioclastic shoals in the Upper Midale (Marly) overlie thin (<5 m thick), eroded lagoonal mudstones and evaporitic mudflat deposits of the Lower Midale. Shoal facies from both the Upper and Lower Midale units produce hydrocarbons in the study area.

Core and geophysical (high-resolution aeromagnetic and well-log) data indicate that facies relationships in the Midale and Frobisher beds along the West Kingsford–Elswick oil-field trend were controlled, in part, by reactivation of a basement lineament zone and dissolution of the Devonian Prairie Evaporite Formation. Local facies and thickness anomalies are roughly coincident with the following features: 1) an unnamed lineament zone interpreted by GEDCO for the geoscience-framework task of the IEA GHG Weyburn CO2 Monitoring and Storage Project; 2) a narrow elongate zone showing anomalous thickening of the Poplar Beds due to local preservation below the sub-Mesozoic unconformity surface; and 3) local areas of partial or complete dissolution of the Prairie Evaporite Formation.

Keywords: Midale Beds, Marly, Vuggy, Frobisher Beds, hydrocarbon trapping, Mississippian, lineaments, Williston Basin.

1. Introduction The Mississippian Midale and Frobisher beds of southeastern Saskatchewan (Figure 1) were deposited across a gently dipping carbonate ramp (Lake, 1998; Kent 1999) that characterized the northeastern flank of the Williston Basin. The Frobisher-Midale succession reflects an overall shallowing and restriction of water circulation with gradual reduction of accommodation space across the carbonate ramp. Carbonate-evaporite sequences within the Frobisher and Midale beds are between 4 and 25 m in thickness. The carbonate facies include shoal, lagoon, restricted ramp, and open marine, and the evaporite facies include mudflat, sabkha, evaporitic pond and playa lake. Many of the depositional sequences show evidence of exposure (e.g., caliche, microkarsting, fenestral porosity) at or near their upper contacts (Burrowes, 2001; Lake, 2001; Kent and Nimegeers; 2002; Nimegeers and Qing, 2002). Facies descriptions and depositional interpretations of the Midale and Frobisher beds of southeastern Saskatchewan have been discussed in several previous studies (Smith, 1980; Crabtree, 1982; Kaldi, 1982; Wegelin, 1984; Kent and Curry, 2002; Lake, 2002; Marsh and Qing, 2002; Nimegeers and Qing, 2002). Both structural and stratigraphic elements have contributed to hydrocarbon entrapment in the study area (Figure 2). The purpose of this paper is to describe stratigraphic relationships that may be helpful for further hydrocarbon exploration. Hydrocarbon trapping in the study area is unrelated to the subcrop edge, unlike most of the larger

Saskatchewan Geological Survey 1 Summary of Investigations 2005, Volume 1 Midale and Frobisher pools of Mississippian Stratigraphy southeastern Saskatchewan. Southeastern Saskatchewan Local paleotopography related to basement lineament zones was (Modified from Fuzesy, 1960) an important control on lateral Jurassic facies changes and later reservoir Watrous Formation Triassic development. Big Snowy Kibbey Formation Group The study area comprises 15 Informal Midale townships (Tp 3, Rge 6W2 to Tp n Poplar Beds s o subdivisions this study i 4, Rge 13W2, and Tp 5, Rge e t l r a 12W2) and includes the West

a Ratcliffe Oungre Evaporite

m Upper Midale h r Kingsford, Macoun, Elswick, Beds s

o (Marly)

C Midale Evaporite d and Bromhead oil fields (Figure F e

P Midale

B 2). Data from 26 cores and over

N U Beds Lower Midale

Frobisher Evaporite e l A 200 geophysical well logs were O I (Vuggy) n a P R o Frobisher Hastings Evaporite d used to interpret facies i y

P S2 Evaporite G n I

relationships in the Midale and n Beds M

o Winlaw Evaporite S i a N t Frobisher Evaporite Frobisher beds. Structure and S C a I O Kisbey Sandstone isopach maps were constructed S S n m I r Gainsborough Evaporite o S

i for all main units from the I o D Alida Beds s F A M

s Kisbey Sandstone to the Midale i

M Evaporite (Figure 1) in order to M Tilston Beds compare cross-stratal regional trends. Several dip-oriented e n l o o i stratigraphic cross sections are t p a e used to define lateral and vertical g

