Medium and Coarsely Crystalline Dolomites in the Middle

8/17/2019 Medium and Coarsely Crystalline Dolomites in the Middle

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O R I G I N A L A R T I C L E

Medium and coarsely crystalline dolomites in the MiddleDevonian Ratner Formation, southern Saskatchewan, Canada:

origin and pore evolution

Qilong Fu • Hairuo Qing

Accepted: 2 December 2010

Springer-Verlag 2010

Abstract Carbonate rocks of the Middle Devonian

Ratner Formation in southern Saskatchewan vary fromundolomitized, to partly dolomitized, to completely

dolomitized. Two major types of medium and coarsely

crystalline dolomite are differentiated in the formation:

medium crystalline, planar-e to planar-s (ME) dolomite,

and medium to coarsely crystalline, nonplanar-a (CA)

dolomite. ME dolomite, which commonly occurs in the

lower and middle parts of the Ratner Formation, has more

radiogenic Sr (87Sr/ 86Sr ratios averaging 0.7085) and sig-

nificantly higher Fe and Mn concentrations (averaging

2,260 and 160 ppm, respectively) than do associated

limestone. ME dolomite is interpreted as resulting from

dolomitization by ascending basin fluids in a burial envi-

ronment during late Devonian and Carboniferous time. ME

dolomite has relatively high porosity (up to 19.3% based on

point counting); intercrystalline pores are dominant, and

vugs are locally abundant. The improved petrophysical

quality of ME dolomite is due to burial dolomitization

processes and the ability of dolomite to resist compaction

and retain porosity. In contrast, CA dolomite, present

mainly in the top parts (locally below the top parts) of the

formation, has 87Sr/ 86Sr ratios (averaging 0.7081) and Fe

and Mn concentrations (averaging 250 and 104 ppm,

respectively) similar to adjacent micro-finely crystallinedolomite. CA dolomite is considered to have formed by

recrystallization of early-formed microcrystalline dolomite.

Recrystallizing fluids probably consist of gypsum dehy-

dration water mixed with formation waters. CA dolomite

has less than 2% vuggy porosity and rare visible inter-

crystalline pores.

Keywords Dolomite Petrography Geochemistry

Origin Porosity Ratner Formation

Introduction

In the last dozen years, the Middle Devonian Ratner For-

mation (commonly referred to as the ‘‘Ratner Laminites’’)

in southern Saskatchewan has attracted increasing attention

because of discovery of oil in the formation. Commercial

oil has been produced from the formation since 1997

(Kissling and Slingsby 1999), and dolomites are the

primary reservoirs (Nimegeers 2005; Nickel 2008). The

Ratner limestone is fairly tight, but dolomites are relatively

porous, with porosity as high as 28% (Kissling and

Slingsby 1999); dolomitization is therefore considered to

be a crucial diagenetic process in porosity enhancement.

Understanding the origin of dolomites with respect to pore

evolution is important for hydrocarbon exploration and

reservoir development. A recent study by Fu et al. (2006)

addressed Ratner micro-finely crystalline dolomite, which

was interpreted to have formed in near-surface evaporitic

environments. This study integrates core logging, thin section

examination, and geochemical analyses to investigate the

formation and pore evolution of Ratner medium and coarsely

crystalline dolomites in southern Saskatchewan.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s13146-010-0038-x ) contains supplementarymaterial, which is available to authorized users.

Q. Fu (&)

Bureau of Economic Geology, Jackson School of Geosciences,

The University of Texas at Austin, Austin, TX 78713-8924, USA

e-mail: [email protected]

H. Qing

Department of Geology, University of Regina,

Regina SK S4S 0A2, Canada

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Carbonates Evaporites

DOI 10.1007/s13146-010-0038-x

http://dx.doi.org/10.1007/s13146-010-0038-xhttp://dx.doi.org/10.1007/s13146-010-0038-x


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Geological setting

The Middle Devonian Ratner Formation occurs in basinal

areas of the Saskatchewan subbasin, the southern part of

the Elk Point Basin (Fig. 1). The Ratner Formation overlies

the Brightholme Member, the Lower Winnipegosis Mem-

ber (where the Brightholme Member is absent), or the

relatively low buildups (\30 m in height) of the Winni-pegosis Formation, and is commonly overlain by the

Whitkow Member of the Prairie Evaporite Formation

(Fig. 2; Jin and Bergman 2001). The Whitkow Member

consists of a lower anhydrite unit (up to 60 m in thickness)

and an upper halite unit. The Ratner Formation was ini-

tially defined as a basinal facies of the Upper Winnipegosis

Member (Jones 1965). Reinson and Wardlaw (1972)

redefined the Upper Winnipegosis Member to include only

carbonate buildups and named carbonate laminites and

associated anhydrite as the Ratner Member. Recently, the

Ratner Member has been raised to formation status (Jin and

Bergman 2001).

The Ratner Formation that occurs in areas adjacent to

carbonate buildups (especially between closely spaced

buildups) is relatively thick. Generally, the farther away

from the buildups, the thinner the formation becomes. In

the northern part of the study area where the Winnipegosis

mud mounds are well-developed (Fig. 1), the Ratner For-

mation varies from 4 to 17 m in thickness. Wardlaw and

Reinson (1971) identified eight laterally persistent units,and Jin and Bergman (1999) documented three brining-

upward cycles within the formation. A typical cycle begins

with finely laminated carbonate mudstone, which grades

upward into finely interlaminated or interbedded carbonate

mudstone and anhydrite overlain by contorted or entero-

lithic bedded anhydrite. The carbonate laminae vary from

0.3 to 5 mm in thickness. Three types of lithofacies dis-

tribution are observed in the cored intervals of the Ratner

Formation: (1) carbonate rocks in individual cored inter-

vals are composed of pure microcrystalline to finely crys-

talline limestone, and finely to very coarsely crystalline

limestone may be present in the top part of the Ratner; (2)

Fig. 1 Sketch map of the

Middle Devonian Elk Point

Basin in southern Saskatchewan

(modified from Fu et al. 2006

with permission from the

Geological Society). An inset in

the upper right corner shows

paleogeography of the Middle

Devonian Elk Point Basin in

western Canada and the location

of the study area. The study area

is divided into three parts:northern, central, and southern.

