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Low-Permeability Gas Reservoirs in Marine Cretaceous Sandstones of Saskatchewan: 1. Project Outline and Rationale

by Frank Simpson 1

Marine sandstone and sil tstone bodies, referable to the dominantly a r gillaceous

Colorado Gr oup and overlying Montana Group (Middle Albian to Campanian) of

Saskatchewan, exhibit varying degr ees o f shaliness a s mani festations of l ithol ogi c

gradation with the enclosing muds t ones and s ha l es . The sandstones and siltstones

mark stages in the attainmen t of t he graded condi t ion on t he shelf areas o f an

epeiric sea which occupied an asynunetrically subsiding structural basin to the east

of the Cordilleran mountain belt. A rapidly s ubsiding wes tern s he lf was s upplied

with ab undant , terrigenous debris f rom the ris ing , ancestral Rocky Mount ains and

was separa ted from a relative ly quiescent, eas t ern shelf, receiving compara tively

minor amounts of detri tus f rom the Precambrian Shi e ld, by a basin p r oper, which was

a site of mud deposition.

Producti on of both non-associated and associ ated natural gas , as well as light

crude oil in t he case of sandstone bodies in the l ower part of the Colo rado Group,

has been obtai ned from this sequence i n Saskatchewan and more wide l y in adjacen t

Alber ta and Montana . Howeve r, even though t he marine Cre t aceous s uccession i s

penetra t e d by a high pr oportion o f the petroleum-explor a tion wells drilled in the

province, it i s possible that existing production does no t r efl ect t he f ul l potential

of the sequence for petr ol eum discover y. Because of their l ow permeabilities, the

muddy sandstones and s iltstones of the Col o rado and Montana Groups are s usceptible t o

t he plugging of porosity by drilling muds under pressure, which would tend to preclude

the detecti on of hydrocarbon s howings .

Project Outline

The res earch project, designated Low- Permeability Gas Reservoirs in Marine ,

Cretaceous Sandstones of Saskatchewan, is the basis fo r a pending three-yea r

agreemen t be tween t he Saskatchewan Geological Survey and the Department of Geology,

University of Windsor, Ontario. The projec t forms a continuation of earlier resear ch

by the author on the Lower Color ado str ata of west-central Sas ka t chewan (Simpson,

1975, 1979a , i n press , in prep.) . During t he summer of 1979, some 120 selected

cored sec t ions of ma r i ne Cretaceous s trata were descr ibed in detail and the

distr ibution of t he stratigr aphic units r ecogni zed was determined at a reconnaissance

1 Department of Geo logy , Unive rsi t y of Wi ndsor , WINDSOR, Ontar io N9 B 3P4

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l evel by means of cr oss-sections , based lar gely on geophys ical well data . Some of the

main results a re summarized in other pape rs appe aring in t his volume (Simpson, 19 79b,

1979c; Simpson and O'Connell, 1979). Subsequent investigations within the f ramework

of the proj ec t will make t h e comple tion of de tailed s ubs urface mapping and corr elati on

of l owe r Colorado st rata ac ros s the pr ovince as the i ni tial obj e ctive . This will be

followed by a detailed study of their r eser voir potential i n western Saskat chewan .

Di stribution of Sand s tone Bodies

The stratigraphic position of the Co l orado-Montana succession within the

Mesozoic strata of Saska tchewan and the names of the principal lithostr atigraphic

s ubdivis ions are given in Figure 1.

The main s andstone units r e ferable to the eastern shelf area in west- central

Saskatchewan a re t he Spinney Hil l Sands tone, the Flatten Lake Sand and t he St.

