Petrophysical Characterization of Sedimentary Facies from the Upper Devonian Lower Mississippian Bakken Formation in the Williston Basin, Southeastern Saskatchewan Solange Angulo 1 and Luis Buatois 1 Angulo, S. and Buatois, L. (2011): Petrophysical characterization of sedimentary facies from the Upper Devonian Lower Mississippian Bakken Formation in the Williston Basin, southeastern Saskatchewan; in Summary of Investigations 2011, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2011-4.1, Paper A-6, 16p. Abstract The Bakken Formation represents an ideal hydrocarbon system, comprising source, reservoir, and seal rocks within the same formation. The Middle Member is largely composed of silty to sandy facies that commonly provide productive reservoirs for light oil in southeastern Saskatchewan. Porosity and permeability in the Middle Member are strongly controlled by lithology, diagenesis, and bioturbation. Integration of petrophysical analyses and sedimentological information allows for an improved understanding of reservoir rock qualities which assists in the exploration and production of the Bakken play. In southeastern Saskatchewan, the Bakken Middle Member is informally subdivided into ten sedimentary facies (facies 2 to 11, in ascending order) including several subfacies. Detailed core examination in this study indicates that facies 4, 6, and 7 and subfacies 8C possess good reservoir qualities. In spite of being the cleanest and coarsest facies (fine-grained sandstone), facies 6 is generally not the best reservoir rock due to common calcite/dolomite cementation. Where facies 6 is locally un-cemented or poorly cemented, it exhibits good porosities and permeabilities. Although facies 4 and 7, and subfacies 8C consist of finer grained lithologies (very fine-grained sandstones), they have proven to be producible reservoir rocks for light oil. Facies 4 shows the best potential as a reservoir rock due to relatively good permeabilities and porosities, its wide distribution, and sufficient target thickness in the study area. Keywords: Bakken Formation, Late Devonian, Early Mississippian, Williston Basin, Western Canada Sedimentary Basin, petrophysical, porosity, permeability, core analysis, sedimentary facies, Saskatchewan, reservoir quality. 1. Introduction The number of wells drilled in the Bakken Formation in southeastern Saskatchewan has dramatically increased in the last five years (between January 2005 and September 2010, 1,737 producing wells were drilled (Kohlruss, written comm., 2011)). The Bakken has become one of the most prominent oil-producing reservoirs in the province. The Bakken Formation was deposited in a shallow epicontinental sea that characterized the Williston Basin during the Late Devonian Early Mississippian and covered much of North Dakota, northeastern Montana, southeastern Saskatchewan, and southwestern Manitoba, (Smith and Bustin, 1996; Algeo et al., 2007). The Bakken Formation represents an ideal hydrocarbon system, comprising source, reservoir, and seal rocks within the same formation (Halabura et al., 2007). In southeastern Saskatchewan it is generally less than 30 m thick, but is up to 70 m thick in places where it is linked to local areas of salt-collapse structures (Kreis et al., 2006). The organic-rich black shales from the Lower and Upper members constitute source rocks, while the Middle Member comprises several silty to sandy facies of which some are reservoir rocks. The black shale from the Upper Member, as well as some silty to muddy facies of the Middle Member, may represent seals for the reservoirs of the Middle Member. Several sedimentological studies have been made on the Bakken Formation in Saskatchewan and different sedimentary facies have been defined (Smith et al., 1995; Smith and Bustin, 1995, 1996; Angulo et al., 2008). However, there is not yet a petrophysical characterization of these facies based on core analysis. For this study, core from 31 wells in southeastern Saskatchewan have been used (Figure 1, Table 1). 1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2. Saskatchewan Geological Survey 1 Summary of Investigations 2011, Volume 1
