low-permeability gas reservoirs in marine cretaceous ... · the main sandstone units referable to...
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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.