m Souris Valley Beds r d o

o facies transitions. F L Bakken Formation DEVONIAN 2. Structure

Figure 1 - Stratigraphic chart of Upper Devonian through Triassic strata in Several important regional faults southeastern Saskatchewan. Detailed informal stratigraphy of the Mississippian and tectonic lineaments were Midale unit, as discussed in this paper, is also shown. interpreted by GEDCO using integrated high-resolution aeromagnetic and seismic data within the IEA GHG Weyburn CO2 Monitoring and Storage Project area of southeastern Saskatchewan (Whittaker, 2004; Whittaker et al, 2004). Two of the lineament zones (Figure 2) are present in the current study area: 1) a previously unnamed lineament that trends roughly east-northeast from the Bromhead area to the northwestern corner of the Steelman Field, informally referred to as the Macoun lineament in this study; and 2) a fault zone trending northwest from North Dakota and passing through the eastern margin of the Elswick Field (Nesson Wrench Fault). Evidence from core and geophysical well logs suggests that these lineaments became reactivated during the Mississippian and affected depositional patterns during Frobisher and Midale times. The Frobisher structure map shows structural trends that coincide with these interpreted lineament zones (Figure 3).

3. Facies Relationships in the Midale and Frobisher Beds Detailed core and well-log analyses in the study area identified several important facies relationships associated with hydrocarbon trapping in the Midale and Frobisher beds. Abrupt thickness and facies changes in the Midale Beds occur in the same vicinity as thickness and facies changes in both underlying (Frobisher) and overlying strata (Ratcliffe and Poplar beds). Recognition of trends exhibiting abrupt changes in thickness or facies in shallower strata (Midale-Poplar) may provide a potential exploration tool for predicting facies changes and, potentially, hydrocarbon traps in the underlying Frobisher Beds, especially in areas where deep well control is lacking. a) Midale: West Kingsford–Macoun Area Along the West Kingsford–Macoun oil-field trend (Figure 2), oolitic shoals in the Lower Midale (Vuggy) unit were deposited preferentially along the flanks of coated-grain banks in the underlying Frobisher Beds. Isopach maps (Figures 4A and 4B) show somewhat coincident areas of thickening in the Lower Midale and Frobisher beds. Based on core and well-log analyses, these localized “thicks” are interpreted as shoals and banks. The spatial association of thickened strata with basement lineaments trends (Figure 3) suggests that development of the shoals and banks was, in some manner, related to these underlying structural features.

Saskatchewan Geological Survey 2 Summary of Investigations 2005, Volume 1

WEYBURN

WEYBURN POOL MIDALE BENSON POOL T6 POOL

T5 d t tren ELSWI men CK C’ oun linea A’ STEELMAN Y’ Mac X’ B’ C POOL T4 WEST N MACOUN B A e KINGSFORD s

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F ESTEVAN a u

l t T1 CANADA R2 R1W2 R15 R14 R13 R12 R11 R10 R9 R8 R7 R6 R5 R4 R3 Seaward edge of S2 Anhydrite 0 10 20 30 40 kilometres Midale Beds subcrop edge

Study area Lineament zones interpreted by GEDCO A’ Location of cross sections Midale oil pools A Figure 2 - Map of the regional study area showing the various Midale pools and local geological features that affected depositional trends and hydrocarbon trapping in Midale and Frobisher strata. The map also shows locations of cross sections discussed in this study.

nt eame N e un lin s Maco s o n

W r e n c h T3

F a u l t

R8W2 R13 R12 R11 R10 R9 C.I. = 5 m

0 2 4 6 8 10 Kkilometresilometers Figure 3 - Structure map of the Frobisher Beds. Irregularities in the surface structure are evident in the region of basement lineaments (in red) interpreted from high-resolution aeromagnetic data by GEDCO.

Saskatchewan Geological Survey 3 Summary of Investigations 2005, Volume 1

t amen n line Macou N e s s o n

r T3 e n c h

F a u l t

R10 R9 R8W2 R13 R12 R11 Frobisher Beds C.I. = 1 m 0 2 4 6 8 10 kilometres

R13 R12 R11 R10 R9 R8W2 Lower Midale S2 Anhydrite Edge C.I. = 1 m 0 2 4 6 8 10 kilometres

R13 R12 R11 R10 R9 R8W2 S2 Anhydrite Edge C.I. = 1 m Upper Midale 0 2 4 6 8 10 kilometres Figure 4 - Isopach maps of the Frobisher, Lower Midale, and Upper Midale units across most of the study area. A) Isopach map of the Frobisher Beds. Frobisher “thicks” (>60 m thick, within shaded area) follow the general trend of the Macoun lineament (east-west red line) and are dominated by bank facies. B) Isopach map of the Lower Midale unit showing areas of thickening (>16 m thick, shaded area) and thinning. The thicker sections commonly correspond to areas of oolitic shoal development. C) Isopach map of the Upper Midale unit. Shaded areas represent regions where the unit is less than 12 m thick. Where the Upper Midale is thin, the Lower Midale is generally thick.

