Prospect saskatchewan

The Mississippian Looking Beyond the Subcrop Plays

Issue No. 4 April 200619 48

INTRODUCTION

Throughout the history of Mississippian exploration in Saskatchewan, plays have been focused on exploiting oil stratigraphically trapped against the regional Sub-Mesozoic Unconformity (Kent, 1987). This strategy has been effective for many years, and has yielded abundant reserves in southeastern Saskatchewan. Although this proven concept may result in further incremental production in some fields, new Mississippian targets are more subtle, and require detailed knowledge of the interplay between facies and structure.

This issue of Prospect Saskatchewan familiarizes the reader with facies, structures, and trapping mechanisms that should be considered when embarking upon Mississippian exploration in Saskatchewan.

STUDY AREA AND INTERVAL

The sequences studied here are the Frobisher Beds, (uppermost Mission Canyon Formation), and the Midale Beds at the base of the Charles Formation (Figure 1).

One useful area in which to examine trapping mechanisms in Mississippian strata down-dip from the subcrop lies within Townships 3 to 5, Ranges 7 to 13 W2M and encompasses the West Kingsford, Macoun, Elswick, and Bromhead oilfields

Figure 1: Stratigraphic chart of Mississippian units in southeastern Saskatchewan.

Figure 2: Location map for study area shown in red. Major Mississippian oilfields are labeled. Green and purple lines indicate fault zones identified using high resolution aeromagnetics (courtesy Gedco). The subcrop edges of various Mississippian strata are shown by thin coloured lines.

(Figure 2). Here, the effect of underlying structure influencing facies distribution and ultimately hydrocarbon accumulation in the Frobisher and Midale Beds can be demonstrated.

ESTEVAN

WEYBURN

CANADAT1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12Saskatchewan

R1W2R2R3R4R5R6R7R8R9R10R11R12R13R14R15R16R17R18R19R20R21R22R23R24W2

WEYBURN POOL MIDALEPOOL

BENSON POOL

BIENFAIT

MACOUN WESTKINGSFORD

BROMHEAD

ELSWICK STEELMAN POOL

Frobisher Beds

Alida Beds

Big Snowy Group

Poplar Beds

Midale Beds

Ratcliffe Beds

MA

DIS

ON

GR

OU

P

BIG SNOWYGROUPN

AIP

PIS

SIS

SIM

CHESTERIAN

MERAMECIAN

OSAGIAN

KINDERHOOKIAN

360

334

329

314

SU

OR

EFI

NO

BR

AC 351

EPOCHPERIOD STAGE/AGE(Ma)

SOUTHEASTERNSASKATCHEWAN

Sub-Mesozoic Unconformity

KIBBEY

CHARLES

MISSIONCANYON

LODGEPOLE

POPLAR

RATCLIFFE

MIDALE

TILSTON

SOURIS VALLEY

FROBISHERALIDAKISBEY

2

Lower Midale Isopach Map

Frobisher Isopach Map

Frobisher Structure Map

C.I. = 1 m

C.I. = 1 m

C.I. = 1 m

0 10 20

Kilometres

0 10 20

Kilometres

0 10 20

Kilometres

MAPPING

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

FACIES

The multi-layer shoal pattern developed in the Frobisher-Midale sequence (Figure 4), has led to a configuration wherein specific facies serve as reservoirs, whereas others act as cap rocks. Potential reservoirs can, for example, occur in the ‘washover facies’ consisting of reworked crinoidal and oolitic grainstones (Figure 5A) deposited in a high-energy environment proximal to shoal developments. Shoals also may have well-developed vuggy and interparticle porosity (Figure 5B). Cap rocks are generally formed by lower energy, landward, lagoonal or intershoal facies mudstones with little porosity or permeability. The seaward edges of evaporites may also act as sealing units.

Figure 3: The upper two isopach maps of the Lower Midale and Frobisher Beds are coloured to indicate regions of potential oolitic shoal development. The lower structure map of the Frobisher Beds includes the location of regional basement faults identified by high-resolution aeromagnetic data (courtesy Gedco) and superimposed Frobisher (blue) and Midale (green) shoals. Cross-section A-A’ is shown on the isopach maps.