O T11 T12 T13 R25 R23 R21 R19 R17 R15 R13 R11 R9 R7 R5 31 R3W2 T7 T8 T9 T10 T1 T2 T3 T4 T5 T6 EXSHAW FORMATION Eastern limit of deformation 60 ALBERTA 110 O MANITOBA SASKATCHEWAN 30 29 N 11? MONTANA BAKKEN FORMATION 49 O 49 O? 22 0 10 20 km NORTH DAKOTA 8 5 4 17 16 15 7 10 6 26 20 19 21 14 13 9 25 24 3 2 23 1 28 18 12 27 Figure 1 - Map of the study area showing the location of the well cores used for the petrophysical characterization of sedimentary facies from the Bakken s Middle Member (reference map based on Smith et al., 1995). The purpose of this study is to better understand the distribution of the porosity and permeability in the different sedimentary facies of the Bakken Middle Member in an effort to characterize reservoir rocks within the Middle Bakken. 2. Methodology For this study, well cores were slabbed and described in detail. Core-slabbing enhanced the visibility of many sedimentary features and resulted in improved core description and interpretation of the facies. The focus of this work was on detailed descriptions of sedimentological (e.g., lithology, sedimentary structures, and fossils) and ichnological features (e.g., trace fossil content and bioturbation index). Although 62 wells were described, only 31 of them were used for the petrophysical characterization (Figure 1, Table 1). For an accurate petrophysical characterization, petrophysical values measured in each plug must be correctly assigned to the sedimentary facies in which the plug was taken. Calibration of the plugs with the sedimentary facies based only on depths can result in inaccurate petrophysical characterization of facies due to errors in measurement and assignment of depths to the cored interval. The core and its core analyses were only used in this study when it was possible to confidently and precisely identify the depth from which each plug was taken. Consequently, only the core analyses from 31 wells were used. Once the calibration of core plug and sedimentary facies was made, the porosity and permeability (k max ) values corresponding to each facies were grouped. Subsequently, the arithmetic mean of the porosities and the harmonic mean of the permeabilities for each facies were calculated. Additionally, standard deviations for both porosity and permeability were calculated and cross-plot charts of porosity versus permeability were made (Table 2, Appendix A). The number of wells and plug values used to characterize each sedimentary facies is shown in the right bottom corner of each cross-plot. The standard deviations for porosity and permeability are also presented in Appendix A, in parenthesis beside the arithmetic mean for porosity and the harmonic mean for permeability. Statistically, the greater the number of wells and core plugs taken for a particular facies, the greater the confidence in the results. For calculation purposes, all permeability values less than 1 md (<1 md) reported in the core analyses were considered to be 1 md. Finally, charts were constructed to compare the porosity and permeability for each facies. Saskatchewan Geological Survey 2 Summary of Investigations 2011, Volume 1
Table 1 - List of well cores used for the petrophysical characterization of the sedimentary facies of the Bakken Formation in southeastern Saskatchewan. No. Well Identification Well License Well Name Core Interval (m) 1 131/04-16-001-08W2/00 05I208 TRISTAR BOUNDARY DAM 4C4-16-1-8 2187.0 to 2205.0 2 141/15-31-001-09W2/00 81A036 ROXY ET AL TABLELAND 15-31-1-9 2128.1 to 2146.1 3 121/04-02-001-10W2/00 05E423 WAVEFORM TABLELAND 4-2-1-10 223 to 229 4 101/12-21-001-16W2/00 66K007 MOBIL FLAT LAKE 12 21 1 16 2325.6 to 2343.9 5 131/10-01-002-19W2/00 94J138 NORTHROCK HUMMINGBIRD S 10-1-2-19 2267.6 to 2285.6 6 141/15-31-003-11W2/00 81D003 NAL ET AL TORQUAY 15-31-3-11 1970.2 to 198 7 101/03-18-003-13W2/00 05C446 HYGAIT OUNGRE 3-18-3-13 2056.7 to 2070.4 8 101/06-11-004-21W2/00 72K044 TENN MCGRATH 6 11 4 21 2141.5 to 2159.5 9 121/05-04-005-11W2/00 82K079 ROXY ET AL MAINPRIZE 5-4-5-11 1882.7 to 1900.7 10 141/09-13-005-13W2/00 81K044 CLARION ET AL GOODWATER 9 13 5 13 1828.3 to 184 11 101/06-20-005-24W2/00 97F372 RIGEL BENGOUGH 6-20-5-24 1821.0 to 1839.0 12 141/02-18-006-05W2/00 06G090 RTOG WEIR HILL 2-18-6-5 163 to 1648.1 13 101/07-28-006-11W2/00 65D028 APACHE MIDALE 7-28-6-11 1728.2 to 1746.5 14 101/03-36-006-12W2/00 97J413 ADVANTAGE WEYBURN 3-36-6-12 1727.2 to 1742.5 15 101/05-31-006-13W2/00 91A005 STARTECH ET AL WEYBURN 5-31-6-13 1701.0 to 171 16 101/16-35-006-14W2/00 84I243 POSTELL WEYBURN 16-35-6-14 1705.1 to 1723.0 17 101/03-25-006-16W2/00 84G025 CANETIC TATAGWA 3-25-6-16 1776.4 to 1792.5 18 111/16-29-007-07W2/00 05H099 INNOVA VIEWFIELD 3A16-29-7-7 1585.0 to 1603.4 19 101/03-04-007-11W2/00 65E033 MICH WIS CANSO MIDALE 3 4 7 11 1719.7 to 173 20 101/01-11-007-11W2/00 65G076 ZARGON MIDALE 1-11-7-11 1693.8 to 1712.1 21 191/07-12-007-11W2/00 04G223 CPEC MIDALE DD 1A10-12-3D7-12-7-11 1693.6 to 1710.3 22 111/01-24-007-23W2/00 08F163 CPG GLASNEVIN 2A1-24-7-23 179 to 182 23 111/07-06-008-08W2/00 03K036 BISON VIEWFIELD 7-6-8-8 1585.0 to 159 24 121/05-05-008-09W2/00 06G152 MISSION VIEWFIELD SWD 5-5-8-9 1617.2 to 1635.2 25 101/07-11-008-10W2/00 67F006 B.A. GRIFFIN GEIGER 7 11 8 10 1591.1 to 1609.3 26 101/01-10-008-11W2/00 69C029 MICH WIS AMARILLO GRIFFIN 1 10 8 11 1609.0 to 1623.7 27 141/03-23-009-03W2/00 94C071 TRI LINK WBOG BIG MARSH LK 3-23-9-3 143 to 145 28 131/13-21-009-07W2/00 05I223 INNOVA MELROSE 1C13-21-9-7 142 to 1445.6 29 101/15-25-010-08W2/00 65G163 G & W ET AL N HNDSWRTH 15-25-10-8 138 to 1400.3 30 121/12-05-011-05W2/00 96E088 TRI LINK PCP HAZELWOOD 