Saskatchewan Geological Survey 4 Summary of Investigations 2005, Volume 1 Shoal facies in the Midale Beds consist of well sorted oolitic grainstones and packstones (Figure 5A) between 1 and 3 m in thickness interbedded with algal/peloidal packstones and wackestones. The axes of oolite shoals generally parallel the paleoshoreline, and are 0.5 to 3 km in length and 0.5 km in width. Vertical relief of the shoals relative to the adjacent lagoon was probably no more than 3 m. Oolites having the best reservoir qualities occur in the middle and upper parts of the Lower Midale. Oolite reservoirs have intergranular porosities from 10 to 15% and permeabilities between 40 and 55 mD. The shoal facies commonly shows evidence of periodic exposure such as laminar crusts, microkarsting, fenestral porosity, and desiccation features (Figure 5B). Lower Midale shoals can often be identified through careful examination of geophysical well logs. Where shoals are present, the Lower Midale is typically thicker (4 to 5 m) than adjacent lagoonal sediments and produces a “cleaner” gamma-ray response (Figure 6). Where the Upper Midale (Marly) overlies shoals, it is often thin (<7 m thick) relative to inter-shoal regions (Figures 4C and 6) suggesting that topographic relief and lack of accommodation space across the shoals affected deposition of the Upper Midale. As a result, dolomudstone reservoirs in the Upper Midale Beds are typically thin or absent over the crests of Lower Midale shoals and thicken over adjacent lagoonal sediments (Figure 6). Oolitic shoals grade landward into washover fans and restricted-lagoon facies consisting of poorly sorted intraclastic/peloidal packstones and algal/peloidal wackestones and packstones, respectively. The lagoonal facies becomes interbedded with nodular anhydrite (Figures 6 and 7) and stromatolitic mudstones in a landward direction (Nimegeers and Nickel, 2003). Along the West Kingsford–Macoun trend, the transition from shoal to evaporite facies (S2 Anhydrite) typically occurs over distances of less than 5 to 6 km. Most previous studies have included the S2 Anhydrite in the Frobisher Evaporite, implying that it was deposited prior to Lower Midale time. Interfingering of the evaporite with laterally equivalent Lower Midale lagoonal facies was first recognized by Smith (1980) near the western margin of the Benson Field. Nimegeers and Qing (2002) reported similar facies transitions in the southwestern part of the Steelman Field. The seaward edge of the S2 Anhydrite has been mapped in southeastern Saskatchewan from the Steelman area to the Weyburn Field (Nimegeers and Nickel, 2003). The distribution of the evaporite (Figure 2) is important for identifying potential plays related to carbonate-evaporite facies changes (e.g., Macoun Pool). Better producing shoals (e.g., 1/15-20-4-7W2) appear to occur where the lateral transition from shoal to evaporite facies is relatively abrupt (<3 km).

A B C

Figure 5 - Core photographs of the Lower and Upper Midale beds in the West Kingsford area. A) Oolitic grainstone reservoir with interparticle and vuggy porosity [Lower Midale unit, 15-20-4-7W2, 1496.4 m (4909.3 ft)]. B) Discontinuous vertical fracturing, colour mottling, and fenestral porosity development associated with microkarsting in a peloidal packstone interval below the Lower Midale upper contact [Lower Midale unit, 15-20-4-7W2, 1484.5 m (4870.4 ft)]. C) Fine bioclastic packstone in the Upper Midale unit [7-26-4-7W2, 1468.2 m (4817 ft)].

Saskatchewan Geological Survey 5 Summary of Investigations 2005, Volume 1

r nds h voi S t ’ r sa

o A N ser 2 rier

W re 7 - ) 4 bar n - 5 0 0 ne 7 7 0 5 4 5 4 rite Midale. Units are in 2 1 - 1 1 rite oo 5 ck- 1 sto / 1 apo 4 hyd ba lag cke G Ev ted stic wa 2 An ale tric cla lo- S e (S ervoir in the Upper Mid Do res Bio ell 10-14-4-7W2 associated with Lower h drit 2 wit W 7

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F K t d a s d e B e l a d i M i l a S Rat D A Figure 6 - Dip-oriented stratigraphic cross- Midale shoal facies and thinning of Upper Mi metres.

