Geology of the Scimitar Lake Area

Preliminary Study of Birdbear Reservoirs in West-central Saskatchewan

Chao Yang and Don Kent 1

Yang, C. and Kent, D. (2010): Preliminary study of Birdbear reservoirs in west-central Saskatchewan; in Summary of Investigations 2010, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2010-4.1, Paper A-6, 12p.

Abstract Although oil has been intermittently produced from the Birdbear Formation in west-central Saskatchewan since 1968, the recent application of horizontal technology has resulted in a sharp increase of average production from 6 m3/day in January 2008 to over 165 m3/day in December 2009. This success has sparked renewed interest in the Birdbear, resulting in re-evaluation of the formation’s potential as an oil resource for the province. In west-central Saskatchewan, Birdbear oil has been recovered along the subcrop of the Birdbear Formation (Tp. 38 and 39, Rge. 24W3 to 27W3) and in the area north of Kindersley (Tp. 30 to 32, Rge. 23W3 to 28W3). Core examination, thin-section studies, and log-to-core correlation of the Birdbear pay zones are being carried out to improve understanding of reservoir quality and trapping mechanisms. North-south– and west-east–trending cross sections have been constructed to correlate the upper and lower members of the Birdbear Formation.

Keywords: Devonian, Birdbear, oil production, reservoir, stratigraphy, lithology, facies, structure, diagenesis, low resistivity, west-central Saskatchewan.

1. Introduction The Birdbear Formation (Figure 1) marks the last major phase of upper Devonian carbonate sedimentation in Saskatchewan. The upper portion of the Birdbear correlates with the Nisku Formation of the Alberta Basin. In Saskatchewan, hydrocarbons are produced from this formation in two widely separated areas, the southeast and west-central. Up to 2007, cumulative production of heavy oil from the Birdbear in the west-central area was only 26 x 103 m3, but this increased to 107 x 103 m3 to May 2010, largely due to utilization of horizontal drilling technology.

Kent (1968) applied a two-fold subdivision to the Birdbear of west-central Saskatchewan. He identified a lower member largely made up of argillaceous carbonate, and an upper member composed of both non-argillaceous and argillaceous dolostones. In a detailed study of the upper Birdbear in the Kindersley area, Cisyk (1991) identified lithologies that he interpreted as representing marginal bank and inter-platform basin successions, the latter being the oil-bearing rocks. He also provided a detailed analysis of the diagenesis of the various facies. A detailed study of diagenesis in the Birdbear and Nisku formations of southeastern Alberta and southwestern Saskatchewan was published by Whittaker and Mountjoy in 1996.

The origin of oil in the Birdbear is not clear. It is possible that that oil has the same source as the Nisku of Alberta. Winterburn Group source rocks in central to southern Alberta, which have high total organic carbon contents (up to 15%) and contain Type I and Type II organic matter deposited in open marine to lagoonal paleoenvironments, are correlated to oil within the Nisku Formation of east-central Alberta and Camrose Member/Nisku Formation of southern Alberta (Fowler et al., 2001). Their relationship to the Birdbear oil in west-central Saskatchewan requires further investigation. This paper presents preliminary results of a study initiated to better understand Birdbear reservoirs in west-central Saskatchewan from Townships 30 to 40, Ranges 23W3 to 29W3 (Figure 2). The study is focused on stratigraphic correlation of units in the upper member and on reservoir characterization using geophysical well logs, well cores and samples, and thin sections.