12-5-11-5 1415.5 to 1427.5 31 101/11-36-013-11W2/00 80J106 TRICENT ET AL MUTRIE 11 36 13 11 1259.4 to 1271.3 3. Results The Bakken is subdivided into three members: Lower and Upper members composed of organic-rich black shale, and a dolomitic siltstone and sandstone Middle Member. The Lower and Upper members are lithologically homogeneous, consisting exclusively of one sedimentary facies (facies 1). The Middle Member is much more vertically and laterally heterogeneous and consists of a wider variety of sedimentary facies (facies 2 to 11) that can be subdivided into units and subunits (Table 2, Appendix A, Figure 2). Only sedimentary facies from the Middle Member have been characterized in this study. Facies 1 from the Lower and Upper Members and facies 11 from the Middle Member could not be characterized due to the absence of plugs. In the Middle Member, ten sedimentary facies were recognized based on sedimentological and ichnological data (facies 2 to 11). Two of these facies were subdivided into subfacies (facies 3 was subdivided into subfacies 3A and 3B; facies 8 into subfacies 8A, 8B, and 8C). Detailed facies descriptions are presented in Angulo et al. (2008), and Angulo and Buatois (2009), and their spatial distribution is shown in the isopach maps in Angulo and Buatois (2010). In this paper, Appendix A and Table 2 summarize the most important features of these facies/subfacies (including lithology, sedimentary structures, trace fossil content, bioturbation index, and interpretation of the sedimentary environments), as well as petrophysical characteristics (cross-plot of relationship between average porosity and permeability (Appendix A only), arithmetic mean porosity and harmonic mean permeability, and their respective standard deviations). Arithmetic mean porosities for the sedimentary facies of the Bakken Middle Member vary from 4.1 to %. Facies 2, 9, and 10, as well as subfacies 3B are typified by the lowest porosities (4.1 to 5.5%). Facies 5 and 6, and subfacies 3A, 8A, and 8B are distinguished by moderate porosities (7.9 to 9.1%). Finally, facies 4 and 7, and subfacies 8C are characterized by the highest porosities (10.6% to %) (Figure 3). Saskatchewan Geological Survey 3 Summary of Investigations 2011, Volume 1
Saskatchewan Geological Survey 4 Summary of Investigations 2011, Volume 1 Table 2 - Sedimentological and ichnological characteristics of the sedimentary facies defined in the Bakken Formation in southeastern Saskatchewan (modified from Angulo and Buatois, 2010). Coloured shading indicates reservoir quantity (see Figure 5): purple, poor quality; green, moderate quality; and blue, good quality. Facies Lithology Sedimentary Structures 1 Black shale, pyrite, and rare fragments of shells locally present. 2 Greenish grey, burrowmottled siltstone, commonly calcareous, with fragments of brachiopod shells and crinoids. 3A 3B Light grey or greenish grey, burrow-mottled, sandy siltstone to silty very fine-grained sandstone, commonly calcareous, pyritic, locally with brachiopod shell remains and discontinuous thin laminae of shale. Interbedded dark grey, highly bioturbated siltstone and light grey, very finegrained sandstone. 4 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 (<1 mm) and thinly laminated very finegrained sandstone. 6 Light brownish grey, finegrained sandstone, well sorted, calcareous, locally with oolites and pyrite. 7 Light grey, very finegrained sandstone, well sorted, with mud drapes. 8A 8B 8C Light to dark grey, beige and locally light red, commonly pyritic, in places slightly calcareous, very fine-grained sandstone. Light to dark grey, very fine-grained sandstone, shale laminae are common; locally mud clasts occur (<5 mm). Light grey, very finegrained sandstone with common shale laminae. 9 Dark grey, very thinly interlaminated, very finegrained sandstone and muddy siltstone, locally calcareous. 10 Very thinly interlaminated, dark grey mudstone and light grey, very finegrained silty sandstone. 11 Sharp-based and poorly sorted coquina with sandy matrix. Massive, locally parallel lamination and injection cracks. Massive with burrow-mottled texture. 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. 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 (1 mm to 8 cm thick) are also common. Wavy lamination; mudstone drapes; microfaults occur rarely. Burrow-mottled texture, irregular and discontinuous shale laminations, common soft deformation structures and rare microfaults occur. Mudstone drapes are common (<3 mm) and occur rhythmically. Locally inclined heterolithic stratification is also present. Parallel lamination; locally current ripple crosslamination and mudstone drapes. Horizontal thin parallel lamination; locally mudstone drapes. Syneresis cracks are commonly present. Sandstone lenses and wave ripples occur. Massive with burrow-mottled texture. Bioturbation Index 0; locally 1 at the top. Ichnofossils Chondrites isp. and Thalassinoides isp. Zoophycos isp. occurs in outcrops of the Exshaw Formation. 