T3

T4

T5

R7W2R8R9R10R11R12R13

T3

T4

T5

R7W2R8R9R10R11R12R13

A

A’

T3

T4

T5

R7W2R8R9R10R11R12R13

A

A’

3

1525

1550

1575

1600

1625

21/07-16-04-08W2

1475

1500

1525

1550

1575

1600

21/11-21-04-08W2

Datum

Mid

ale

Bed

sFr

obis

her B

eds

Rat

cliff

e a

nd P

opla

r Bed

s

Midale Evaporite

LowerMidale(Vuggy)

UpperMidale(Marly)

1525

1550

1575

1600

1625Kisbey

Alida Beds

11/13-09-04-08W2

Datum

1550

1575

1600

1625

1650

41/09-34-03-09W2DG

Sub-Mesozoic Unconformity Surface

DGG D DG

A’Northeast

ASouthwest

Evaporites (Salina/Sabkha)

Restricted Shelf Environment

Shoal Environment

Intershoal Environment

‘Washover’ Environment

Kisbey Sandstone

Midale/Frobisher Contact

Hydrocarbon Trap

Midale Evaporite

Frobisher Evaporite

Upper Frobisher EvaporiteUpper Midale “Marly”

Lower Midale “Vuggy”

Frobisher Beds

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.

Upper Frobisher Evaporite

4

References:

Kent, D.M., 1987, Mississippian facies, depositional history, and oil occurrences in Williston Basin, Manitoba and Saskatchewan: Rocky Mountain Association of Geologists Symposium, p., 157-170.

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

To download these and other documents related to hydrocarbon resources of Saskatchewan, visit: http://www.er.gov.sk.ca/prospectsask

SUMMARYAnalogs:West Kingsford, View Hill, and Macoun Pools.

Trap:Stratigraphic with facies distribution partially influenced by underlying structure

Reservoir:Skeletal/oolitic/crinoidal grainstones and vuggy oolitic wacke/packstones

Seal:Lagoonal mudstones and sabkha/salina evaporites

Oil Source:Lodgepole Formation

PREDICTIVE MODELING

Traps resulting from favourable spatial distribution of appropriate facies likely developed associated with underlying paleotopography. Paleotopography in southern Saskatchewan is usually controlled by either a) dissolution along the edge and within the underlying Prairie Evaporite and other Devonian salts, or b) movement along regional linear fault zones in underlying strata. Two such fault zones are shown on the Frobisher structure map of Figure 3. Many other regional fault zones (Figure 2) may have impacted depositional history, resulting in the development of potential Mississippian traps.

Inverse thickness relationships of overlying beds may be a predictor of the location of Mississippian shoal developments. For example, the overlying Poplar and Ratcliffe Beds ‘thin’ above Frobisher shoals as shown in well 13-09-04-08W2 and 11-21-4-8W2 of Figure 4. This correlation occurs in many areas, and can be used as a predictive tool for Mississippian exploration.

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.

The example provided here can be used to assist in exploring similarly faulted areas, but with less well control.

Figure 5: A) Coated-grain intraclastic packstone reservoir in the Frobisher Beds of the Elswick Pool. This is interpreted to be a “washover” facies. 5-6-5-12W2 (1540.8m), B) Lower Midale shoal facies oolitic grainstone reservoir in 15-20-4-7W2 (1496.3m) in the West Kingsford area.

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-

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CONCLUSIONS

A B

This is the fourth in a series of informational brochures released by the Petroleum Geology Branch of Saskatchewan Ministry of Energy and Resources. These brochures are intended to highlight prospective areas in the province as identified by staff geologists. It is hoped these will be useful introductions to regions and concepts for explorationists to further develop the oil and natural gas resources of Saskatchewan. To receive additional copies or new releases, please email Dan Kohlruss at dan.kohlruss@gov.sk.ca.

Although the Ministry of Energy and Resources has exercised all reasonable care in the compilation, interpretation and production of this brochure, it is not possible to ensure total accuracy, and all persons who rely on the information contained herein do so at their own risk. The Ministry of Energy and Resources and the Government of Saskatchewan do not accept liability for any errors, omissions or inaccuracies that may be included in, or derived from, this brochure.