1 D.M. Kent Consulting Ltd., 86 Metcalfe Road, Regina, SK S4V 0H8.

Saskatchewan Geological Survey 1 Summary of Investigations 2010, Volume 1 ERA PERIOD EPOCH 2. Birdbear Formation Production History in West-central Bakken Three Saskatchewan Forks Big Valley Group The Upper Devonian Nisku Torquay Formation has been one of the Upper Birdbear major oil producers in Alberta Devonian for many years. In west-central Saskatchewan Saskatchewan, before 2007, only Group Duperow 11 wells had produced from the Birdbear Formation with a cumulative output of about 26 000 m3 of oil. By May 2010, Souris River Manitoba the number of wells with Group 1st Red Bed Birdbear production history had Dawson Bay increased to 37 with total oil DEVONIAN 2nd Red Bed 3 PALEOZOIC Prairie production of almost 107 000 m Middle Evaporite (Table 1). Production is derived Devonian Elk from two regions, the first Point Winnipegosis associated with the subcrop of Group the Birdbear Formation (Tp. 38 Ashern and 39, Rge. 24W3 to 27W3), and the second located north of Lower Kindersley (Tp. 30 to 32, Rge. Devonian 23W3 to 28W3) (Figure 2). Application of horizontal well drilling has increased yearly production from 1050 m3 in 2006 to 47 760 m3 in 2009 (Figure 3). Sandstone Evaporite Carbonate Red Bed Over the 24-month period from Figure 1 - Stratigraphy of Devonian strata in the study area. January 2008 to December 2009, the number of producing wells in the Birdbear rose from five to 18 (Figure 3), increasing the monthly production from 180 m3 to 5135 m3 (Figure 4).

3. Stratigraphy, Lithology, and Facies In west-central Saskatchewan, the Frasnian Birdbear Formation is conformable with the underlying Duperow Formation and overlying Torquay Formation (Figure 1) except near the northern margin of the area where it is truncated by the sub-Mesozoic unconformity and overlain by the Lower Cretaceous Mannville Group. The Birdbear Formation is divisible into a lower member of argillaceous carbonate and an upper member containing both non- argillaceous and argillaceous dolostones (Kent, 1968). The lower member represents a single shoaling-upward sequence consisting of a basal argillaceous open marine to subtidal dolomitized mudstone unit, and an upper subtidal to intertidal argillaceous dolomitized wackestone unit. Argillaceous content generally decreases upwards. The boundary between the lower to upper members can be recognized best on the gamma-ray log where it is placed at the contact between an interval of higher gamma radiation below and one of lower gamma radiation above. In core, the contact is commonly placed at the colour change from mainly grey to beige and grey, and the increasing amount and variety of the fossil debris. This boundary is indicated in the log and core correlation of reference well Rife et al Eureka 131/14-02-032-23W3 (Figure 5) and logs of Nuvista Macklin 111/12-04-039-27W3 well (Figure 6). Regional structural cross sections of the Birdbear Formation incorporating the boundary between the lower and upper members are presented in Figures 7 and 8. In this study, the upper member of the Birdbear Formation in west-central Saskatchewan is subdivided into two informal units based on log analysis and core examination of Rife et al Eureka 131/14-02-032-23W3 well: a basal open marine sequence and an upper restricted marine sequence (Figure 5). These units are recognizable on geophysical well logs and can, therefore, be correlated across the study area (Figures 7 and 8). Unit 1 conformably overlies the lower Birdbear member. It is an open-marine subtidal to intertidal sequence composed of two fine crystalline dolomitized wackestone intervals separated by a laminated argillaceous dolostones. The lithology seen in core is easily correlated to gamma-ray log, as shown in the reference well of Rife et al Eureka 131/14-02-032-23W3 well (Figure 5).

Saskatchewan Geological Survey 2 Summary of Investigations 2010, Volume 1 R28 R27 R26 R25 R24 R23W3

T40 T40 Unity

14 N T39 Core T39 Macklin

T38 T38

T37 T37

21

T36 T36

T35 T35

Saskatchewan

Lloydminster T34 Kerrobert T34

T33 T33 Saskatoon

Kindersley T32 Core E T32 W Regina

B' A' Swift Current

Weyburn T31 T31 S

kilometres 0 5 10 15 T30 T30 0 5 10 Birdbear subcrop (Marsh et al., 2011) miles R28 R27 R26 R25 R24 R23W3 Figure 2 - Map illustrating the location of the study area, wells with Birdbear oil production (green dots), and the location of the two cores referred to in this paper; note that in Township 39, Range 27W3, 19 wells have produced from the Birdbear. Also shown is the location of the Birdbear subcrop edge and north-south and west-east cross sections shown in Figures 7 and 8. The lower interval (Unit 1A) contains light beige to green-grey very fine crystalline dolostone in the lower part and dolomitized fine crystalline wackestone with light grey, very fine crystalline dolostone interbeds in the upper part. Diversified fauna, including brachiopods, corals, crinoids and algae, are observed in matrix-supported wackestone proportions. Some are partially dissolved, forming moldic porosity (Figure 9A). Dissolution vugs and fractures are well developed in the upper part. Average porosity, based on core analysis, is around 9% with variable maximum permeability values ranging from 1 to 1820 md. Oil staining is observed. The middle interval (Unit 1B) contains beige to grey laminated argillaceous very fine crystalline dolostone. Faunal diversity is limited. Brachiopods are observed to be concentrated along bedding plane (Figure 9B). This interval has higher porosity than unit 1A (averaging around 13%) and very low permeability (around 39 md).