5 to 6 Phycosiphon incertum, burrow mottlings. 5 Dominant ichnotaxa: Phycosiphon incertum and Nereites missouriensis. Subordinate ichnotaxa: Asterosoma isp., Teichichnus rectus, and Planolites montanus. Rare elements: Rosselia isp. Highly variable: in the siltstones 6; in the sandstones 0 to 1. Dominant elements in the siltstone: Phycosiphon incertum and Nereites missouriensis. Dominant elements in sandstone: Teichichnus rectus. Rare elements: Siphonichnus eccaensis. 4 to 5 Dominant elements: Nereites missouriensis and Planolites montanus. Subordinate ichnotaxa: Phycosiphon incertum and Asterosoma isp. Rare elements: Rosselia isp. Highly variable: in the massive intervals 4 to 5; in the laminated intervals 0 to 1. Dominant elements: Planolites montanus. Subordinate: Nereites missouriensis, Phycosiphon incertum, and Asterosoma isp. Sedimentary Environment Arithmetic Mean Porosity (%) Standard Deviation Permeability (md) Harmonic Mean Standard Deviation Shelf Lower offshore Upper offshore (low intensity and frequency of storms). Upper offshore (moderate intensity and frequency of storms). Offshore transition Lower shoreface 4.1 1.7 2 8 3.0 4 0.17 5.5 2.6 3 0.27 10.6 2.2 9 0.63 1.9 5 0.1 0 None Barrier bar 8.6 3.3 0.27 5.34 0 None Wavedominated tidal flat 1 to 2 Planolites montanus and burrow mottlings. 3 to 4 Dominant elements: Planolites montanus, Phycosiphon incertum. Rare elements: Nereites missouriensis and Teichichnus rectus. 11.0 2.8 9 1.14 Distal bay 7.9 2.2 9 1.82 Distal bay 9.1 5 0.11 2 to 3 Planolites montanus Tidal flats 1.8 0.2 0.15 0 to 1 Planolites montanus Distal bay 5.2 3.0 4 8 3 to 4 Dominant ichnotaxa: Planolites montanus and Teichichnus rectus. Rare elements: Rosselia isp. and Siphonichnus eccaensis. 0 to 1 Burrow mottlings High-energy ravinement during drowning of the bay. Distal bay 4.9 2.7 7 8
LOG OF THE BAKKEN FORMATION WELL 15-31-003-11W2 (LICENSE 81D003) DEVONIAN MISSISSIPPIAN AGE? BAKKEN FORMATION Members STRATIGRAPHIC SUBDIVISIONS Units/Subunits Upper Member Middle Member Kreis et al. (2005) C B A1 A This paper B A C B2 B1 A2 A1 LOG DEPTH (m) 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 GAMMA RAY SONIC FACIES 1 3B 10 8A 9 8A 9 8A 6 5 4 3A 2 1 LITHOLOGIC COLUMN 1992 Lower Member 1993 1994 1995 1996 1997 Figure 2 - Schematic log showing core description, sedimentary facies, gamma-ray and sonic log signatures, stratigraphic subdivisions, and age for the well core from 15-31-003-11W2 (license 81D003). Note that facies 7 does not occur in this location. For description of the sedimentary facies, see Appendix A. The stratigraphic subdivisions used here for the Middle Member are very similar to those used by Kreis et al. (2005). However, unit A of Kreis et al. (2005) is here called subunit A1, which together with subunit A2 forms a new unit A. In addition, unit B of Kreis et al. (2005) has been subdivided into subunits B1 and B2. Cl vf f m c vc Sl Sandstone Saskatchewan Geological Survey 5 Summary of Investigations 2011, Volume 1
Arithmetic Mean Porosity (%) Figure 3 - Chart comparing the arithmetic mean porosities of the sedimentary facies (F) from the Bakken Middle Member. Harmonic Mean Permeability (md) 10 0 0 0 0 0 0 0 0.30 0.25 0.20 0.15 0.10 5 0 4.1 4.9 5.2 5.5 7.9 Figure 4 - Chart comparing the harmonic mean permeabilities of the sedimentary facies (F) from the Bakken Middle Member. 8.6 9.1 10.6 11.0 F2 F10 F9 F3B F8A F3A F5 F6 F8B F4 F7 F8C 2 3 4 4 Sedimentary Facies 5 5 7 9 9 9 0.20 0.27 F2 F3B F3A F9 F5 F8B F10 F8A F4 F7 F8C F6 Sedimentary Facies Harmonic mean permeabilities in the Middle Member are very low, from 2 to 0.27 md. Facies 2, 5, and 9, as well as subfacies 3A, 3B, and 8B are distinguished by the lowest permeabilities (<6 md). Facies 4, 7, and 10, as well as subfacies 8A are characterized by intermediate permeabilities (7 to 9 md). Facies 6 and subfacies 8C are typified by the highest permeabilities (0.27 and 0.2, respectively) (Figure 4). When both porosity and permeability are considered for each sedimentary facies, three categories of reservoir rock quality can be defined (Figure 5): 1) poor quality (<8 md and <5.6% porosity), 2) moderate quality (4 to 9 md and 7.9 to 9.1% porosity), and 3) good quality (9 to 0.27 md and 8.6 to % porosity). As a result, facies 2, 9, and 10, as well as subfacies 3B are characterized as poor quality reservoirs. Facies 5 and subfacies 3A, 8A and 8B fall within the moderate reservoir quality range. Relatively good reservoir quality is observed in facies 4, 6, and 7, as well as subfacies 8C (Figure 5). 