Saskatchewan Geological Survey 6 Summary of Investigations 2005, Volume 1 B B’ South North 11/13-9-4-8W2 21/7-16-4-8W2 21/11-21-4-8W2 G D G D G D

1475 Ratcliffe 1525

Midale 1525 Datum Evaporite

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d (Vuggy) i M 1550 Datum 1525 1575

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1575 1625 Kisbey 1625 Alida Beds 1600

Lagoon and Bioclastic back-barrier sands washover Lagoonal sediments with Midale Evaporite replacive (S2) nodular anhydrite Perforated intervals Kisbey Sandstone

Figure 7 - Dip-oriented stratigraphic cross-section B-B' in the Macoun area. Hydrocarbon trapping in well 13-9-4-8W2 may be related to landward thinning and pinchout of the Frobisher and Lower Midale cycles. Units are in metres. The distribution of Lower Midale shoals controlled later facies development in the Upper Midale. In the Kingsford- Macoun area, bioclastic back-barrier sands (Figure 5C) in the Upper Midale were deposited preferentially on the landward flanks of Lower Midale shoals. Moderately sorted skeletal material in these packstones mainly consists of sand-sized fragments (<2 mm) suggesting frequent reworking and abrasion. Skeletal packstones are usually observed at or near the base of the Upper Midale unit and rarely exceed about 3 m in thickness. A dolomitic skeletal wackestone reservoir occurs within the bioclastic sands and is an important producer in the West Kingsford Pool (Figure 8). The reservoir averages 1 to 2 m in thickness and pinches out in a landward direction (Figure 6). b) Midale: Elswick-Bromhead Area The Lower Midale unit in the Elswick-Bromhead area (Figure 2) differs considerably from facies transitions observed in the Macoun-Kingsford area. The primary difference is the general absence of oolitic shoals in these oil pools. Lower Midale shoals are observed in the Weyburn and Midale pools (Kaldi, 1982; Burrowes, 2001) suggesting a northward shift of the paleocoastline in this area. This does not imply, however, that deeper marine conditions characterized the Elswick and Bromhead areas during Lower Midale times. Core and well-log data indicate that shallow-water conditions associated with an uplifted region or paleotopographic ridge existed in the Elswick area, and this trend can be traced eastward into the Macoun area. The Lower Midale is anomalously thin in the Elswick and Bromhead areas. In several cores (5-6-5-12W2; Figure 9), only a thin (2.5 m thick) patterned to laminated mudstone (Figures 10A and 10B) is present in the Lower Midale. Often these mudstones are associated with inclined bedding, soft-sediment deformation features and brecciated intervals including large (8 cm), rounded to subangular intraclasts (Figure 10C). The mudstone is overlain by low- angle bedded, crinoidal, and skeletal packstones (Figures 10D, 10E, and 10F) of the Upper Midale unit. These packstones are interpreted as bioclastic shoals that unconformably overlie evaporitic mudflat facies. The Upper Midale shoal facies is productive in the Elswick area and grades laterally into less porous, bioturbated skeletal wackestones (Figures 9 and 11C).

Saskatchewan Geological Survey 7 Summary of Investigations 2005, Volume 1 Typical algal/peloidal wackestones of the Lower Midale were either eroded or never deposited in areas Top underlying these shoals. Lower Midale lagoonal facies were observed in wells adjacent to the shoals and contain reservoirs with pinpoint vuggy porosity (Figure 11A). The reservoir rock is capped by a non-porous, mottled mudstone/wackestone facies at the top of the Lower Midale (Figure 11B). The reservoir facies Upper pinches out updip toward the Upper Midale shoals (e.g., well 3-6-5-12W2; Figure 9).

c) Frobisher Beds Lower Several previous studies have discussed depositional facies in the Frobisher Beds (Crabtree, 1982; Lake, 1991; Kent and Curry, 2002; and Marsh and Qing, 2002). The Frobisher Beds have not typically been targeted for oil in the study area, and therefore core data are generally lacking. Stacked, coated-grain banks in the Frobisher Beds generally occur where the unit is thickest (>60 m thick) and where gamma-ray logs show relatively “clean” carbonate sequences separated by thin (<2 m thick) argillaceous beds (Figures 4A and 7). This contrasts with off-bank facies where the Frobisher is typically thinner, dominated by mudstones and Upper Midale wackestones, and is more argillaceous (well 7-16-4- Base Dolomitic Wackestone Reservoir 8W2; Figure 6). In the study area, Frobisher banks appear to parallel the Macoun lineament zone shown in Figures 2 and 3.