Saskatchewan Geological Survey 3 Summary of Investigations 2010, Volume 1 Table 1 - Production data for wells in west-central area in Saskatchewan that have produced oil from the Birdbear (to May 2010). Data are compiled from information provided by the Petroleum Statistics Brach, Saskatchewan Ministry of Energy and Resources. Cumulative oil production Last year Year to May 2010 on Well Name Well ID Licence # Drilled (m3) production

HUSKY HOMESTEAD SMILEY 10 29 31 26 101/10-29-031-26W3/00 68F030 1968 76 1968 I N C HUSKY N HOOSIER 13 21 32 28 101/13-21-032-28W3/00 69C023 1969 383 1969

RENAISSANCE UNITY A8-28-39-24 101/08-28-039-24W3/00 83E114 1983 30 1983 RIFE ET AL EUREKA 12-2-32-23 101/12-02-032-23W3/00 84K097 1984 955 1986

PVR COLEVILLE B12-20T-31-23 122/12-20-031-23W3/00 85B056 1985 2 1986 RIFE ET AL EUREKA 14 2 32 23 131/14-02-032-23W3/00 85E014 1985 2,796 1988 BARR ZALLER LAKE A16-1-39-26 111/16-01-039-26W3/00 85K264 1985 2,283 1994 NUVISTA MACKLIN 12-4-39-27 111/12-04-039-27W3/00 87C016 1987 1,616 1991 RENAISSANCE ET AL ZALLER L HZ 3B4-6-2C13-6-39-25 191/13-06-039-25W3/00 93A079 1993 15,761 2010 NUVISTA EVESHAM S HZ 3C4-4-4C6-4-39-27 191/06-04-039-27W3/00 93G037 1993 34 1994 PVR ET AL MILTON INJ A7-24-30-28 111/07-24-030-28W3/00 97C210 1997 138 1997 BAYTEX FCE EUREKA N A7-10-32-23 111/07-10-032-23W3/00 97D060 1997 1,062 2009 WASCANA FUSILIER A8-29-32-28 111/08-29-032-28W3/00 97G006 1997 259 1997 ISH HOOSIER RE A10-29-31-26 101/10-29-031-26W3/01 97L243 1998 1,058 2010 NUVISTA MACKLIN C10-4-39-27 131/10-04-039-27W3/00 06L322 2007 3,654 2010 NUVISTA MACKLIN 2HZ 3A6-4-2D14-4-39-27 191/14-04-039-27W3/00 07K200 2007 3,901 2010 NUVISTA HALLAM NORTH A13-5-39-27 111/13-05-039-27W3/00 07E079 2007 312 2007 NUVISTA MACKLIN A16-5-39-27 111/16-05-039-27W3/00 07G099 2007 955 2009 BAYTEX FUSILIER NORTH HZ 3B4-28-4B4-29-32-28 191/04-29-032-28W3/00 08E247 2008 3,219 2010 TALISMAN SENLAC HZ 4D4-1-2D10-34-38-26 191/10-34-038-26W3/00 08B349 2008 4,235 2010 TALISMAN SENLAC HZ 1D3-12-3A2-1-39-26 191/02-01-039-26W3/00 08H351 2008 13,057 2010 NUVISTA HALLAM NORTH HZ 3D4-9-1D5-4-39-27 191/05-04-039-27W3/00 08J339 2008 7,341 2010 NUVISTA HALLAM NORTH HZ 3B2-9-1D6-4-39-27 192/06-04-039-27W3/00 08J353 2008 7,134 2010 NUVISTA HALLUM NORTH RE HZ 4A2-9-3A7-4-39-27 191/07-04-039-27W3/00 08K266 2008 8,312 2010 HUSKY SALT LAKE HZ 1A8-29-1C4-29-39-24 191/04-29-039-24W3/00 09I096 2009 292 2010 NUVISTA HALLAM NORTH HZ 4C4-9-1B5-4-39-27 192/05-04-039-27W3/00 09B057 2009 5,618 2010 NUVISTA HALLAM NORTH HZ 3B3-9-3B6-4-39-27 193/06-04-039-27W3/00 09B055 2009 1,594 2010 NUVISTA HALLAM NORTH HZ 3B1-9-2C8-4-39-27 191/08-04-039-27W3/00 09B056 2009 3,609 2010 NUVISTA HALLAM NORTH HZ 3A1-9-2D8-4-39-27 192/08-04-039-27W3/00 09I133 2009 2,633 2010 NUVISTA MACKLIN HZ 1A1-8-4D8-5-39-27 191/08-05-039-27W3/00 09A100 2009 1,946 2010 NUVISTA HALLAM NORTH HZ 2A1-8-1B9-5-39-27 191/09-05-039-27W3/00 09B058 2009 2,967 2010 NUVISTA HALLAM NORTH HZ 8A16-5-4C12-5-39-27 191/12-05-039-27W3/00 09I143 2009 4,535 2010 CULANE COLEVILLE HZ 1C8-20-1C5-20-31-23 191/05-20-031-23W3/00 10A132 2010 23 2010 HUSKY ZOLLER LAKE HZ 3C13-6-1C1-1-39-26 192/01-01-039-26W3/00 09L167 2010 1,838 2010 NUVISTA HALLAM NORTH HZ 8C4-9-2A5-4-39-27 193/05-04-039-27W3/00 09L260 2010 928 2010

