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Geology of the Mississippian Frobisher Beds, Innes Field Southeastern Saskatchewan.

by H. T. Crabtree

This paper is intended as a brief summary of the stratigraphy, structure, and

reservoir characteristics of the Innes Field.

The Innes Field produces hydrocarbons from the Frobisher Beds of the

Missi s sippian Frobisher-Alida Beds, and covers a pproximately 2960 hectares (29.6 km2)

in eighteen sections straddling Townships 7 and 8 and Ranges 10 and 11 west of the

Second Meridian in southeastern Saskatchewan (Fig. 1). The field presently

contains 68 wells of which 54 have produced oil. Reservoir pressure is created by

a primary basal wa ter drive. No comprehensive enhanced recovery scheme is in

operation even though approximately 70 per cent of the estimated total reserves

available from primary drive have been exhausted.

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Fig. l - Structure contour map on the Frobisher Beds, Innes Fiel d . Contour interval is 4 m; datum is sea level.

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Geological Setting

The gently dipping Mississippian shelf carbonates of the northeast part of the

Williston Basin have been affected by Laramide epeirogenesis that established the

present dip in the Innes area at approximately 8-12 m/km towards the centre of the

basin. Pre-Mesozoic epeirogenesis and erosion caused truncation of progressively

older Mississippian strata outward from the centre of the basin. Thus the

Frobisher Beds, along with seven other conformable, cyclic, marker-defined units,

have northwest-southeast trending subcrop traces on the sub-Mesozoic unconformity.

Hydrocarbons are trapped where porous and permeable carbonate beds meet the

anhydritized unconformity surface and oil pools are roughly aligned with the

subparallel subcrops of the beds. The Innes Field is located on the periphery of

the Frobisher Beds subcrop and can be considered to be within the Frobisher trend of

oilfields.

Method of Study

Cores were examined directly with a binocular microscope, and thin sections are

being prepared for detailed petrological examination. Also used was relevant

information from geophysical well logs, stratigraphic logs, and core analyses. Rock

type and porosity were classified after Dunham (1962) and Choquette and Pray (1970)

respectively. A visual estimation was made of porosity percentage, aided by

available core analyses. Porosity of the rock was categorized: 0-6 per cent-poor,

7-12 per cent-fair, 13-18 per cent-good, greater than 18 per cent-excellent.

Stratigraphy of the Frobisher Beds

The Frobisher-Alida beds in southeastern Saskatchewan are separated into two

carbonate units, the Alida Beds below, and the Frobisher Beds above, by an inter­

vening arenaceous or argillaceous unit, the Kisbey Sandstone (Fig. 2). In the Innes

area,~he Kisbey Sandstone consists of a reddish stained, dolomitic quartzose sandstone

or a buff argillaceous microsucrosic dolomite, both characterized by high gamma

ray counts on the geophysical well logs.

The Frobisher Beds are overl ain by the Frobisher Evaporite of the Midale Beds.

This evaporitic unit consists of patterned dolomite and nodular anhydrite, suggestive

of extremely shallow, perhaps supratidal deposition.

Four major rock types make up the Frobisher Beds in this area: oolitic­

pelletoidal packstone, skeletal packstone, argillaceous microsucrosic dolomite, and

sucrosic (marly) dolomite.

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Fig. 2 - Typica l mi crofacies and geophys ica l log correla t ion of the Frobisher Beds, Innes Field .

Oolitic- pelletoidal packstone : The oolitic pelletoida l packstone i s t he main

r eservoir rock (algal, oolitic, pis olitic limes tone of Fuzes y , (1966)) . I t

consists o f oolites , poorly de fined oolites (laminations degr aded by r ecr ystalliz­

ation?), pelletoids and pisolites. The unit has been t e rmed a pseudo- ooli te by

various authots, becaus e t he origin of the pe lletoids is difficult to de t ermine .

It is not c lea r whe t her they are poorl y defined ooli tes , or fecal pellets . The

oolites and pisolites are commonly veneers, and some of the l ar ger nuc lei a r e

broken dasycladacean or codiacean a l gal pl a tes. Many of the non-ske l e tal and ske letal

components have been broken and some of the f r a gments form nuc lei of other ooli tes

and pi soli tes.

