sequence stratigraphy and preliminary diagenetic study of ... · resulted in progradation of coarse...

Saskatchewan Geological Survey 1 Summary of Investigations 2005, Volume 1

Sequence Stratigraphy and Preliminary Diagenetic Study of the Lower Cretaceous Viking Formation, Hoosier Area, West-Central

Saskatchewan

A. Tong 1, G. Chi 1, and P.K. Pedersen 2

Tong, A., Chi, G., and Pedersen , P.K. (2005): Sequence stratigraphy and preliminary diagenetic study of the Lower Cretaceous Viking Formation, Hoosier area, west-central Saskatchewan; in Summary of Investigations 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-16, 8p.

Abstract This paper reports on preliminary results of sequence stratigraphic and diagenetic studies of the Lower Cretaceous Viking Formation in the Hoosier area, west-central Saskatchewan. In the Hoosier area, the Viking Formation is represented by two major transgressive-regressive sandstone cycles, within which six sequence stratigraphic surfaces have been recognized. A transgressively reworked sequence boundary, characterized by a chert pebble layer, truncates the Joli Fou shale and is overlain by sandstones of the Viking Formation. Each transgressive-regressive sandstone cycle is capped by an amalgamated sequence boundary and/or transgressive surface. The final transgression initiated deposition of the Westgate Formation marine shales.

The major diagenetic processes affecting porosity and permeability in the Viking Formation are quartz cementation and dissolution. Within the Hoosier area, primary porosity in Viking sandstones is enhanced by detrital-grain dissolution. South of the Hoosier region, reduction of porosity and permeability in the sandstones is related to compaction and pore-filling quartz cementation.

Keywords: Viking Formation, Hoosier, sequence stratigraphy, diagenesis, reservoir development.

1. Introduction The Viking Formation in the Hoosier area of west-central Saskatchewan contains prolific hydrocarbon-producing sandstones that have produced in excess of 2.0 million m3 (12.6 million bbl) of oil and 4.7 million E3m3 (165.9 million mmcf) of gas. Previous studies on the Viking Formation in this area have focused on sedimentological and stratigraphic characteristics (Evans, 1970; Pozzobon and Walker, 1990), whereas little has been published regarding the influence of diagenesis on the reservoir quality of these rocks. As part of an on-going project designed to evaluate the mechanisms that influence porosity and permeability in the Viking sandstones, and their relationship to hydrocarbon charging, this paper re-examines the Viking Formation using sequence stratigraphic concepts and reports on diagenetic features based on petrographic studies of thin sections. The area of interest is in the Hoosier region of west-central Saskatchewan (Figure 1) and covers approximately 6700 km2.

2. Geological Background The Lower Cretaceous Viking Formation is generally the coarsest unit in the predominantly argillaceous Colorado Group of western Saskatchewan (Figure 2). The Viking Formation forms an eastward-thinning clastic wedge that extends from British Columbia towards central Saskatchewan and formed in response to the Cordillera uplift (MacEachern et al., 1999). Internally, the Viking Formation is stratigraphically complex with numerous unconformities related to two major marine transgressive-regressive cycles (Caldwell, 1984). Each major cycle consists of higher order transgressive-regressive cycles. Marine and coastal sediments are represented in Viking strata as marine shales, mudstones, sandstones, and pebble beds (Evans, 1970; Pedersen, 2004). The Viking Formation is time equivalent with the Bow Island Formation in the southern Alberta Plains and the Mill Creek Formation in the southern Alberta Foothills (Pedersen, 2004). Previous work in the Hoosier area (Evans, 1970; Pozzobon and Walker, 1990) indicates southward-prograding Viking sandstone bodies. This contrasts with the northeast-prograding Viking sandstone bodies typically found in Alberta and Saskatchewan (Evans, 1970; Walz et al., this volume).

1 Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2; E-mails: [email protected] or [email protected]. 2 Currently with Apache Canada Ltd., #1000, 700 - 9th Avenue SW, Calgary, AB T2P 3V4; E-mail: [email protected].