NUVISTA HALLAM NORTH HZ 8B2-9-1B10-4-39-27 191/10-04-039-27W3/00 09L246 2010 928 2010 NUVISTA HALLAM NORTH HZ 4A9-5-4B12-5-39-27 192/12-05-039-27W3/00 09L254 2010 1,480 2010

Total Production 106,964

The upper interval (Unit 1C) is composed of interbedded beige fine crystalline dolomitized wackstone and grey very fine crystalline dolostone with a wispy nodular texture. Much of the fossil debris can be recognized only by the molds left following their dissolution. Intercrystalline dissolution vugs and fractures are the major porosity types in wackestone. The vugs and fractures are coated by heavy oil stains, and locally partially filled by hydrocarbon residues (Figure 9C). This interval has variable porosity (10 to 24%) and permeability (22 to 2050 md) which can be attributed to low permeability dolostone interbedded with the more permeable wackstone containing solution- enlarged vugs and fractures. Unit 2 is represented by a restricted lagoon sequence with laminated argillaceous dolostone at the base and a shallowing upward sequence of intertidal to supertidal deposits. The fauna are sparse in this unit. This unit is overlain by anhydrites and claystones of the Upper Devonian Torquay Formation. The Rife et al Eureka 131/14-02- 032-23W3 well clearly demonstrates that this unit can be subdivided into three intervals (Figure 5).

Saskatchewan Geological Survey 4 Summary of Investigations 2010, Volume 1 25 The lower interval (Unit 2A) contains light beige to grey Oil Production laminated argillaceous very fine Producing Wells 40,000 20 crystalline dolostones that is easily recognized on gamma logs by its high radiation peak that ) 3 immediately overlies the open 30,000 15 marine sequence. Oil staining is not evident in hand specimens. The middle interval (Unit 2B) 20,000 10

ProducingWells consists of beige laminated fine Oil Production(m Oil crystalline dolostones. Intercrystal pores, solution- 10,000 5 enhanced vugs and fractures are well developed, resulting in good porosity (14%) and permeability (312 md). Locally the vug and 0 0 fracture porosity is partially filled by calcite (Figure 9D) and 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 recrystallized dolomite. This Years interval is heavily oil stained Figure 3 - Birdbear oil production and number of producing wells in west-central with hydrocarbon residues filling Saskatchewan from 1968 to 2009. Data are compiled from information provided by in some pore spaces. the Petroleum Statistics Brach, Saskatchewan Ministry of Energy and Resources. The upper interval (Unit 2C) is a Oil Production shallowing-upward sequence. 5,500 The lower contact is placed at the base of a thick argillaceous 5,000 dolostone characterized by a 4,500 high gamma-ray response

