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Low-Permeability Gas Reservoirs in Marine Cretaceous Sandstones of Saskatchewan:

4. Upper Colorado and Montana (Turonian to Campanian) Strata of Western Saskatchewan

by Frank Simpson1

The study area is that part of western Saskatchewan, bounded by the northern

limit of Township 40 and l a titude 49° North and extending from the Third Meridian

to the Albert a border (Fig. 1).

In this paper, the relationships between northward extensions of the Phillips

Sandstone (Gre enhorn Formation), Bowdoin Sandstone (Carlile Formation) and Martin

Sandy zone (Niobrara Formation) of northern Montana and the enclosing Upper Colorado

(Turonian to Santonia n) shaly chalks and calareous shales are described for the first

time. In addition, observations on lithologic variations of considerable significance

to future studies of reservoir dynamics are made with respect to the Medicine Hat

Sandstone, the Milk River Formation and the Ribstone Creek Tongue of the Belly

River Formation.

Stratigraphy

In western Saskatchewan, the youngest strata yielding production of hydro-

carbons are referable to the Ribstone Creek Tongue of the Belly River Formation

(Campanian) at the Whiteside field (Nichols and Wyman, 1969). Figure 2 shows litho-

logic variation in the Oldman, Ribstone Creek and Victoria Tongues of the Belly River

elastic wedge and associated sediments of the dominantly argillaceous Lea Pa rk

Formation (Shandro, Vanesti and Grizzly Bear Tongues) across the Whiteside

production locale. The Ribstone Creek Tongue consists of type-IV and type-III

elements, made up of fine-grained, micaceous sandstone in a coarsening-upward

sequence, incorporating siderite and calcite-cemented, strongly indurated sandstone

in the uppermost 30 cm or so. Electric logs of the unit display the inverted bell

shape, characteristic for upward coarsening. Ribstone Creek sandstones are

typically greenish grey (SGY 6/1) and dark greenish grey (5GY 4/1), variably

glauconitic, micaceous and for the most part friable. The tongue undergoes

progressive thickness decrea se eastwards, from 21 m to 8 m along the line of

section, whereas only minor thickness variation is observed either in the Victoria

and Oldman Tongues of the Belly River Formation or in the Shandro, Vanesti and

Gr izzly Bear Tongues of the Lea Park Formation. The Ribstone Creek Tongue appears

to reflect deposition in a nearshore marine setting during a regressive-transgressive

l Department of Geology, University of Windsor, WINDSOR, Ontario N9B 3P4

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~ 11111 100 !')() (> 100 WO ______ ...,._.

SCAL E IN MIU.S

Fi g. l - Sketch map showing location of study area.

pulse within the framework of the larger-scale Belly River r egress ive-transgressive

event .

The Milk River Forma tion (C ampanian) is made up of bioturbated, muddy siltstones

and fine-grained sands tones ( t ype- III e l ements), which incorporate scarce , fla ttened

lenses of fine- and very f i ne-grained sands tone . The thickness of the uni t in south-

western Saskatchewan i s in the orde r of 131 m. The vertical distribution of cored

sections of the Mi lk River Format i on in s outhwestern Saska tchewan is s hown in Figure 3.

IMPERIAL DE WAR 7 - 13V • 3 1 - 26

LSD.7-1 3 - 3 1- 26W 3 ,< 8:B91i"7?8i:i )

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West-east cross - section from Impe ri a l Dewar 7-13-31-26 well (Lsd 7-13- 31- 26W3) to Phil l ips-Husky Sawde No. l wel l (Lsd 16-1 9- 29-24W3 ), showi ng lithologic vari at ion in Oldman, Ribs t one Creek and Vi ct or ia To ng ues (Bell y Rive r Formati on, Cretaceous ) of Whi t es ide fi el d, west- cen t ra l Saskatchewan.

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194

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- 197 -

The Medicine Hat Sandstone (Fig. 4) comprises up to 13 m of muddy siltstone and

very fine- and fine-grained sandstone in type-II and type-III elements, which form a

dominantly coarsening-upward sequence some 33 m below the top of the Colorado Group

in southwestern Saskatchewan. It too is thought to represent the climax condition

of grade on the western shelf of the Colorado sea (Simpson, 1975). Definition of

the top of the unit is sharp, in that a pebbly sandstone, incorporating reworked,

relict fragments of concretionary siderite, is frequently succeeded by one or more

calcite concretionary layers associated with thin bentonite beds (Kendall and

Simpson, 1974), which in turn are overlain by the calcareous shales and shaly

chalks of the First White-Speckled Shale. The Medicine Hat Sandstone maintains a

fairly constant thickness along the line of section (Fig. 4) before shaling out

between the AEG Buttes Horsham 7-26-17-27 well (Lsd 7-26-17-27 W3) and Fox Valley

No. 13-11 well (Lsd 13-ll-19-26W3).