Walburg Sandstone, listed in order of decreasing age and r efer able t o the Early

Cretaceous pa rt of t he Lower Colorado success ion. Thes e un i ts s ubcrop beneath

Quaterna r y deposits in c entral Saskat chewan and ar e unlikely to t ake on any measure

of r ese rvoir s igni ficance , e xcept possibly f or exploita tion of ground water . The

Spinney Hill Sandstone is a possible exception i n that it ex tends far south o f

t he s ub crop and hydrocarbon s how i ngs have been reported from it . In eas tern

Saskatchewan, s andstone bodies of the Viking Formation a ppear to be genetical l y

r e la ted to t he eastern sh e l f , as is the Okla Sands tone , which appear£ to be the

younges t o f Lowe r Colorado sands t ones of east ern (Shi eld) provenance . In addi t ion ,

t he Boyne Sand , a unit of r estric t ed distribution within the calcareous sha l es and

s haly chalks o f the First White-S peckled Shale was l aid down on t he eas t e rn s he l f .

I n wes tern Saska t chewan, sed imentation of sands on t he western shelf i s

r eflec ted in the distal end of a relat i vely coars e- gr ained , r egressive-transgr essive

wedge (Bow Is l and of southern Alberta) , r efer able t o the Viking Formation. The

Viking Forma tion yi elds produc tion of non-associated natural gas and l ight crude oil

in west-cen t r al Sas kat chewan . Sandstone and silts t on e i ntercal cations in the mudstones

and shales f r om the base of t he Fish-Sca l e Marker to t he top of t he First Bow I s land

Sand in the Lower Col or a do s equence of northe rn Montana have been des i gnated the

Spikes zone by Campen ( 1975) . The l ower limi t of the Spikes Sandstone i n Saskatchewan

occurs a t the top of a r elat i vely t hick Viking s uccession, whi ch may be equiva l ent to

the Second and Third Bow I s l and Sands . The Fi s h- Scale Marke r consists of graded

sandstones, made up of fish-skeletal debris , a lternat ing wi t h mudstone l ayers. The

Upper Colorado s uccess ion i s divisible into two main cal car eous marke r units , the

First (Upper) and Second (Lower) White Speckl ed Shales , separated by an unnamed,

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ERA PERIOD EPOCH

u

0 N 0 z w u

u

0 N

0 Cf)

w ::;

U) ::, 0 w u

"' ,-.. w

"' u

Fig. 1 - Stratigraphic correlation chart for Mesozoic strata of the northern Williston basin region.

noncalcareous shale sequence. In southwestern Saskatchewan, each of the three

divisions incorporates monotonous alternations of sandstones and rnudstones termed

the Martin Sandy zone, the Bowdoin Sandstone and the Phillips Sandstone respectively

in order of increasing age; the basal part of the Second White-Speckled Shale also

includes abundant bioclastic limestone and shaly chalk layers interbedded with

bituminous, calcareous shales and mudstones. A prominent reservoir unit in the upper-

most part of the Martin Sandy zone is the Medicine Hat Sandstone which contains

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commercial quantities of non-associated natural gas near the Fourth Meridian. The

Milk River Formation is a monotonous sequence of muddy sandstones and siltstones,

yielding production of non-associated natural gas near the base of the Montana Group

in southwestern Saskatchewan. The marine part of the Montana Group in the area

incorporates a prominent eastward- thinning wedge of relatively coarse- grained, elastic

sediments , referable to the Belly River Formation. A pulsatory, regressive-trans­

gress ive pattern of sedimentation is i ndicated by the presence of several Belly River

tongues; one of these, the Ribstone Creek Tonge yields non- associated natural gas in

west-central Saskatchewan.

Sequence Elements

The gradational relationships between fine- gr a ined lithologies making up the

marine deposits under consider ation present cer tain difficulties with r egard to

their systematic description, which are to some extent alleviated when convent ional

r ock classifications a r e augmented by use of the scheme of sequence elements

introduced by Simpson (1971). Use of this scheme is continued in the present study

and it is s ununarized in Table 1. A particular advantage t o be gained from use of the

scheme lies in its ready application to detailed reservoir studi es , in that it

places emphasi s on abundance and degree of continuity of shale breaks. The sandstone

bodies of the western shelf in gener a l tend to exhibit dominantly coarsening-upward

arrangements of sequence elements; in the Bow Island- Viking succession of southwestern

Saskatchewan, stacking of sandstone bodies give vertical repetition of dominantly

coarsening-upward sequences . Farther eas t, fining-upward a rrangements of sequence

elements become connnon, as well as repetitive , i n the Viking succession.