4. Discussion Lithology plays the most important role in the rock quality of the sedimentary facies of the Bakken Formation. Fine-grained facies present low porosity and permeability values, while coarse-grained facies display the highest values. Poor-quality reservoir facies consist predominantly of fine-grained lithologies and, although these facies include very fine-grained sandstone, they are mostly dominated by siltstone and/or mudstone. For example: facies 2 consists of siltstone; facies 10 of very thinly interlaminated mudstone and very fine-grained silty sandstone; facies 9 of very thinly interlaminated, very fine-grained sandstone and muddy siltstone; and subfacies 3B is comprised of interbedded siltstone and very fine-grained sandstone. Although dominated by very fine-grained sandstone, facies with moderate reservoir quality rocks still have a relatively high content of mud or silt. Subfacies 3A is mostly siltstone, facies 5 has muddy partings, subfacies 8A typically has mud drapes, and 8B commonly includes discontinuous muddy and shale laminae. These lithological characteristics account for the relatively poorer permeabilities observed in this category. Facies with good reservoir quality are characterized by relatively cleaner sandstones compared to those in the poorand moderate quality reservoir categories. Facies 6 is composed of fine-grained sandstone, whereas facies 4 and 7 and subfacies 8C are composed of very fine-grained sandstone. It is important to note that facies 7 and subfacies 8C commonly show mud and siltstone drapes that likely play a strong role in suppressing permeabilities in these facies. Saskatchewan Geological Survey 6 Summary of Investigations 2011, Volume 1
Arithmetic Mean Porosity (%) 13.00 0 11.00 0 9.00 0 7.00 0 5.00 0 3.00 0 1.00 0 F2 F8B F3A F5 F3B F9 F10 F7 F8A F4 Harmonic Mean Permeability (md) Figure 5 - Cross-plot of the relationship between porosity and permeability for the various sedimentary facies of the Bakken Middle Member. Sedimentary facies are grouped into three categories of reservoir rock quality: 1) poor quality, with low porosities and permeabilities; 2) moderate quality, with moderate porosities and low permeabilities; and 3) good quality, with high porosities and relatively high permeabilities (the same colour coding is seen in Table 2). Abbreviation: F, facies. F8C 0 5 0.10 0.15 0.20 0.25 0.30 Poor quality Moderate quality Good quality Our results support previous work by Kreis et al. (2005), who mentioned that light-oil production in the Viewfield area is interpreted to come from the very fine-grained sandstone of facies 4, at the top of Kreis (2005) unit A1. These authors also stated that in some localities, where the sandstones are less argillaceous and sedimentary structures such as flaser bedding are present (facies 7 of this study), the reservoir quality can be very good. Ferdous (2001) wrote that diagenetic processes played a significant role in modifying the rocks and led to variable cement and porosity distribution in the Middle Bakken Member across southern Saskatchewan. The dominant, widespread cements are dolomite, calcite, and pyrite. Carbonate cement was generated in several stages during diagenesis. According to Ferdous (2001), their precipitation and dissolution control most of the porosity and permeability in the Bakken Middle Member. Kohlruss and Nickel (2009) stated that thin sections made from unit A revealed the local presence of up to 70% dolomite, suggesting that in places this unit is a silty dolostone. Therefore, the petrophysical properties of subfacies 3A and facies 4 are likely strongly controlled by diagenetic processes. Kreis et al. (2005) wrote that exploration companies first targeted clean sandstones from unit B (facies 6 in this paper) for being relatively free of argillaceous content and showing high resistivities on geophysical logs. However, this facies is commonly calcite-cemented, resulting in moderate to poor reservoir characteristics. In addition, although facies 6 is characterized by relatively good arithmetic mean porosities and harmonic mean permeabilities, these values can vary significantly both laterally and vertically within the facies. The more heterogeneous nature of this facies is illustrated by the highest values of standard deviation for both porosity (3.3%) and permeability (5.34 md) (Table 2, Appendix A). Although claims of destruction of sediment porosity and permeability by bioturbation has been presented in reservoir characterization for many years, recent studies have demonstrated that this is not always the case (e.g., Buatois et al., 1999; Gingras et al., 1999; Pemberton and Gingras, 2005; Tomkin et al., 2010). Selective feeding in Nereites missouriensis and Phycosiphon incertum, dominant in subfacies 3A and facies 4, may have played a key role in enhancing permeability and promoting light-oil mobility in these facies. Nereites missouriensis and Phycosiphon incertum consist, in general terms, of horizontal tunnels with a cores filled by clay and envelope zones characterized by cleaner sandstone that results in better permeability, improving the mobility of light oil, particularly in the horizontal (parallel bedding) direction (Figure 6). Spatial distribution is another consideration when evaluating the reservoir potential of these rocks. According to the isopach maps published by Angulo and Buatois (2010), facies 4 is mostly present throughout the study area and reaches a maximum thickness of 4 m in the central-northeast region of the study area. Facies 6 and 7, and subfacies 8C, in contrast, are much more locally distributed. F6 Saskatchewan Geological Survey 7 Summary of Investigations 2011, Volume 1