) (Figure 5). From base to top, this 3 4,000 sequence includes grey to beige laminated argillaceous very fine 3,500 crystalline dolostones, yellowish chalky dolostone, and anhydritic 3,000 shale. The porosity and permeability are very low except 2,500 toward the top of the interval in

Oil Production(m Oil 2,000 the chalky dolostone where porosity increases to 23%, but 1,500 permeability remains low (31 md). This high porosity and 1,000 low permeability chalky dolostone is easily identified in 500 density/neutron logs (Figure 5) and is areally extensive in 0 2008 2009 Township 32. Figure 4 - Birdbear oil production and number of producing wells in west-central Saskatchewan from January 2008 to December 2009. Data are compiled from information provided by the Petroleum Statistics Brach, Saskatchewan Ministry of 4. Diagenesis Energy and Resources. Core and thin section examinations indicate that the Birdbear Formation in west-central Saskatchewan is heavily dolomitized and the original depositional fabric could not be recognized in most cases (e.g., Nuvista Macklin 111/12-04-039-27W3 well). The massive dolomitization occurred in the early stage of diagenesis, possibly shortly after burial. Significant intercrystalline porosity was created during dolomitization (Figure 10A). In some intervals, the intercrystalline porosity has been enhanced by dolomite dissolution (Figure 10B). High production values are from dolostone reservoirs in which dissolution has increased permeability horizontally and vertically through development of cavity, vuggy, and fracture porosity. Dissolution may be related to an influx of

Saskatchewan Geological Survey 5 Summary of Investigations 2010, Volume 1 14-02-032-23W3 KB=701.1m Rife et al Eureka LIC 85E014 Top Crystal Lithology Porosity K-max Sizes Oil Porosity Sed. Fauna Diag. Facies Description (%) (md) (100µ m) Stain Type Structures 0 10 20 30 0 1000 2000 1 2 3 Torquay 890 Alternating grey- green to brown laminated Torquay Birdbear fine siltstones and shales Anhydrite and shale 5 to 127 Unit 2C 894 Chalk dolostones Avg. 31 IC 00900 Shallowing- Light beige 15 to 30% upward Avg. 23% IC sequence laminated very fine crystalline M dolostones Perforated Grey laminated 898 argillaceous very IC fine crystalline Unit 2B dolostones M Unit 2 Beige laminated M fine crystalline 10 to 24% Avg.14% Restricted dolostones M 902 42 to 674 marine Avg. 312 M Unit 2A IC sequence 11 12 Perforated Upper Birdbear Light beige to Unit 1C 906 grey laminated argillaceous very IC fine crystalline

Unit 1 Unit 2 dolostones

Unit 1B M Karst 910 Interbeded beige 10 to 24% wackstone and M Avg.16% 22 to 2050 Mold grey fine Avg. 606 M M crystalline IC dolostones 00925 Beige to grey 10 to 15% 914 laminated 1 to 364 Avg.13% argillaceous very Avg. 39 M IC Unit 1A fine crystalline dolostones Unit 1 Normal marine 918 Light beige to green-grey very sequence fine crystalline M M dolostones 1 to 12% 1 to 1820 M to dolomitized Avg. 9% Avg. 478 IC wackstones upwards 922

Lower Birdbear Grey argillaceous Lower very fine IC Birdbear crystalline Member dolostones Birdbear 926 Legend Duperow Open vug Dolomite recrystallization Coral M Medium oil stain Partial-filled vug Calcite cement Crinoid Dark oil stain

Open fracture Pyrite Brachipod Very light oil stain 00950 Bottom Partial-filled fracture IC Intercrystalline pore Stromatoporoids Laminae Amphipora Limestone Moldic Stylolyte Ripple 0.2 R(LLA) Ω-M 2000 0 GR (API) 150 45 Porosity-CDL -15 125 CAL-X (MM) 375 0.2 R(ILA) Ω-M 2000 125 CAL-Y (MM) 375 45 Porosity-CNS, LM -15

0.2 R(ILM) Ω-M 2000

Figure 5 - Log and core correlation of Birdbear strata in Rife et al Eureka 131/14-02-032-23W3 well. In 1985, this well was drilled through the Birdbear Formation to a depth of 950 m. The upper member of the formation was cored from 890 to 926.25 m. To 1991, the well had a cumulative oil production of 2796 m3 from two perforated intervals, the first from 900.5 to 903.5 m, and the second from 910.5 to 913.5 m. The oil has a heavy specific gravity of 1.003 (API 9.6°). Porosity and permeability data are taken from historic core analysis.