Towards a Unifying Search Rationale

The genesis of known petroleum accumulations in the marine Cretaceous sand­

stones of Saska tchewan, which appears most likely and has greatest appeal on account

of its simplicity, is by migration from the envel oping shales and mudstones. The

s tackin g of these accumulations at successive l evels throughout the sequence

indicates the s trong likelihood of structural control . Major basement linears,

notably t he Sweetgrass - North Battleford arch, have exerted an important influence,

as have salt-solution structures and compaction-related undulations of correlation

surfaces. This is especially so along palaeotopographic prominences beneath the

Cretaceous succession, such as those defined by the northern limits of both

J urassic and Mississippian str a t a (Christopher et al ., 1971). It is of interest

to note that the cap rocks of many hydrocarbon reservoirs in the succession under

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Table l. Sequence Elements , Based on Gross Lithologic Associations and Layer Properites, i n Cores of Colorado Group in Saskatchewan

SEQUENCE1 ELEMENT

v conglomerati c element

IV

sandstone el ement

I II

muddy sandstone­siltstone element

II

siltstone-sandstone element

mudstone element

(c ) (b) (a)

(e)

(d)

{c)

(b)

(a)

(c) (b) (c)

(d)

(c)

(b)

(a)

(c)

(b)

(a)

SUB-ELEMENT2

pebbly mudstone conglomerate pebbly sandstone

sandstone with dune- scale cross-lamination sandstone with horizontal laminat ion sandstone with trough cross-lamination sandstone with ripple-drift cross-lamination flaser-bedded sandstone

biot urbated sandstone bioturbated muddy sandstone bioturbated muddy siltstone

wavy-bedded, composite layers of sandstone, silt­stone and mudstone wavy-bedded simple layers of sandstone, siltstone and muds tone alternating mudstone and sandstone/siltstone with low-angle, planar cross­lamination lenticular-bedded siltstone and sandstone in mudstone

subordinate siltstone and sandstone in lenses and scarce continuous layers subordinate siltstone and sandstone in flattened lenses structureless mudstone

REMARKS

coarse t r action load and laa concentrates of nodular and broken con­cretionary material

continuous mudstone layers frequently intercalated; best original porosity usually plugged by cement (sider ite, calcite, pyrite)

continuity of mudstone layers mostly disrupted by biogenic reworking

coquinoidal layers of pelecypod and fish-skeletal debris common

frequently incorporate coquinoidal layers, benton­itic mudstones, siderite and calcite concretions, nodular phosphorite

Arbitrary lower limit of thickness set at 1 .0 ft. for convenience in core desc r iption

2 No preferred vertical order of occurrence implied

consideration are not the enclosing argillaceous rocks but are cementation barriers.

These were formed at or near the tops of coarsening-upward sequences by precipitation

of mineral cements (calcite, siderite, pyrite) from formation waters in the sites

of best original porosity and permeability, usually in type-III and type IV elements.

In a given reservoir, the density differences existing between formation fluids

suggests that the cementation barriers were formed relatively early in diagenesis in

the absence of a hydrocarbon column. However, microscopic bodies occurring

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sporadically around tightly cemented sand grains and interpreted as mineralized

hydrocarbon globules, indicate some early expulsion from the source rocks (Simpson,

in prep.). It is suggested that retention of the hydrocarbon accumulations after

Laramide tilting of the region is due in large measure to the existence of these

cementation barriers which augment the b a sinward decrease in grain size of the

reservoir units. Thus the traps are regarded as diagenetic, in the meaning of

Wilson (1977).