In addition to the calcium carbonate laminated particles, t his unit contains

considerable lime mud matrix and minor amounts o f crinoidal debris , a lgal fragments ,

and foraminifera l t es ts . I t grades vert ically and l ateral ly i nto wackestone s with

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oolitic and pelletoidal components. As present-day ooids fo nn in agitated

environments with wate r depths of less than two meters (Milliman, 1974), the

oolitic-pelletoidal packstone was probably deposited in a shelf environment. Broken

and reworked components could represent l ocal shoals in an intertidal or subtidal

zone.

Skeletal packstone: The skeletal packstone contains a variety of organic remains

including brachiopod valves, fragments of corals, crinoid ossicles and plates, and

algal debris, as well as unidentifiable fossil material. It grades vertically and

l aterally into a wackestonecomposed of a matrix of finely cornminuted skeletal debris

with isolated large rugose coral fragments.

This packstone is a reservoir rock in four wells in the Innes Field.

Argillaceous microsucrosic dolomite: The argillaceous microsucrosic dolomite is a

friab l e, light coloured rock with variable calcite content. It contains rare

skeletal fragments a nd often shows distinct bioturbat ion and mottling. It character­

istically has excellent porosity (30 per cent)and extremely low permeabilities

(.5-3 md.) with residual water saturations of 75 to 80 per cent common.

The argillaceous microsucrosic dolomite interval in places is represented by

lime muds tone, patterned dolomite, hematite stained dolomi tic quartzose sandstone,

or by argillaceous microsucrosic dolomite with mottled hematite or limonite staining.

The lat ter l ithology possibly represents the early s t ages of formation of cryp to­

crystalline patterned dolomite.

Sucrosic (marly) dolomite: The sucrosic (marly) dolomite has the same excellent

i ntercrystalli ne porosity as the argillaceous rnicrosucrosic dolomite, but shows

higher permeabilities (7-8 md.). It isahydrocarbon reservoir r ock in the northwest

portion of the Innes Field.

The Frobisher Beds of the study area have been divided into four stratigraphic

units, each containing one or more of the four rock types. These units are designated,

oldest to youngest, IFBP One through Four (Innes Frobisher Beds Pool 1-4) (Fig. 2) .

Each is thought to represent one complete depositional cycle(marine transgression

followed by regression).

The basal unit (IFBP-1), approxima t ely 23 m thick, is poorly represented in

cores b ecause the main oil pools are located in the superjacent IFBP -2 and -3.

The lower portion of the unit contains skeletal packstone and wackestone grading

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upwards through ool itic-pe lletoidal pa ckstone. The top o f the uni t i s de fined by an

a r gillaceous microsucrosic dolomi te (A- 1), i n terpreted a s being later ally equival ent

t o the Wi nlaw Evaporite of Fuzesy , (1960) . The unit a lso inc l udes a non-a r gillaceous

micros ucrosic dolomite which increases i n thi ckness t owards the northwes tern por tion

o f t he I nnes Field.

Units IFBP-2, - 3 and -4 each app roxi ma tely 8 m t hi ck, are predomi nan t l y

oolitic- pelletoidal packs t ones and wackes t ones. Ar gillaceous dolomi tes A- 3 and A- 4

define t he t op of IFBP-3 and IFBP- 4 respect ively . Although an argillaceous

micr osucros i c dol omi t e is local l y pr esen t within A- 2 , t he interval is occupied by

an a rgil l aceous bed throu ghout most of the fie l d . A- 3 is t hough t t o be l a t e r a l ly

equiva l en t t o t he Has tings Evapori te of Fuzesy (1960).

Rese rvoir

The ma j ority of Innes produc t i on comes fr om a domal st r uc ture in the western

porti on of the f i eld ( Fi g . 1) . This s tructure is bounded on the nor t hwest by a

facies change t hroughout t he Frobisher s ucces s ion, and on t he east by a northwes t­

sout heast trend i ng depress ion . The present dep r ess ion had at l east two stages of

subs i dence , as indica t ed by anoma l ous thi ckening o f t wo unit s .