Saskatchewan Geological Survey 6 Summary of Investigations 2010, Volume 1 12-04-039-27W3 fresh groundwater during the Nuvista Macklin latest Mississippian to the Early KB=684.1m LIC 87C016 Cretaceous when the basin was subjected to uplift and erosion. During a later stage of diagenesis, some solution- Mannville enlarged intercrystalline, vuggy, moldic and fracture porosity has been reduced by dolomite Birdbear recrystallization and precipitation of calcite (Figures 10C and 10D).

Core These diagenetic features strongly influence lateral and Perf. Upper Birdbear vertical changes in porosity and Zone permeability and thus play a very important role in variation of reservoir quality. The diagenesis events and their effect on reservoir quality are summarized in Table 2.

5. Low Resistivity Pay Zone

Lower Birdbear Figure 6 shows that the usual high resistivity signature is absent from the Birdbear producing zone of the Nuvista Macklin 111/12-04-039-27W3 well. Careful core and thin section examination indicates that an abundance of pyrite is present in the pay zone. Two Birdbear types of pyrite are recognizable: pyrite framboids and large cubic Duperow crystals. The pyrite framboids, Limestone consisting of small and

0 GR (API) 150 0.2 R(LLA) -M 2000 45 Porosity-CDL -15 interlocking pyrite crystals that are randomly distributed in the 125 CAL-X (MM) 375 0.2 R(ILA) -M 2000 dolostones (Figure 11A), are 125 CAL-Y (MM) 375 45 Porosity-CNS, LM -15 commonly subspherical and the 0.2 R(ILM) -M 2000 sizes range between 20 µm and 1 cm. They are easily recognized Figure 6 - From left to right, gamma, density/neutron, and resistivity geophysical under reflected light or a logs from the Nuvista Macklin 111/12-04-039-27W3 well showing the Birdbear pay combination of reflected and zone without high resistivity reading. In 1985, this well was drilled into the subcrop transmitted light during thin of the Birdbear Formation to a total depth of 785 m. The top part of the Birdbear is section examination. This type of truncated by the sub-Cretaceous unconformity as the Birdbear is overlain by the pyrite indicates a reducing Lower Cretaceous Mannville Group. The cores (742 to 760 m) were taken from the upper member of the Birdbear Formation immediately below the unconformity. The environment, or at least a local well was perforated from 750 to 753 m, and had a cumulative production of 1616 m3 reducing environment associated of heavy oil (specific gravity of 0.985 (API 12.2°)) from 1985 to 1988. Further with decomposing organic matter research is needed to correlate the upper Birdbear units and subunits to these during which the pyrite was observed in Rife et al Eureka 131/14-02-032-23W3 well. formed shortly after burial. The large cubic pyrite crystals are thought to be diagenetic as they coat the fracture surfaces, line vugs, and form an interlocking mosaic with the dolomite crystals in the pay zone (Figure 11B). The high electrical conductivity of pyrite may result in reduced resistivity readings of the Birdbear producing zone of the Nuvista Macklin 111/12-04-039-27W3 well (Figure 6).

Saskatchewan Geological Survey 7 Summary of Investigations 2010, Volume 1

101/13-21-032-28W3 101/07-30-031-26W3 111/06-23-031-26W3 131/01-27-031-25W3 111/08-34-031-24W3 131/14-02-032-23W3 West East <=6.5km=> <=19.3km=> <=8.8km=> <=10.2km=> <=10.7km=> KB=734.9m LIC 69H008 KB=708.1m LIC 87I043 KB=711.9m LIC 86B012 KB=704.7m LIC 87A006 KB=707.7m LIC 97D054 KB=701.1m LIC 85E014

875 900 875

900 900 900

Torquay 925 900

00900

900

Cored Interval Upper 925 925

Birdbear 925 950 Unit 2 Member 925 00925 925 Lower

Birdbear Unit 1 950

Member 950 950 975 TD 952.0m Duperow 950 TVD 949.8m

975

975 Cored interval: 890.0 to 926.25 m 1000 TD 980.0m 975 (see Figure 5)

TD 988.0m

Legend

Dry oil well Active oil well Suspended multizone oil well ABD oil well Water disposal well

Figure 7 - West-east structural cross section illustrating distribution of the Birdbear Formation in west-central Saskatchewan, constructed in March 2010. Note a consistent thickness of the Birdbear Formation. Datum is subsea level of 39 m. Location of the cross section is shown in Figure 2.