Frobisher bank development along the West Kingsford–Elswick oil-field trend appears primarily to Figure 8 - Core box photograph showing dolomitic wackestone reservoir of the Upper Midale underlying have been an aggradational process. This interpretation bioclastic packstone facies in the West Kingsford area. Note is supported by core data showing abrupt lateral the Upper-Lower Midale contact below the reservoir facies transitions from stacked-bank facies to thinner, mud- [7-26-4-7W2, 1472.4 to 1467.9 m]. dominated off-bank facies (e.g., wells 15-15-4-6W2 and 7-21-4-6W2; Marsh and Qing, 2002). The upward persistence of lower energy mudstones and wackestones in off-bank areas suggests very little progradation of adjacent banks.

An off-shoal setting was interpreted from core in the 11-31-4-12W2 well (Figure 9) in an area where the Frobisher Beds are unusually thin (<45 m thick). A 5 m interval of patterned carbonate at this location (Figure 12A) overlies bioturbated skeletal wackestone and represents a transition from normal-marine to evaporitic-lagoon conditions. The wackestone contains thin beds (10 to 15 cm) of crinoidal/skeletal packstone. This facies is similar to off-shoal facies described in the South Heward Pool (Kent and Curry, 2002).

In the 5-6-5-12W2 well, core from the upper Frobisher contains 7 m of crinoidal/skeletal packstone-grainstone (Figure 12B) overlying a poorly sorted intraclastic/skeletal packstone facies (Figure 12C). The upper part of the crinoidal facies contains fair (10 to 15%) interparticle porosity. Approximately 3 m above the crinoidal/skeletal facies, a peloidal/oolitic packestone with vuggy porosity forms another reservoir in the Frobisher. Facies in this well are more grain dominated than stratigraphically equivalent facies in 11-31-4-12W2. The poorly sorted packstone facies may be similar to washover facies interpreted in the South Heward Pool (Kent and Curry, 2002). The crinoidal packstone facies contains faint planar bedding (Figure 12B) and is interpreted as a shoal facies. Coated-grain facies in the Frobisher Beds are potential targets in the study area and remain underexploited. They may be more common in the West Kingsford and Macoun areas than in the Elswick region where production is mainly from porous lenses of crinoidal grainstone and peloidal packstone. To date, production from these thin reservoirs, which interfinger updip with nonporous mudstones and wackestones, has been minimal.

Saskatchewan Geological Survey 8 Summary of Investigations 2005, Volume 1

2 W h 2

t ’ 1 r - 0 5 0 0 5 C o 5 5 2 0 0 7 - 5 5 5 6 5 1 1 1 1 1 N 7 - 5 / 1 2 G

metres. 2 w Upper Midale crinoidal shoals in wells 3-6- W 2 1 5 0 - 2 0 5 5 5 1 1 - 6

- 5 / G 1

shoals. Units are in

2 W

2 5 0 1 2 0 - 5 5 5 1 1 - 6 - er the Upper Midale

3 Kisbey Sandstone / Perforated intervals Evaporitic mudflat/lagoon (patterned- laminated mudstones) G Off-shoal (bioturbated skeletal wackestone) 1

inning of Lower Midale due to erosion belo S 2 W 2

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2 W 2 r in 11-31-4-12W2 thins and pinches out und

1 - 0 5 0 4 0 2 5 - 5 5 5 1 1 1 1 3 section C-C' in the Elswick area. Note th - Lagoon and washover (peloidal/algal packstones) Crinoidal shoal Midale Evaporite Sabkha (nodular anhdyrite) 1 1 / 1 G

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4 G

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A Datum R 5-12W2 and 5-7-5-12W2. Lower Midale reservoi Figure 9 - Dip-oriented stratigraphic cross-

Saskatchewan Geological Survey 9 Summary of Investigations 2005, Volume 1

A B C

D E F

Figure 10 - Core photographs of Upper and Lower Midale beds in the Elswick area. A) Laminated mudstone overlying patterned mudstone in the Lower Midale. Note soft-sediment deformation and displacement of laminated bedding [5-6-5- 12W2, 1524.7 m (5002.5 ft)]. B) Laminated/brecciated mudflat facies in the Lower Midale. Note abundant flat-pebble intraclasts typical of storm erosion of tidal-flat sediments [5-35-4-13W2, 1555.2 m]. C) Brecciation and slumping of mudflat sediments in the Lower Midale. Note highly inclined bedding [5-6-5-12W2, 1525 m (5003.5 ft)]. D) Eroded upper contact of the Lower Midale. Note elongate burrow-like structures or borings filled with skeletal wackestone [5-6-5-12W2, 1523.7 m (4999 ft)]. E) Bedded crinoidal grainstone reservoir in Upper Midale Beds [5-6-5-12W2, 1520 m (4986.8 ft)]. F) Inclined bedding in crinoidal shoal facies of the Upper Midale Beds [5-35-4-13W2, 1552.7 m (5094 ft)].