The reservoir a t Innes has a high water sat ur ation (app r oximately 35 per

cent), no gas cap , and ther e are s i x di s t inct hydr ocarbon pools varying i n area from

120 t o 480 h ectare s .

Four oil poo l s a r e enclosed by t he ooli tic- pelletoi dal packstones of t he four

s t rat i gr aphic uni ts on t h e ma i n pr oducing structure . An addit iona l pool, loca t ed i n

the northeast of t he f i e ld, occupies a single oo l i t ic- pelletoidal packstone i n t h e

IFBP -3.

In the northwest of the f i e ld oolitic- pelleto i dal packs t one of the southeas t

changes to argillaceous d ol omites , dolomitic pell etoidal wackestones , and skeletal

packstone s and wackes t ones . The s i xth poo l i s l oca t ed here in a s uc r osic (marly)

dolomi te and skel etal packstone.

The ooli t i c- pe l let oida l packstone reser voir rock s hows fair to good int erpart­

icle , fenestra l and vuggy porosity whi ch appears to be t he r esul t of two s t ages of

deve l opment. A pr i mary porosi t y was created by wave energy r emoving much of the

lime mud matrix. A l arge portion of t his primary porosi ty was l ater los t due t o

post-depositional cal ci te and anhydri t e void-filling cement. In the second stage ,

por os i ty was cr ea t ed when the rock was exposed , and cir c ulati ng gr oundwat ers

dissolved the cement and intersti t i al carbonate mud mate rial. Most of t he ooli tic-

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pelletoidal packstone in the area retains a vuggy nature, however the best porosity

and permeability is located where there i s a s i gnificant primary porosity in the rock.

Whereve r t he individual porous oolitic-pelletoidal packstones are separated by

water-saturated impermeable argillaceous microsucrosic dolomites, separate reservoirs

exist. Thus, each pool has a unique oil/water contact and its own pressure system.

The poo ls are sealed up-dip by permeability changes within the beds or by the anhyd­

ritized sub-Mesozoic erosion surface. The top and bottom seals to each pool are

c r eated by the water saturated argill aceous zones . This type of strat i fied pool

system has been called a multi-level reservoir (Edie 1958, Fuller 1967) .

Hydrocarbons are also produced from a sucros ic dolomite with excellent inter­

crystalline porosity, and a skel etal packstone with fair interparticle porosi t y and

minor moldic and shelter porosity. The in tercrystalline porosity and permeability

in the dolomites are primarily controlled by the size of the dolomit e rhombs, and

the hydrocarbons in this reservoir rock are trapped by permeability r eduction up-dip.

References

Choquette, P.W., and Pray, L.C., 1970, Geologi cal Nomenclature and Classificati on of Porosity i n Sedimentary Carbonates: AAPG Bull., vol . 54, pp. 207-250 .

Dunham, R.J., 1962, Classification of Carbonate Rocks According to Depositional Texture: in, Ham, W. E. (editor), Classification of Carbonate Rocks: AAPG Mem. No. 1, pp. 108-121.

Edie, R.W., 1958, Mississippian Sedimentat ion and Oil Fields in Southeaste rn Saskatchewan: in , Goodman, A.J . (editor) , Jurassic and Ca rboniferous of Western Canada: All an Memorial Vol ume, AAPG, pp. 331- 363.

Fuller, J.G.C.M., Illing, L.V., and Wood, G.V ., 1967, Reservoir Rocks and Strati ­graphic Traps in Non-Reef Carbonates: Pro c . Seventh World Petrol. Cong., vol . 2, pp. 487-499 .

Fuzesy, L.M., 1960, Correlation and Subcrops of the Mississippian Strata in South­eastern and South-Central Saskatchewan: Sask. Dept. Min. Res. Rept. No.51, 63 p.

Fuzesy, L.M. , 1966, Geol ogy of the Frobisher-Alida Beds Southeastern Saskatchewan: Sask. Dept • .tfin. Res. Rep t. No. · 104, 59p.

Milliman, J .D. , 1974, Marine Carbonates: Springer-Verlag, New York, 375p.