Figure 1 - The Hoosier oil and gas field in west-central Saskatchewan. The orange polygon outlines the Hoosier oil pool and the green polygon the Hoosier gas pool. Locations of wells that penetrate the Viking Formation and the available cores are shown. Location of cross sections constructed during this study (only cross-section A presented in this report) are also shown.

Figure 2 - Stratigraphic chart of the Lower Viking Formation in west-central Saskatchewan (Saskatchewan Industry and Resources Stratigraphic Correlation Chart, 2003).

3. Database and Methods Fifteen cores, in conjunction with sixty-three resistivity and gamma ray logs, were examined in detail in this study. Integration of sedimentological descriptions with geophysical wireline logs established a correlation between sedimentary facies and petrophysical signatures. One hundred and five samples were collected and forty-one thin sections prepared. Four detailed cross sections were constructed to identify and map sequence stratigraphic surfaces.

Two types of sequence stratigraphic surfaces have been identified: sequence boundaries (SB), and transgressive surfaces (TS). Sequence boundaries are characterized by a basinward shift in facies or by regional, erosional unconformities. Transgressive surfaces are characterized by a landward shift in facies and an environmental change to increasingly marine conditions. The transgressive surface is also the boundary between lowstand and transgressive system tracts (Van Wagoner et al., 1988).

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4. Facies Descriptions

a) Facies A – Fissile Shale with Minor Sandstone Interlaminae The lower portion of Facies A is characterized by grey, mudstone to fissile shale (Figure 3). This facies can be identified regionally with only minor variations in composition or structure. Sandstone interlaminae, approximately 1 mm thick, with grain sizes ranging from lower very fine (62 to 88 µm) to lower fine (125 to 177 µm), constitute approximately 5% of the facies. Sandstone beds are most abundant in the upper 50 cm of this facies below the Viking Formation. Facies A comprises the shaly deposits of the Joli Fou and Westgate formations, which respectively underlie and overlie the Viking Formation.

b) Facies B - -Pebbly Sandstone Facies B is represented by laterally extensive 10 to 20 cm thick beds in which dark grey chert pebbles comprise 10 to 40% and the remaining matrix is mudstone and lower fine- (125 to 177 µm) to lower medium- (250 to 350 µm) grained sandstone (Figure 4). Pebbles are moderately rounded and spherical, and range from 1 to 6 mm in diameter. In the northern portion of the study area, Facies B occurs as three discrete beds, whereas only two were observed to the south.

c) Facies C – Interlaminated Sandstone and Siltstone Facies C is a matrix-supported rock made up of about 50% sandstone and 50% mudstone and siltstone. Framework grains, from lower fine (125 to 177 µm) to lower medium (250 to 350 µm) in size, are approximately 80% quartz, 5% feldspar, 14% lithic grains, and 1% carbonate shell fragments. Primary physical sedimentary structures are rare and include very thin (less than 2 mm thick) trough cross-stratification (Figure 5). Intense bioturbation, including Planolites, Teichichnus, and Zoophycos traces, probably destroyed most primary physical sedimentary structures through reworking of sands, silts, and muds. The ratio of sand to silt and mud content was observed to increase towards the top of the facies in wireline logs and core.

5. Sequence Stratigraphy Within the Hoosier area, six sequence stratigraphic surfaces have been recognized to date within the Viking Formation. The basinward shift in facies between the underlying Joli Fou marine shale (Facies A) and interlaminated sandstone and siltstone of the Viking Formation (Facies C) represents the first sequence boundary (VikSB-1), which developed in response to a relative sea-level fall (Figures 6 and 7). Although the fall in sea level resulted in progradation of coarse sediments into the area, a subsequent sea-level rise caused reworking of deposits at the sequence boundary (VikSB-1) and winnowed all but the coarsest sediments, leaving a chert pebble bed (Facies B). This chert pebble bed represents the first transgressive surface, VikTS-1 (Figures 6 and 7). Thus, within the Hoosier area, VikSB-1 and VikTS-1 are somewhat correlative and form the sequence boundary between the Joli Fou and Viking formations.