Saskatchewan Geological Survey 10 Summary of Investigations 2005, Volume 1

A B C

5027 ft

Figure 11 - Depositional facies in the Lower and Upper Midale Beds in off-shoal areas of the Elswick region. A) Algal- peloidal wackestone reservoir in the Lower Midale, Elswick area [11-31-4-12W2, 1532 m (5027 ft]). B) Nonporous mottled mudstone wackestone facies near top of Lower Midale; this facies caps reservoir rock observed in (A) [11-31-4-12W2, 1528.9 m (5016 ft)]. C) Bioturbated skeletal-wackestone facies of the Upper Midale; this facies occurs adjacent to Upper Midale bioclastic shoal facies shown in photographs 10E and 10F [11-31-4-12W2, 1528.3 m, (5014 ft)].

4. Evidence for Syndepositional Reactivation of Lineament Zones Several lines of evidence indicate syndepositional structural control impacted facies trends in the Midale and Frobisher beds. Lower Midale shoals and the S2 Anhydrite edge are roughly coincident with the Macoun lineament interpreted by GEDCO from high-resolution aeromagnetic and seismic data (Whittaker, 2004). Lateral changes from shoal to evaporitic environments were likely controlled by local paleotopography along this trend (Figure 13). It is also important to note that Lower Midale shoals are restricted to areas south of the S2 Anhydrite edge (Figure 4B). Frobisher banks also appear to have developed parallel to this trend (Figure 4A). A northward shift in depositional facies (e.g., the paleocoastline) occurs where the Nesson Wrench Fault intersects the Macoun lineament. The S2 Anhydrite edge shifts northward (Figure 13) and Lower Midale shoals were able to develop north of the study area in the Weyburn and Midale Fields (Kaldi, 1982; Burrowes, 2001). An area of Devonian Prairie Evaporite dissolution is also near the intersection of these lineament zones. In well 1-4- 4-11W2 (Figures 13 and 14A), the Prairie Evaporite is absent and the Mississippian Poplar Beds are approximately 30 m thicker than in adjacent wells. This suggests that dissolution occurred prior to any significant erosion at the sub-Mesozoic unconformity surface, and was possibly syndepositional with Poplar Bed accumulation. Differential dissolution of the Prairie Evaporite also likely produced relative paleotopographic differences such that the geological setting that resulted in the development of reservoir rocks of the Elswick and Bromhead Pools was along the upper flanks of a salt-dissolution trend. Isopach maps of the Prairie Evaporite (Kreis et al., 2003) show local thinning in the area south of the Elswick Field and east of the Bromhead area (Figure 13).

Saskatchewan Geological Survey 11 Summary of Investigations 2005, Volume 1

A B C

Figure 12 - Depositional facies in the Frobisher Beds of the Elswick area. A) Patterned carbonate in the upper Frobisher Beds, Elswick area, likely deposited in a shallow evaporitic-lagoon setting [11-31-4-12W2, 1551 m (5090 ft)]. B) Crinoidal/skeletal packstone/grainstone interpreted as a shoal facies in the upper Frobisher Beds, Elswick area [5-6-5- 12W2, 1547.7 m, (5078 ft)]. C) Poorly-sorted, intraclastic/skeletal packstone facies in the upper Frobisher Beds, Elswick area, interpreted as a washover facies [5-6-5-12W2, 1549.9 m, (5085 ft)]. Figure 13 shows trends where Poplar Beds are relatively thick. The increased thickness of the Poplar Beds along these trends may be related more to local preservation below the erosional surface represented by the sub-Mesozoic unconformity than to depositional processes. It is noteworthy that the linear zone of thickening is spatially associated with abrupt facies and thickness changes in the Midale and Frobisher beds and the Macoun lineament. In the western part of the study area, there is a spatial relationship between Prairie Evaporite dissolution and Poplar Bed thickening, whereas in the Macoun and West Kingsford areas, Poplar thickening is more aligned with the Macoun lineament.

5. A Predictive Model Stratigraphic cross-sections X-X' and Y-Y' (Figure 14) show facies and thickness changes observed near the Macoun lineament. Across the lineament, the Poplar Beds are anomalously thick; however, the Frobisher Beds show local thinning in this same area. Core data indicate that Frobisher “thins” are dominated by muddier, off-bank facies that were deposited in structurally lower lying areas. Thickening of the Frobisher toward the north and south (Figure 14A) is related to coated-grain bank aggradation over structural highs in these areas. Lower Midale shoals developed on the flanks of Frobisher banks, and in these areas the Lower Midale is relatively thick compared to the Upper Midale (Figure 14). The Upper Midale thickens in a landward direction away from the shoals, while Lower Midale lagoonal sediments thin and grade into the S2 Evaporite. The S2 Anhydrite edge typically occurs in areas immediately north of the postulated structural lows (Figure 14A). Figure 15 illustrates several stratigraphic traps related to the facies changes described above. Revealing the relationships among facies laterally and vertically may provide a predictive model for identifying potential areas of hydrocarbon accumulation in Mississippian strata away from the subcrop edge.