A Ne Ph Ph 5. Conclusions The most significant factor that controls the rock quality of the different sedimentary facies in the Bakken Middle Member is lithology. As would be expected, coarser grained facies display higher porosity and permeability values, while finer grained facies present petrophysically poorer quality reservoirs. Accordingly, the petrophysical characterization based on core analyses of the sedimentary facies from the Bakken Middle Member s facies 4, 6, and 7, and subfacies 8C display the best reservoir qualities (highest porosities and permeabilities). B Ph Ne Muddy core Ne Ne Muddy core Figure 6 - Core photographs showing Nereites missouriensis in A) bedding plane (well 07-12-007-11W2, license 04G223, depth 1701.1 m) view and B) in cross section (well 07-28-006-11W2, license 65D028, depth 1738 m) view. Note in B, how Nereites missouriensis might enhance horizontal permeabilities through the pale mantle of cleaner sandstone that surrounds the muddy core. Abbreviations: Ne, Nereites missouriensis; and Ph, Phycosiphon incertum. o However, due to the low permeabilities that characterized these facies, horizontal drilling and large sand-fracture completions are proving to be necessary strategies to overcome dominantly low-permeability reservoirs within the Middle Bakken. These completion techniques play a pivotal role in obtaining economic production from relatively poor quality reservoir rock. Diagenesis also played an important role in the petrophysical properties of the sedimentary facies of the Bakken Middle Member. Precipitation and dissolution of carbonate cement control most of the porosity and permeability in the Bakken Middle Member. According to Kohlruss and Nickel (2009), thin-section study revealed the presence of up to 70% dolomite, suggesting that these facies in fact locally consist of silty dolostone, and consequently, their petrophysical properties are strongly controlled by diagenetic processes. Additionally, although facies 6 is the cleanest and coarsest facies (fine-grained sandstone), it is not the best reservoir because of its heterogeneous nature and common occurrence of pore-plugging calcite cement. Bioturbation may have played an important role in enhancing the petrophysical properties in facies 4 and subfacies 3A. These two facies are characterized by the abundant Nereites missouriensis and Phycosiphon incertum, which can promote light-oil mobility in low-permeability reservoirs. Finally, when considering the spatial distribution of the sedimentary facies, reservoir potential is significantly enhanced in facies 4 due to its wide distribution and thickness. 6. Recommendations Ne Ph Cleaner sand envelope Cleaner sand envelope Ne Ph For a better understanding of the different parameters that control the petrophysical properties of the sedimentary facies of the Bakken Formation, a detailed petrographic study of the sedimentary facies analyzed in this study is recommended. This would help to quantify the influence of lithology and diagenesis in the rock quality. Additionally, it would be advisable to consider the clay content as another parameter used for petrophysical characterization. Ideally, the clay content should be obtained from the same plugs used for the core analyses. Finally, further research, involving mini-permeameter measurements should be carried out to understand and determine the role that Nereites missouriensis and Phycosiphon incertum may have played in the reservoir quality in subfacies 3A and facies 4. Saskatchewan Geological Survey 8 Summary of Investigations 2011, Volume 1