Saskatchewan Geological Survey 8 Summary of Investigations 2010, Volume 1

111/08-24-039-27W3 111/16-01-039-26W3 141/03-18-036-25W3 111/08-01-035-26W3 101/15-07-033-25W3 111/06-23-031-26W3 North <=10.6km=> <=27.1km=> <=17.1km=> <=17.4km=> South KB=687.9m LIC 97F239 KB=671.1m LIC 85K264 KB=692.6m LIC 97I411 <=12.8m=> KB=741.0m LIC 97A307 KB=719.0m LIC 82I061 KB=711.9m LIC 86B012

725 775

750 800 750 775

750 800

Mannville 775 825

775 800

Duperow 775

825 TD 795.0m 800 850

825

800 850 825 875 850

TD 815.0m

Unit 2

875 850 900

875 Unit 1

900 875 925 900 Legend

Gas well

925 Torquay 900 950 Water disposal well TD 954.0m 925

Cased oil well Upper Birdbear 950 ABD oil well 925 Member 950 Active oil well

ABD Hz strat test well Lower 975

950 Birdbear Member 975

TD 980.0m Duperow

Figure 8 - North-south structural cross section illustrating distribution of the Birdbear Formation in west-central Saskatchewan, constructed in March 2010. Note a general southward-deepening trend of the Birdbear. Datum is subsea level of 160 m. Location of the cross section is shown in Figure 2.

Saskatchewan Geological Survey 9 Summary of Investigations 2010, Volume 1

A B

C D

Figure 9 - Core photographs of dolostones in the upper member of the Birdbear Formation, Rife et al Eureka 131/14-02-032- 23W3 well. A) Moldic porosity generated by partial dissolution of the fossil debris. Sample depth is 921.1 m. B) Brachiopods concentrated along bedding plane. Sample depth is 914.7 m. C) The vugs and fractures coated by heavy oil stains. Sample depth is 911.5 m. D) The excellent intercrystal, fracture and vuggy porosities that are partially filled by oil-stained calcite crystals. Note the intensity of oil stain. Sample depth is 900.8 m.

6. Discussion of Trapping Mechanisms for Birdbear Reservoirs a) Stratigraphic and Diagenetic Controls Core examination of the producing zones of the Birdbear suggests that the interval represents shallow-water, fine crystalline dolostones and dolomitized wackstones. The reservoir rock makes a rapid transition to argillaceous, laminated impermeable dolostones or dolomitic shale locally. This transition from porous and permeable dolostone to a non-reservoir argillaceous layer may form a regional stratigraphic trap and may be the primary trapping mechanism locally. The lithology, porosity, and permeability of each pay zone are also observed to be heterogeneous in some cases. Distribution of solution-enlarged intercrystalline, vuggy, moldic, and fracture porosity varies vertically and horizontally. The argillaceous dolostone interbeds in the pay zone and late diagenetic features subdivide the reservoir vertically, resulting in reduced reservoir permeability and significant changes of porosity and permeability vertically in the Birdbear pay zone (Figure 5). Core examination of the Nuvista Macklin 111/12-04-039-27W3 well and the Rife et al Eureka 131/14-02-032-23W3 well indicate lateral changes of lithology, porosity, and permeability.

Saskatchewan Geological Survey 10 Summary of Investigations 2010, Volume 1

A B

450µm 300µm

C D

3000µm 200µm

Figure 10 - Thin-section photographs showing the porosity types and the effects of diagenesis on the porosity. A) Intercrystal porosity created during dolomitization. Sample location: Coleville Unit No 1 5-30-31-23W3 well at depth of 914.4 m. B) Solution-enlarged intercrystal porosity in fine-crystalline subhedral dolomite, an example of vuggy porosity when the size of the pores is greatly larger than the size of surrounding crystals. Sample location: Rife et al Eureka 131/14-02-032-23W3 well at depth of 917.8 m. C) Vuggy porosity created by dissolution has been reduced by late-stage calcite. Hydrocarbon residues filled some of pore space between the calcite, indicating oil emplacement post-dated calcite cementation. Sample location: Rife et al Eureka 131/14-02-032-23W3 well at depth of 902.8 m. D) Vuggy porosity created by dissolution was reduced by dolomite recrystallization. Note the early stage dolomites are mostly anhedral or subhedral, but late stage dolomite crystals growing into the available vugs are euhedral in shape and larger in size. Sample location: Rife et al Eureka 131/14- 02-032-23W3 well at depth of 917.8 m.