Saskatchewan Geological Survey 12 Summary of Investigations 2005, Volume 1

WEST T5 ELSWICK KINGSFORD MACOUN C’ A’ STEELMAN Y’ X’ B’ POOL C T4 A B

Y X T3

BROMHEAD

T2 Estevan

R13 R12 R11 R10 R9 R8 R7 R6 R5

0 10 20 30 40 Faults interpreted by GEDCO kilometres A’ Oilfield Location of well-log cross sections A Seaward edge of S2 Anhydrite X’ Trend of anomalous Poplar Bed thickening Location of line cross sections

Lower Midale oolitic shoal facies X Area where Devonian Prairie Upper Midale skeletal/crinoidal shoals Evaporite is anomalously thin

Figure 13 - Map of the study area showing facies and thickness trends in the Midale, Frobisher and Poplar beds an the location of lineament zones and regions of Devonian Prairie Evaporite dissolution.

6. References

Burrowes, G. (2001): Investigating CO2 storage potential of carbonate rocks during tertiary recovery from a billion barrel oil field, Weyburn, Saskatchewan: Part 2 – reservoir geology (IEA Weyburn CO2 Monitoring and Storage Project); in Summary of Investigations 2001, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1, p64-71.

Crabtree, H.T. (1982): Lithologic types, depositional environment, and reservoir properties of the Mississippian Frobisher Beds, Innes Field, southeastern Saskatchewan; in Christopher, J.E. and Kaldi J. (eds.), Fourth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 6, p203-210. Fuzesy, L.M. (1960): Correlation and Subcrops of the Mississippian Strata in Southeastern and South Central Saskatchewan; Sask. Dep. Miner. Resour., Rep. 51, 63p. Kaldi, J. (1982): Reservoir properties, depositional environments and diagenesis of the Mississippian Midale Beds, Midale field, southeastern Saskatchewan; in Christopher, J.E., and Kaldi, J. (eds.), Fourth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 6, p211-216. Kent, D.M. (1987): Mississippian facies, depositional history, and oil occurrences in Williston Basin, Saskatchewan and Manitoba; in Peterson, J.A., Kent, D.M., Anderson, S.B., Pilatzke, R.H., and Longman, M.W. (eds.), Williston Basin: Anatomy of a Cratonic Oil Province, Rky. Mtn. Assoc. Geol., Denver, p157-170.

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A X X’

S 11/1-35-3-9W2 11/13-11-4-9W2 31/5-13-4-9W2 41/2-24-4-9W2 N Lower Watrous 1450 1500

s d

e 1500 1450 B

r a l p o

P 1500

Ratcliffe 1550 Beds 1550 1500

s Midale Evaporite

e Upper Midale

B (Marly)

e l 1550 a Lower Midale d i (Vuggy) M 1600 s

d 1600 1550 e B

S2 Anhydrite r e h s i 1600 b o r 1650 F

Kisbey 1600 Alida Beds Low High Approximate location of High Macoun lineament Y Y’

41/7-32-3-11W2 21/1-4-4-11W2 1/9-7-4-11W2 31/13-20-4-11W2 B S N

Lower 1550 Watrous 1500

1550 s

d 1600 e B

1500 r 1550 a l p o P 1600 Ratcliffe Beds 1650 1575 s Midale Evaporite 1600 d

e Upper Midale B

(Marly) e l

a Lower Midale d i (Vuggy) 1650 M 1700

s 1650 1650 d e B

e h s

i 1650 b o r

F 1750 1650 1650 Kisbey

Alida Beds Low High High Approximate location of Macoun lineament

Figure 14 - Dip-oriented stratigraphic cross sections across the Macoun lineament. Locations of cross sections are show in Figures 1 and 13. Arrows indicate approximate locations of interpreted paleostructural highs and lows. A) Stratigraphic cross section X-X' showing thickness and facies relationships in the Macoun area. Note thickening of the Poplar Beds (13-11-4- 9W2) is coincident with an area of thinning in the underlying Frobisher Beds. B) Stratigraphic cross section Y-Y' showing thickness and facies relationships associated in the Bromhead area. Anomalous Poplar thickening in well 1-4-4-11W2 is related to an area of complete dissolution of the Prairie Evaporite that occurs near the intersection of the Macoun lineament with the Nesson Wrench Fault. Depths in metres.