The Upper Colorado succession (Turonian to Santonian) in the Bowdoin dome

region of northern Montana includes three main sandstone sequences listed in order

of increasing age: The Martin Sandy Zone of the Niobrara Formation, the Bowdoin

Sandstone of the Carlile Formation and the Phillips Sandstone of the Greenhorn

Formation (Campen, 1975). In each case the sandstone "unit" seems to refer to the

total thickness of sandstone occurring within the corresponding, largely argillaceous

formation. The equivalent stratigraphic units in western Saskatchewan are the First

White-Speckled Shale, an unnamed Upper Colorado shale (here termed the Carlile

equivalent), and the Second White-Speckled Shale. Sandstones equivalent to the

Martin Sandy zone are encountered in the Amurex Albercan Odenbach No. 1-10 well

(Lsd 10-23-14-30W3); the uppermost sandstone body of the unit is about 7 m thick and

occurs some 16 m below the top of the First White-Speckled Shale (Fig. 4). The

Martin sandstone bodies are composed of interbedded, muddy siltstones and fine-

grained, micaceous sandstones occurring as type-II elements and are seen for

example at 500 min the SPC Hatton 10-23-14-30 well (Lsd 10-23-126-30W3). Figure 5

illustrates well-log responses to lithologic variation in some 492 m of marine

Cretaceous strata from the top of the Milk River Formation to below the Fish-Scale

Marker. Each unit within the Upper Colorado sequence registers responses indicative

of interbedded sandstone and these were verified by examination of drill cuttings.

The Bowdoin Sandstone does not appear to have been cored in Saskatchewan, but drill

cuttings suggest that it is similar to both the Martin Sandy Zone and Phillips

Sandstone. The latter unit occurs some 6.5 m below the top of the Second White-

Speckled Shale. The Phillips Sandstone consists of alternating layers of fine-

grained sandstone and mudstone, in the order of a few centimetres thick, making

up sandy type-II elements with a tendency towards upward coarsening: type-III

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(bioturbated) sandstones occur at the top of the sequence in some cores. The top

of the Phillips Sandstone is marked by calcite concretionary layers formed beneath

bentonite beds up to 70 cm thick. These lithologic relationships are well displayed

in the POC Noel Supreme 6-20-2-27 well (Lsd 6-20-2-27W3) from 902 m to 919 m. In

conclusion, it is noted that unnamed bioclastic limestones and shaly chalks, widely

interbedded with calcareous shales near the base of the Second White-Speckled Shale,

may also be prospective for natural gas.

Discussion

It is clear from the preceding account that the Montana and Upper Colorado

sequences of southwestern Saskatchewan incorporate numerous potential pay zones and

that possibilities exist for production from two or more of these at any given

location. The existence of strongly cemented sandstones near the tops of different,

coarsening-upward sequences, occupying closely comparable structural settings,

suggests that the traps discovered thus far in marine Cretaceous deposits are mainly

diagenetic (Simpson, 1979). Deeper drilling in areas where production comes from

the Medicine Hat Sandstone might prove worthwhile. The production trend of the

Bowdoin field in northern Montana might be extended across the border into Saskatchewan.

References

Campen, E.B., 1975. Well log analysis in the Cretaceous gas sands of northern Montana: Montana Geol. Soc., Twenty-Second Ann. Publication, p.15-25.

Kendall, A.C., and Simpson, F., 1974. Calcite layers in the uppennost Medicine Hat Sandstone (Upper Cretaceous) of southwestern Saskatchewan: Canadian Bull. Petroleum Geol., v. 22, p. 34-41.

Kent, D.M., and Simpson, F., 1973. Geological road log of the Cypress Hills -Milk River area, southeastern Alberta (Second Edition): Saskatchewan Geol. Soc., Special Publ. No. 1, p. 297-318.

Nichols, R. A.H., and Wyman, J, M., 1959. Interdigitation versus arbitrary cutoff: resolution of an Upper Cretaceous stratigraphic problem, western Saskatchewan: Am. Assoc. Petroleum Geologists Bull., v. 53, p. 1880-1893.

Simpson, F., 1975. Marine lithofacies and biofacies of the Colorado Group (Middle Albian to Santonian) in Saskatchewan: p. 553-588, in Caldwell, W.G.E. (Editor), The Cretaceous System in the Western Interior~ North America, Geol. Assoc. Canada, Special Paper 13, 666 p.

Simpson, F., 1979. Low-permeability gas reservoirs in marine, Cretaceous sandstones of Saskatchewan: 1. Project outline and rationale, in Christopher, J.E., and Macdonald, R. (Editors), Saskatchewan Geol. Survey, Summary of Investigations, 1979.