VikSand-1 forms the basal Viking sandstone above the VikSB-1/VikTS-1 within the Hoosier area (Figures 3, 6 and 7). Continued progradation of coarse sediments is represented by cycles of sanding upward, bioturbated, interlaminated sandstones and siltstones (Facies C) reflecting alternating offshore transitional to lower shoreface environments. VikSand-1 is capped by the overlying sequence boundary VikSB-2 (Figures 6 and 7).

The merging of VikSB-2 with VikSB-1/VikTS-1 in the southern part of the study area (Figures 6 and 7) is likely related to the southward thinning of VikSand-1 and erosion associated with VikSB-2. In the southern part of the study area, VikSand-2 forms a lowstand, southward-prograding sandstone succession (Facies C) which overlies VikSB-2 and onlaps the sequence boundary toward the north (Figures 6 and 7). The following transgression (VikTS-2) is recognized as offshore marine shales overlying VikSand-2 in the south or as chert pebble beds of Facies B to the north (Figure 6 and 7). In the northern part of the study area, partly due to transgressive erosion, VikTS-2 is coincident with VikSB-2. The transgressive-regressive cycle initiated during the formation of VikTS-2 represents the final deposition of Viking sediments in the study area. The sanding upwards succession of VikSand-3 (Facies C) indicates deposition occurred during sea-level highstand or fall. Deposition of VikSand-3 is terminated by a coinciding sequence boundary and transgressive surface (VikSB-3 and VikTS-3) (Figures 6 and 7), which is identified as a chert pebble bed (Facies B) (Figure 4).

6. Diagenesis Dissolution is the major diagenetic process affecting reservoir quality of the Viking Formation within the Hoosier area. Quartz cementation is absent within the Hoosier area, probably because of post-cementation quartz


Figure 3 - Sharp based sandstones of the Viking Formation in well 8-19-31-26W3, depth 721 to 736 m. a) Core photograph illustrating the sequence stratigraphic surfaces. b) Sedimentological log showing sequence-stratigraphic surfaces and ichnology.

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dissolution, indicated by the irregular grain boundaries shown by most detrital grains (Figure 8a). Outside the Hoosier area, minor amounts of quartz occur as pore-filling cement in the Viking sandstones (Figure 8b), and later dissolution has been relatively minor as detrital grains mainly exhibit unaltered grain boundaries.

7. Discussion Porosity and permeability characteristics of the Viking Formation within and adjacent to the Hoosier area are variable and related to differences in lithology arising from changes in relative sea level, compaction, quartz cementation, and dissolution. Texturally, Viking strata above and below VikSB-2 are different. Below VikSB-2, VikSand-1 is characterized by a slightly larger grain size (lower fine [125 to 177 µm]) relative to the framework grains (upper very fine [88 to 125 µm]) of VikSand-2 and -3. VikSand-1 also has a higher ratio of sandstone to siltstone thanVikSand-2 and -3. This is significant in that higher siltstone contents occupy more pore space thereby reducing primary porosity. Quartz cementation outside the Hoosier area further degrades porosity. Secondary porosity enhancement of VikSand-1 within the study area occurred as a result of quartz dissolution. The difference in porosity and permeability of Viking strata above and below VikSB-2 can be illustrated through production data. Of the core examined for this study, oil and gas production from one well within the Hoosier study area reaches values of 44 000 m3 and 280 000 E3m3, respectively, within VikSand-1. Comparatively, strata above VikSB-2 produced only 140 e3m3 of gas and 1450 m3 of oil.