Strongly indurated sandstones were observed in the uppermost parts of coarsening­

upward sequences referable to the Viking Formation of southwestern Saskatchewan and

in the lowermost parts of fining-upward sequences belonging to the same unit farther

east. In the latter instance, the strongly indurated sandstones might serve as

sea t rocks for the single, fining-upward unit in which they are found but might also

be regarded as possible cap rocks for the sandstones immediately below in multistory

arra ngements of sandstone bodies. Thus the multistory Viking sandstones of

southern Saskatchewan merit particular attention as hydrocarbon prospects. Further­

more, since tidal-channel activity appears to have been important in disperal of

the eastern Viking sandstones, prospects may be localized low on the flanks of

major arch forms or in linear features of negative relief. The study of cored

sections o f Viking sandstones from southern Saskatchewan revealed the widespread

distribution of kaolinitic lithologies in contrast to the relatively low kaolinite

content of sandstones along the distal (producing) edge of the sequence in west­

central Saskatchewan. This may reflect a difference in diagenetic processes

obtained at up-dip locations inside the hydrocarbon column, as compared with those

where the rocks were wetted only by formation waters. It follows that for a given

Viking sandstone body, up-dip occurrences of non-kaolinitized sandstone are likely

to be most prospective for hydrocarbons.

Acknowledgements

The cost of these studies are in part defrayed by operating grant No. A9174 from

the Natural Sciences and Engineering Research Council of Canada.

Computer printouts of well data, supplied by D. F. Paterson and J. V. Buller,

permi t t ed optimum use of the time spent in Regina. Special thanks go to C. A.

Balster of Burlington Northern in Billings for discussing problems connected with

the Upper Colorado sandstones of northern Montana; and to N. C. Meijer-Drees and

L. L. Price of the Geological Survey of Canada in Calgary for helpful discussions

concerning the Milk River Formation and Lower Colorado succession respectively.

References

Campen, E.B., 1975. Montana:

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

Christopher, J.E., Kent, D.M., and Simpson, F., 1971 Hydrocarbon potential of Saskatchewan: Saskatchewan Dept. Mineral Resources, Rept. No. 151, 47 p.

Simpson, F., 1971. Sequence elements in stratigraphic analysis of Lower Colorado Cretaceous) sediments, west-central Saskatchewan (Abstract). Am. Assoc. Petroleum Geologists, Bull., v. 55, pg. 541-542.

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 of North America, Geol. Assoc. Canada, Special Paper 13, 666p.

Simpson, F., 1979a. Evolution of a graded Cretaceous Shelf, p. 423-434, in Podwysocki, M.H., and Earle, J.L. (Editors), Proc. Second Inter;;t. Conference on Basement Tectonics, Newark, De, July 13-17, 1976, 575 p.

Simpson, F., 1979b. Low-permeability gas reservoirs in marine Cretaceous sandstones of Saskatchewan: 3. Lower Colorado (middle Albian to Cenomanian) strata of east-central Saskatchewan: in Christopher, J.E., and Macdonald, R. (Editors), Saskatchewan Geol. Survey, Sununary of Investigations, 1979.

Simpson, F., 1979c. Low-permeability gas reservoirs in marine Cretaceous sandstones of Saskatchewan: 4. Upper Colorado and Montana (Turonian to Campanian) strata of western Saskatchewan: in Christopher, J.E., and Macdonald, R. (Editors),Saskatchewan Geol. Survey, Summary of Investigation, 1979.

Simpson, F., in press. Lithologic descriptions of selected, cored sections from Lower Colorado (Cretaceous) strata of west-central Saskatchewan. Saskatchewan Dept. Mineral Resources, Rept. no. 160.

Simpson, F., in prep. Sedimentology, palaeoecology and economic ecology of Lower Colorado (Cretaceous) strata, west-central Saskatchewan. Saskatchewan Dept. Mineral Resources, Rept. no. 150.

Simpson, F., and O'Connell, S., 1979. Low-permeability gas reservoirs in marine, Cretaceous sandstones of Saskatchewan: 2. Lower Colorado (middle Albian to Cenomanian) strata of southern Saskatchewan: in Christopher, J.E., and Macdonald, R. (Editors), Saskatchewan Geol. Survey, Summary of Investig­ations 1979.

Wilson, H.H., 1977. "Frozen-in" hydrocarbon accumulations or diagenetic traps­exploration targets. Am. Assoc. Petroleum Geologists, Bull., v.61, p. 483-491.