Prospect Saskatchewan the Mississippian Looking Beyond the Subcrop Plays

Prospect saskatchewan The Mississippian Looking Beyond the Subcrop Plays

INTRODUCTION

Throughout the history of Mississippian exploration in PERIOD EPOCH STAGE/ SOUTHEASTERN Saskatchewan, plays have been focused on exploiting oil AGE(Ma) SASKATCHEWAN stratigraphically trapped against the regional 314 Sub-Mesozoic Sub-Mesozoic Unconformity (Kent, 1987). This strategy CHESTERIAN

S BIG SNOWY U N Unconformity

has been effective for many years, and has yielded 329 GROUP A

O KIBBEY

R P

abundant reserves in southeastern Saskatchewan. E MERAMECIAN

P POPLAR

F I

Although this proven concept may result in further S 334 CHARLES RATCLIFFE

I O

incremental production in some fields, new Mississippian S MIDALE B

S OSAGIAN FROBISHER I R KISBEY

MISSION ALIDA A targets are more subtle, and require detailed knowledge of M 351 CANYON

the interplay between facies and structure. C TILSTON MADISON GROUP KINDERHOOKIAN LODGEPOLE SOURIS VALLEY This issue of Prospect Saskatchewan familiarizes the 360 reader with facies, structures, and trapping mechanisms Figure 1: Stratigraphic chart of Mississippian units in that should be considered when embarking upon southeastern Saskatchewan. Mississippian exploration in Saskatchewan. (Figure 2). Here, the effect of underlying structure influencing facies distribution and ultimately STUDY AREA AND INTERVAL hydrocarbon accumulation in the Frobisher and Midale Beds can be demonstrated. The sequences studied here are the Frobisher R24W2 R23 R22 R21 R20 R19 R18 R17 R16 R15 R14 R13 R12 R11 R10 R9 R8 R7 R6 R5 R4 R3 R2 R1W2 Beds, (uppermost Saskatchewan T12 Mission Canyon T11 Formation), and the T10 Midale Beds at the base of the Charles Formation T9 WEYBURN (Figure 1). T8 Alida Beds T7 One useful area in WEYBURN POOL MIDALE BENSON POOL T6 which to examine POOL Frobisher Beds trapping mechanisms in T5 ELSWICK STEELMAN Mississippian strata POOL Midale Beds T4 WEST MACOUN KINGSFORD Ratcliffe Beds down-dip from the T3 subcrop lies within BROMHEAD BIENFAIT Poplar Beds T2 Townships 3 to 5, ESTEVAN T1 CANADA Ranges 7 to 13 W2M Big Snowy Group and encompasses the West Kingsford, Figure 2: Location map for study area shown in red. Major Mississippian oilfields are Macoun, Elswick, and labeled. Green and purple lines indicate fault zones identified using high resolution Bromhead oilfields aeromagnetics (courtesy Gedco). The subcrop edges of various Mississippian strata are shown by thin coloured lines.

19 48 Issue No. 4 April 2006 Lower Midale Isopach Map MAPPING

An isopach map of the Frobisher Beds T5 (Figure 3) shows the unit thickness to be highly variable; this is due largely to the presence of shoals commonly A’ T4 developed in this package. The isopach map of the lower Midale or “Vuggy” Beds in the same area also A shows anomalous “thicks” that reflect T3 oolitic shoals. Oolitic shoals in the younger Midale strata tend to be found R13 R12 R11 R10 R9 R8 R7W2 0 10 20 farther south than the shoals in the C.I. = 1 m Frobisher Beds, suggesting the Kilometres distribution of this facies migrated Frobisher Isopach Map progressively basinward with continued basin-filling. To illustrate the relationship of structure on T5 sedimentation patterns, regional basement faults identified by A’ high-resolution aeromagnetic data are T4 displayed on the structure map of the Frobisher Beds in Figure 3. A close A spatial correspondence of the east-west fault trend and shoal T3 development is evident.

R13 R12 R11 R10 R9 R8 R7W2 FACIES 0 10 20 C.I. = 1 m Kilometres The multi-layer shoal pattern Frobisher Structure Map developed in the Frobisher-Midale sequence (Figure 4), has led to a configuration wherein specific facies T5 serve as reservoirs, whereas others act as cap rocks. Potential reservoirs can, for example, occur in the ‘washover

T4 facies’ consisting of reworked crinoidal and oolitic grainstones (Figure 5A) deposited in a high-energy environment proximal to T3 shoal developments. Shoals also may have well-developed vuggy and R13 R12 R11 R10 R9 R8 R7W2 interparticle porosity (Figure 5B). Cap 0 10 20 C.I. = 1 m rocks are generally formed by lower Kilometres energy, landward, lagoonal or intershoal facies mudstones with little Figure 3: The upper two isopach maps of the Lower Midale and porosity or permeability. The seaward Frobisher Beds are coloured to indicate regions of potential oolitic shoal development. The lower structure map of the Frobisher Beds edges of evaporites may also act as includes the location of regional basement faults identified by sealing units. high-resolution aeromagnetic data (courtesy Gedco) and superimposed Frobisher (blue) and Midale (green) shoals. Cross-section A-A’ is shown on the isopach maps.