7. Acknowledgments We want to sincerely thank the Saskatchewan Ministry of Energy and Resources and the University of Saskatchewan for providing financial support. Additional funds have been provided by the American Association of Petroleum Geologists and the International Association of Sedimentologists. We are also very grateful to the Subsurface Geological Laboratory and its staff in Regina, and to Andrew Nimegeers, Kim Kreis, Erik Nickel, and Melinda Yurkowski for reviewing this paper and for their useful comments. 8. References Algeo, T., Lyons T., Blakey R., and Over D. (2007): Hydrographic conditions of the Devono-Carboniferous North American Seaway inferred from sedimentary Mo-TOC relationships; Palaeo, v256, p204-230. Angulo, S. and Buatois, L. (2009): Sedimentological and ichnological aspects of a sandy low-energy coast: Upper Devonian Lower Mississippian Bakken Formation, Williston Basin, southeastern Saskatchewan; in Summary of Investigations 2009, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2009-4.1, Paper A-5, 17p, URL <http://er.gov.sk.ca/adx/aspx/adxgetmedia.aspx?docid=11859, 11858,11458,11455,11228,3385,5460,2936,Documents&MediaID=36784&Filename=angulo.pdf>. (2010): 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 2010, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2010-4.1, Paper A-4, 18p, URL <http://er.gov.sk.ca/adx/aspx/adxgetmedia.aspx?docid=11867,11866,11458,11455,11228,3385,5460,2936, Documents&MediaID=36829&Filename=A-4+Angulo_Buatois.pdf>. Angulo, S., Buatois, L., and Halabura, S. (2008): 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 2008, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2008-4.1, CD-ROM, Paper A-3, 24p. Buatois, L.A., Mángano, M.G., and Carr, T.R. (1999): Sedimentology and ichnology of Paleozoic estuarine and shoreface reservoirs, Morrow Sandstone, Lower Pennsylvanian of Southwest Kansas, U.S.A.; Current Resear. Earth Sci., v241, p1-27. Ferdous, H. (2001): Regional sedimentology and diagenesis of the Middle Bakken Member: implications for reservoir rock distribution in southern Saskatchewan; unpubl. Ph.D. thesis, Univ. Sask., Saskatoon, 392p. Gingras, M.K., Pemberton, S.G., Mendoza, C.A., and Henk, F. (1999): Assessing the anisotropic permeability of Glossifungites surfaces; Petrol. Geosci., v5, p349-357. Halabura, S., Buatois, L., Angulo, S., and Piché, L. (2007): From source to trap: a review of the Bakken petroleum system, Upper Devonian Mississippian, southeast Saskatchewan; in Summary of Investigations 2007, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2007-4.1, CD-ROM, Paper A-4, 8p. Kohlruss, D. and Nickel, E. (2009): Facies analysis of the Upper Devonian Lower Mississippian Bakken Formation, southeastern Saskatchewan; in Summary of Investigations 2009, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2009-4.1, Paper A-6, 11p. Kreis, L.K., Costa, A., and Osadetz, K.G. (2005): New perspectives on the hydrocarbon potential of Bakken and Torquay formations, southeastern Saskatchewan; in Summary of Investigations 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-10, 10p. (2006): Hydrocarbon potential of Bakken and Torquay formations, southeastern Saskatchewan; in Gilboy, C.F. and Whittaker, S.G. (eds.), Saskatchewan and Northern Plains Oil & Gas Symposium 2006, Sask. Geol. Soc., Spec. Publ. No. 19, p118-137. Pemberton, S.G. and Gingras, M.K. (2005): Classification and characterizations of biogenically enhanced permeability; Amer. Assoc. Petrol. Geol. Bull., v89, p1493-1517. Smith, M.G. and Bustin, R.M. (1995): Sedimentology of the Late Devonian and Early Mississippian Bakken Formation, Williston Basin; in Hunter, L.D.V. and Schalla, R.A. (eds.), 7th International Williston Basin Symposium, Mont. Geol. Soc., Spec. Publ., p103-114. Saskatchewan Geological Survey 9 Summary of Investigations 2011, Volume 1
(1996): Lithofacies and paleoenvironments of the Upper Devonian and Lower Mississippian Bakken Formation, Williston Basin; Bull. Can. Petrol. Geol., v44, no3, p495-507. Smith, M.G., Bustin, R.M., and Caplan, M.L. (1995): Sequence stratigraphy of the Bakken and Exshaw formations: a continuum of black shale formations in the Western Canada sedimentary basin; in Hunter, L.D.V. and Schalla, R.A. (eds.), 7th International Williston Basin Symposium, Mont. Geol. Soc., Spec. Publ., p399-409. Tomkin, N.S., McIlroy, D., Meyer, R., and Moore-Turpin, A. (2010): Bioturbation influence of reservoir quality: a case study from the Cretaceous Ben Nevis Formation, Jeanne darc Basin, offshore Newfoundland, Canada; Bull. AAPG, v94, p1059-1078. Saskatchewan Geological Survey 10 Summary of Investigations 2011, Volume 1
Appendix A Summary of Sedimentological and Petrophysical Characteristics of Facies and Subfacies. Abbreviations: Ø (AM), arithmetic mean porosity; and k (HM), harmonic mean permeability. FACIES 2 Greenish grey, burrowmottled siltstone, commonly calcareous, with fragments of brachiopod shells and crinoids. 5 to 6 Massive with burrowmottled texture. Lower offshore Phycosiphon incertum, burrow mottlings. Ø (AM): 4.1% (1.7) k (HM): 2 md (8) 0 5 0.1 0.15 0.2 0.25 4 wells/8 values SUBFACIES 3A Light grey or greenish grey, burrow-mottled, sandy siltstone to silty very finegrained sandstone, commonly calcareous, pyritic, locally with brachiopod shell remains and discontinuous thin laminae of shale. 5 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. Upper offshore Dominant ichnotaxa: Phycosiphon incertum and Nereites missouriensis. Subordinate ichnotaxa: Asterosoma isp., Techichnus rectus, and Planolites montanus. Rare elements: Rosselia isp. Ø (AM): % (3.0) k (HM): 4 md (0.17) 1 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 25 wells/217 values Saskatchewan Geological Survey 11 Summary of Investigations 2011, Volume 1