Table 2 - Summary of the diagenesis events, and their timing and effects on reservoir porosity and permeability.

Major Diagenetic Event Timing Effect on Porosity and Permeability Dolomitization Shortly after burial Increase porosity due to formation of intercrystalline pore Compaction During burial Reduce porosity Dissolution Pre-Cretaceous uplift and Increase porosity and permeability due to formation of vugs,

erosion fractures, and cavities Dolomite re-crystallization Re-burial Decrease porosity and permeability by reducing pore space and blocking pore throats Calcite cementation Re-burial Decrease porosity and permeability by reducing pore space and blocking pore throats

Saskatchewan Geological Survey 11 Summary of Investigations 2010, Volume 1 A B

Figure 11 - Core photographs of dolostones from the upper member of the Birdbear Formation in west-central Saskatchewan. Sample location: Nuvista Mackin 111/12-04-039-27W3 well. A) Pyrite framboids of almost perfectly spherical shape that consist of small and interlocking pyrite crystals randomly distributed in the dolostones. Sample depth is 753.1 m. B) Diagenetic cubic pyrite crystals distributed in dolostone. Sample depth is 752.0 m. b) Structural Control The regional dip of the Birdbear Formation is south (Figure 8). The Birdbear is truncated by erosion beneath the sub-Mesozoic unconformity near the northern margin of the study area. Most of the high producing wells are drilled along the subcrop of the Birdbear in the vicinity of Township 39 (Figure 2). Further work is required to investigate the effects of the sub-Mesozoic unconformity on Birdbear oil migration and entrapment. Detailed mapping will be helpful to identify structural highs and noses that are favourable for hydrocarbon accumulation.

7. Summary Porosity and permeability in the Birdbear change significantly both laterally and vertically, indicating a combination of variations in depositional texture, dolomite crystal fabric, and dissolution features. Stratigraphy, structure, and diagenesis play their own roles in the formation and quality of the Birdbear reservoirs. A good combination of stratigraphy, structure, and diagenesis is the key factor for high productivity reservoirs. Future work should be focused on identification of structural highs, updip closure by unconformity, stratigraphic seals formed as a result of interfingering, and upward transition from porous dolostones to impermeable argillaceous dolomudstones, and solution-enhanced secondary porosities that play important roles on hydrocarbon accumulation of the Birdbear.

8. Acknowledgments We are grateful to Fran Haidl and Melinda Yurkowski for their thoughtful reviews of the paper, and Tyler Music for helping with the diagrams.

9. References Cisyk, D.E. (1991): Stratigraphy, depositional setting and diagenesis of carbonate rocks of the Upper Devonian Birdbear Formation, west-central Saskatchewan; unpubl. M.Sc. thesis, Univ. Regina, Regina, 148p. Fowler, M.G., Stasiuk, L.D., Hearn, M., and Obermajer, M. (2001): Devonian hydrocarbon source rocks and their derived oils in the Western Canada Sedimentary Basin; Bull. Can. Petrol. Geol., v49, p117-148. Kent, D.M. (1968): The Geology of the Upper Devonian Saskatchewan Group and Equivalent Rock in Western Saskatchewan and Adjacent Areas; Sask. Dep. Miner. Resour., Rep. 99, 224p. Marsh, A., Yang, C., and Kohlruss, D. (2011) Sub-Mesozoic unconformity subcrop map, west-central Saskatchewan; Sask. Ministry of Energy and Resources, Open File 2011-1, 1:350 000-scale map. Whittaker, S.G. and Mountjoy, E.W. (1996): Diagenesis of an Upper Devonian carbonate-evaporite sequence: Birdbear Formation, southern Interior Plains, Canada; J. Sed. Resear., v66, p965-975.

Saskatchewan Geological Survey 12 Summary of Investigations 2010, Volume 1