2 Sub-Mesozoic Unconformity Surface

A A’ Southwest Northeast 41/09-34-03-09W2 11/13-09-04-08W2 21/07-16-04-08W2 21/11-21-04-08W2

fe and Poplar Beds G D D G G D 1550 G D

Ratcli f 1475 1525

Midale Evaporite 1525 Datum Upper 1575 Midale 1500 (Marly) 1550 Lower Midale

Midale Beds (Vuggy) 1550 Upper Frobisher Evaporite 1600 1525 Datum

1575

1575 1625 1550

1600

1600 Frobisher Beds 1650 1575

1625

Kisbey 1625

Alida Beds 1600

Midale Evaporite

Upper Midale “Marly”

Frobisher Evaporite Lower Midale “Vuggy” Upper Frobisher Evaporite

Frobisher Beds

Kisbey Sandstone

Evaporites (Salina/Sabkha) Intershoal Environment Hydrocarbon Trap Restricted Shelf Environment ‘Washover’ Environment Midale/Frobisher Contact Shoal Environment Kisbey Sandstone

Figure 4: Cross-section A-A’ through the West Kingsford Pool showing bank/washover relationships in the Frobisher and Midale Beds. Below this is a schematic interpretation of the facies relationships and oil traps in the study area.

3 CONCLUSIONS - Mississippian exploration in Saskatchewan should no longer focus solely on structure and unconformity trapping. Integrated structure and facies analysis will be useful toward finding new Mississippian pools.

- Predictive facies analysis is possible considering that deposition of facies having reservoir potential in Mississippian strata is often controlled by underlying structure, and is reflected in overlying strata. A B - The example provided here can be used to assist in exploring similarly faulted areas, but with less well control.

SUMMARY Analogs: West Kingsford, View Hill, and Macoun Pools. Figure 5: A) Coated-grain intraclastic packstone reservoir in the Frobisher Beds of the Elswick Pool. This is interpreted to be a “washover” facies. Trap: 5-6-5-12W2 (1540.8m), B) Lower Midale shoal facies Stratigraphic with facies distribution partially oolitic grainstone reservoir in 15-20-4-7W2 (1496.3m) in the West Kingsford area. influenced by underlying structure

PREDICTIVE MODELING Reservoir: Skeletal/oolitic/crinoidal grainstones and vuggy Traps resulting from favourable spatial distribution of oolitic wacke/packstones appropriate facies likely developed associated with underlying paleotopography. Paleotopography in Seal: southern Saskatchewan is usually controlled by either a) Lagoonal mudstones and sabkha/salina evaporites dissolution along the edge and within the underlying Prairie Evaporite and other Devonian salts, or b) Oil Source: movement along regional linear fault zones in underlying Lodgepole Formation strata. Two such fault zones are shown on the Frobisher structure map of Figure 3. Many other regional fault References: zones (Figure 2) may have impacted depositional history, resulting in the development of potential Mississippian Kent, D.M., 1987, Mississippian facies, depositional history, and oil occurrences in Williston Basin, Manitoba and Saskatchewan: Rocky traps. Mountain Association of Geologists Symposium, p., 157-170.

Inverse thickness relationships of overlying beds may be Nimegeers, A.R. and Nickel, E. (2005): Hydrocarbon trapping in the Mississippian Midale and Frobisher Beds, southeastern Saskatchewan: a predictor of the location of Mississippian shoal Looking away from the subcrop edge; in Summary of Investigations developments. For example, the overlying Poplar and 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Ratcliffe Beds ‘thin’ above Frobisher shoals as shown in Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-13, 16p. well 13-09-04-08W2 and 11-21-4-8W2 of Figure 4. This To download these and other documents related to hydrocarbon correlation occurs in many areas, and can be used as a resources of Saskatchewan, visit: http://www.er.gov.sk.ca/prospectsask predictive tool for Mississippian exploration.

Although the Ministry of Energy and Resources has exercised all reasonable care in This is the fourth in a series of informational brochures released by the Petroleum the compilation, interpretation and production of this brochure, it is not possible to Geology Branch of Saskatchewan Ministry of Energy and Resources. These ensure total accuracy, and all persons who rely on the information contained herein brochures are intended to highlight prospective areas in the province as identified by do so at their own risk. The Ministry of Energy and Resources and the Government staff geologists. It is hoped these will be useful introductions to regions and of Saskatchewan do not accept liability for any errors, omissions or inaccuracies concepts for explorationists to further develop the oil and natural gas resources of that may be included in, or derived from, this brochure. Saskatchewan. To receive additional copies or new releases, please email Dan Kohlruss at [email protected].