8. Conclusions Based on sedimentological and petrophysical data, six sequence stratigraphic surfaces (VikSB-1, VikTS-1, VikSB-2, VikTS-2, VikSB-3, VikTS-3) have been recognized within the Joli Fou and Viking formations in the Hoosier area. Sequence boundary VikSB-1 records the abrupt basinward shift in facies from the Joli Fou shales to Viking sandstones. In the northern part of the study area, Viking sandstone deposits VikSand-1 are truncated by a sequence boundary (VikSB-2). In the southern part of the area, VikSand-2 forms a detached lowstand unit characterized by northward onlap onto VikSB-2. VikSand-2 is capped by a transgressive surface (VikTS-2), overlain by a chert pebble bed. Towards the northern part of the study area, VikSB-2 and VikTS-2 merge partly due to transgressive erosion. In the southern part of the Hoosier area, marine siltstones and shales overlie VikTS-2 whereas in the northern part of the study area, sandstones of VikSand-3 overlie the coincident VikSB-2/VikTS-2 surfaces. Terminating VikSand-3 is a third sequence boundary (VikSB-3) and transgressive surface (VikTS-3). This surface separates the Viking Formation from the overlying Westgate Formation marine shales.

Figure 4 - Core photograph of chert (Ch) pebble bed in well 9-7-31-25W3, depth 753 m. Note the mudstone interbed (Md).

Figure 5 - Core photograph from well 8-19-31-26W3, depth 673 m, showing trough cross-stratification and Teichichnus burrows (Te).

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Figure 7 - Schematic illustration of relationships between sequence-stratigraphic surfaces and sedimentary facies.

The controls on porosity and permeability within the Viking Formation in the Hoosier area are related to both sedimentary environments and diagenetic processes. Enhancement or reduction of primary porosity is a result of diagenetic processes such as cementation and dissolution. Quartz cementation of the Viking Formation occurred mainly outside the Hoosier area. Within the Hoosier area, enhancement of primary porosity of the Viking Formation is related to quartz dissolution.

9. Acknowledgments The authors are grateful to Profico Energy and National Science and Engineering Research Council of Canada for funding this project. We also thank the staff at Saskatchewan Industry and Resources Subsurface Geological Laboratory for access to core and reference material. Software support from geoLOGIC and M.J. Systems is invaluable and greatly appreciated. Dr. Hairuo Qing and Dr. Osman Salad Hersi are thanked for their critical review of the paper.

10. References Caldwell, W.G.E. (1984): Early Cretaceous

transgressions and regressions in the southern interior plains; in Stott, D.F. and Glass, D.J. (eds.), Mesozoic of Middle North America. Can. Soc. Petrol. Geol., Mem. 9, p173-204.

Evans, W.E. (1970): Imbricate linear sandstone bodies of Viking Formation in Dodsland-Hoosier area of southwester Saskatchewan, Canada; Amer. Assoc. Petrol. Geol. Bull., v54, no3, p469-486.

MacEachern, J.A., Zaitlin, B.A., and Pemberton, S.G. (1999): A sharp-based sandstone of the Viking Formation, Joffre Field, Alberta, Canada: Criteria for recognition of transgressively incised shoreface complexes; J. Sed. Resear., v69, no4, p876-892).

Pedersen, P.K. (2004): Shallow gas research project in southwestern Saskatchewan: Revised lithostratigraphy of the Colorado Group and reservoir architecture of the Belle Fourche and Second White Specks in the Senate Pool; in Summary of Investigations 2004, Volume 1, Saskatchewan Geological Survey, Sask. Industry

Figure 8 - Photomicrographs of sandstones of the Viking Formation. a) Porosity increased through dissolution of quartz grains; qtz, quartz; dis, dissolution; and p, porosity (15-34-30-26W3, depth 734 m, plane-polarized light]. b) Quartz overgrowth (qo) forming along a grain boundary; qtz, quartz (15-34-30-26W3, depth 734 m, plane-polarized light).

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Resources, Misc. Rep. 2004-4.1, CD-ROM, Paper A-16, 15p.

Pozzobon, J.G., and Walker, R.G. (1990): Viking Formation (Albian) at Eureka, Saskatchewan: A transgressed and degraded shelf sand ridge; Amer. Assoc. Petrol. Geol. Bull., v74, no8, p1212-1227.

Saskatchewan Industry and Resources (2003): Stratigraphic Correlation Chart, 1p.

Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S., and Hardenbol, J. (1988): An overview of the fundamentals of sequence stratigraphy and key definitions; in Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C. (eds.), Sea level changes: An integrated approach, Soc. Econ. Paleont. Mineral., Spec. Publ. 42, p37-45.

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