SUBFACIES 3B Interbedded dark grey, highly bioturbated siltstone and light grey, very fine-grained sandstone. Highly variable: in the siltstones 6; in the sandstones 0 to 1. Microhummocky crossstratification and very thin parallel lamination occur in the sandstones. In some cases, wave ripples occur on top of microhummocky beds. Upper offshore Dominant elements in the siltstone: Phycosiphon incertum and Nereites missouriensis. Dominant elements in sandstone: Teichichnus rectus. Rare elements: Siphonichnus eccaensis. Ø (AM): 5.5% (2.6) k (HM): 3 md (0.27) 0 0.2 0.4 0.6 0.8 1 7 wells/19 values FACIES 4 Interbedded light grey, massive, very fine-grained sandstone and siltstone. Deposits are generally slightly to moderately calcareous. 4 to 5 Bed boundaries are diffuse. Locally continuous shale laminae occur. Offshore transition Dominant elements: Nereites missouriensis and Planolites montanus. Subordinate ichnotaxa: Phycosiphon incertum and Asterosoma isp. Rare elements: Rosselia isp. Ø (AM): 10.6% (2.2) k (HM): 9 md (0.63) 1 1 1 0 0.5 1 1.5 2 2.5 3 3.5 17 wells/108 values Saskatchewan Geological Survey 12 Summary of Investigations 2011, Volume 1
FACIES 5 Interbedded massive light grey, very fine-grained sandstone with muddy partings (<1 mm) and thinly laminated very finegrained sandstone. Highly variable: in the massive intervals 4 to 5; in the laminated intervals 0 to 1. Massive with common intervals of wavy or parallel lamination. Locally parallel and wavy lamination. Continuous shale laminae occur. Lower shoreface Dominant elements: Planolites montanus. Subordinate: Nereites missouriensis, Phycosiphon incertum, and Asterosoma isp. Ø (AM): % (1.9) k (HM): 5 md (0.1) 0 5 0.1 0.15 0.2 0.25 0.3 0.35 5 wells/15 values FACIES 6 Light brownish grey, finegrained sandstone, well sorted, calcareous, locally with oolites and pyrite. 0 Erosive-based high-angle planar cross-stratified, some intervals are massive or present parallel lamination/ low-angle crossstratification. Barrier bar None Ø (AM): 8.6% (3.3) k (HM): 0.27 md (5.34) 1 1 1 0 5 10 15 20 25 30 35 40 45 12 wells/107 values Saskatchewan Geological Survey 13 Summary of Investigations 2011, Volume 1
FACIES 7 Light grey, very fine-grained sandstone, well sorted, with mud drapes. Flaser bedded, with wave and current ripples, climbing ripples and mudstone drapes (1 mm to 8 cm thick) are also common. None 0 Wave-dominated tidal flat Ø (AM): 11.0% (2.8) k (HM): 9 md (1.14) 2 1 1 1 2 15.0 5.0 0 0 0.5 1 1 1.5 2 2.5 3 3 3.5 4 4 Average Permeability Permeability (md) (md) 4 wells/31 values Porosity (%) SUBFACIES 8A Light to dark grey, beige and Wavy lamination; mudstone locally light red, commonly drapes; microfaults occur pyritic, in places slightly rarely. calcareous, very fine-grained sandstone. 1 to 2 Distal bay Planolites montanus and burrow mottlings. Ø (AM): 7.9% (2.2) k (HM): 9 md (1.82) 1 0 1 2 3 4 5 6 7 8 15 wells/43 values Saskatchewan Geological Survey 14 Summary of Investigations 2011, Volume 1
SUBFACIES 8B Light to dark grey, very fine-grained sandstone, shale laminae are common; locally mud clasts occur (<5 mm). 3 to 4 Burrow-mottled texture, irregular and discontinuous shale laminations, common soft deformation structures and rare microfaults occur. Distal bay Dominant elements: Planolites montanus, Phycosiphon incertum. Rare elements: Nereites missouriensis and Teichichnus rectus. Ø (AM): 9.1% () k (HM): 5 md (0.11) 1 1 0 5 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 7 wells/21 values SUBFACIES 8C Light grey, very fine-grained sandstone with common shale laminae. Mudstone drapes are common (<3 mm) and occur rhythmically. Locally inclined heterolithic stratification is also present. Planolites montanus 2 to 3 Tidal flats Ø (AM): % (1.8) k (HM): 0.2 md (0.15) 1 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 2 wells/18 values Saskatchewan Geological Survey 15 Summary of Investigations 2011, Volume 1
FACIES 9 Dark grey, very thinly interlaminated, very finegrained sandstone and muddy siltstone, locally calcareous. 0 to 1 Parallel lamination; locally current ripple crosslamination and mudstone drapes. Distal bay Planolites montanus Ø (AM): 5.2% (3.0) k (HM): 4 md (8) 0 5 0.1 0.15 0.2 0.25 0.3 8 wells/15 values FACIES 10 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 lenses and wave ripples occur. Dominant ichnotaxa: Planolites montanus and Teichichnus rectus. Rare elements: Rosselia isp. and Siphonichnus eccaensis. 3 to 4 Distal bay Ø (AM): 4.9% (2.7) k (HM): 7 md (8) 0 5 0.1 0.15 0.2 0.25 0.3 5 wells/8 values Saskatchewan Geological Survey 16 Summary of Investigations 2011, Volume 1