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South North

ite le Evapor e “Marly” Mida Upper Midal e porit rite Eva “Vuggy” nhyd isher wer Midale S2 A Frob Lo

isher Beds Frob

Kisbey Sandstone

Evaporites (salina/sabkha) Intershoal environment Hydrocarbon trap Interbedded limestone and Back-barrier sands Midale/Frobisher contact nodular anhydrite (S2) Kisbey Sandstone Structural high Restricted shelf environment Structural low Shoal/bank environment Figure 15 - Stratigraphic Trapping Model for the Midale and Frobisher beds along the Macoun lineament. Arrows indicate interpreted structural highs and lows. ______(1999): A re-examination of the process of formation and diagenesis of coated-grain accumulations and contiguous facies in the Mississippian of southeastern Saskatchewan; in Summary of Investigations 1999, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.1, p17-26.

Kent, D.M. and Curry C. (2002): Characterization of reservoir rocks in the Mississippian Frobisher Beds of the South Heward Oil Pool, southeastern Saskatchewan; in Summary of Investigations 2002, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.1, CD-ROM, p24-36.

Kent, D.M. and Nimegeers A.R. (2002): Intrastratal depositional breaks in the Mississippian Midale Beds of southern Saskatchewan; their role in reservoir rock creation and hydrocarbon accumulation; in C.S.P.G. Core Convention Abstracts, Diamond Jubilee Convention, June 3-7, Calgary, p47. Kreis, L.K., Thomas, P.L., Burke, R.B., and Whittaker, S.G. (2003): Devonian isopach and structure maps, IEA Weyburn CO2 Monitoring and Storage Project Area; Sask. Industry Resources, Test Version CD-ROM. Lake, J.H. (1991): Transgressive cycles in an overall shallowing upwards sequence, Mississippian, Mission Canyon, Nottingham Unit, Williston Basin, southeast Saskatchewan; in Christopher, J.E. and Haidl, F.M. (eds.), Sixth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 11, p136-141. ______(1998): A Mississippian epeiric shelf model for the Williston Basin; in Christopher, J.E., Gilboy, C.F., Paterson, D.F., and Bend, S.L. (eds.), Eighth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Pub. No. 13, p69-71. ______(2001): Onlap within the Mississippian Midale Beds of southern Saskatchewan; in Summary of Investigations 2001, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1, p56-60.

Saskatchewan Geological Survey 15 Summary of Investigations 2005, Volume 1 ______(2002): Hydrocarbon trapping in the Midale Beds south of Weyburn Unit, southeastern Saskatchewan; in Summary of Investigations 2002, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.1, CD-ROM, p68-72. Marsh, A.K. and Qing, H. (2002): Transgressive-regressive cycles of the Mississippian carbonate-evaporite succession in the Steelman area, Southeastern Saskatchewan; in Summary of Investigations 2002, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.1, CD-ROM, p37-46. Nimegeers, A.R. and Nickel, E. (2003): Horizontal Potential in a Lower Midale (Vuggy) reservoir of the Mississippian Midale Beds, Weyburn-Steelman area, southeastern Saskatchewan; in Sask. Geol. Soc., Core Workshop Volume, Spec. Publ. No.16. Nimegeers, A.R. and Qing, H. (2002): Depositional model of the Mississippian Midale Beds, Steelman Field, southeastern Saskatchewan; in Summary of Investigations 2002, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.1, CD-ROM, p47-67. Smith, S.R. (1980): Petroleum Geology of the Mississippian Midale Beds, Benson Oil Field, Southeastern Saskatchewan; Sask. Energy Mines, Rep. 215, 98p. Wegelin, A. (1984): Geology and reservoir properties of the Weyburn field, Southeastern Saskatchewan; in Lorsong, J.A. and Wilson, M.A. (eds.), Oil and Gas in Saskatchewan, Sask. Geol. Soc., Spec. Publ. No. 7, p71- 82. Whittaker, S.G. (2004): Investigating geological storage of greenhouse gases in southeastern Saskatchewan: The IEA Weyburn CO2 Monitoring and Storage Project; in Summary of Investigations 2004, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.1, CD-ROM, Paper A-2, 12p.

Whittaker, S., Rostron, B., Khan, D., Hajnal, Z., Qing, H., Penner, L., Maathuis, H., and Goussev, S. (2004): Theme 1: Geological Characterization; in Wilson, M. and Monea, M. (eds.), IEA GHG Weyburn CO2 Monitoring and Storage Project Summary Report 2000-2004, Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, Volume III, p15-69.

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