Isopach map of the Ratner plus

Winnipegosis deposits exported

from Accumap (2002 version).

Contour interval = 10 m. ‘‘T ’’

is ‘‘Township’’, and ‘‘ R’’ is

‘‘Range’’. For example,

T40 = Township 40,

R1W2 = Range 1 west of the

Second Meridian. Additional

information (including well list

examined in this study and

detailed explanation of

‘‘Township–Range’’) is given in

Online Resource 1


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individual cored intervals are exclusively dolomite (in

addition to anhydrite), mostly microcrystalline to finely

(micro-finely) crystalline dolomite, but medium to coarsely

crystalline, planar-a dolomite (CA dolomite) may occur at

the top of the Ratner or be associated with anhydrite beds;

(3) the upper and middle cycles of the formation consist

of limestone and anydrite, and the lower cycle contains

medium crystalline, planar-e to planar-s dolomite (ME

dolomite).

In the central study area, the Ratner Formation is thin

(commonly\4 m in thickness) and consists of laminated

or vaguely laminated, microcrystalline dolomudstone.

No depositional cycles are observed. The thickness of

carbonate laminae is commonly less than 1.5 mm. In the

southern study area (Fig. 1), where Winnipegosis pinnacle

reefs are abundant, the Ratner Formation varies from 3 to

20 m in thickness and is commonly composed of vaguely

to wavily laminated carbonate mudstone, with anhydrite

nodules increasing toward the top of the formation.

Depositional cycles are commonly absent or indistinct. The

upper part of the formation is commonly replaced by

micro-finely crystalline dolomite (Fu et al. 2006). The

lower (or base) part is preserved as lime-mudstone in

places. Locally, however, medium crystalline, planar-e to

planar-s dolomite (ME dolomite) occurs in limestone

intervals.

Skeletal fragments and nonskeletal grains are extre-

mely rare in the Ratner lime-mudstone. Bioturbation

features were not observed. Laminites may be separated

by very thin organic-rich partings. Anhydrite laths,

needles, and aggregates are present in lime-mudstone.

Calcite crystals in microcrystalline lime-mudstone may

show slight variation in size (from less than 4 lm to as

much as 20 lm).

The Ratner Formation was initially interpreted to have

been deposited as algal mats in shallow subaqueous to

intertidal environments, analogous to the modern TrucialCoast, Persian Gulf (Shearman and Fuller 1969). Other

researchers, however, argued that Ratner deposits precipi-

tated in a quiet, relatively deep water body with anoxic

bottom conditions (Davies and Ludlam 1973; Jin and

Bergman 1999; Wardlaw and Reinson 1971). This study

supports the latter interpretation on the basis of the rare

presence of skeletal fragments and relative lateral con-

stancy of laminations in Ratner deposits.

Methods

Cored intervals of the Ratner Formation about 285 m long

from 42 wells were examined and sampled, and 74 stan-

dard thin sections were prepared (Online Resource 1). One-

half of each thin section was stained with a mixture of

alizarin red-S and potassium ferricyanide to differentiate

between dolomite and calcite, and between ferroan car-

bonates and non-ferroan carbonates (cf. Dickson 1966).

Visible porosity in thin section is measured by point

counting the visible pores to calculate pore space, and the

thin section is taken as the bulk volume.

All isotopic measurements were performed at the

University of Saskatchewan. Powder samples for C, O, and

Sr isotope analyses were obtained from core slabs using a

Mastercraft drill. Carbon and oxygen isotope values were

measured using a Finnigan Kiel-III carbonate preparation

device directly coupled to the inlet of a Finnigan MAT 253

isotope ratio mass spectrometer. Isotope ratios were

reported in per mil notation relative to the VPDB (Vienna

Pee Dee Belemnite) standard. The 87Sr/ 86Sr measurements

of carbonate powders were performed on a Finnigan MAT-

261 instrument using a multi-dynamic peak-hopping rou-

tine, and 88Sr/ 86Sr of 8.375209 was used to correct for

instrumental mass fractionation.

Elemental analysis of carbonate samples was done using

a Perkin Elmer Optima 3300-DV ICP-Atomic Emission

Spectrometer housed at the University of Western Ontario.

Powdered samples (0.10 g) were digested by 4 ml of aqua

regia mixed acid in a steam bath for 2 h. After being cooled

to room temperature, the solution was diluted to 50 ml.

Multi-element, high-purity solution standards were used

for calibration. The analytical accuracy for Ca2?, Mg2?,

Fe2?, Mn2?, Sr?, and Na? was 0.5, 0.8, 1.1, 1.1, 1.4, and

0.9%, respectively.

n o i t a m r o

F e t i r o p a v

E e i r i a r

P

Ashern Formation

Lower Winnipegosis Member

n o i t a m r o F s i s o g e p i n n i W

r e

b m e

M s i s o g e p i n n i W r e p p

U ( m o u n

d s

) Ratner Formation

BrightholmeMember U

p p e r

W i n

n i p

e g o s

i s

n a i n o v e D e l d d i M

Whitkow Member

Shell Lake Member

Leofnard Member

Fig. 2 Stratigraphic nomenclature of the Winnipegosis, Ratner, and

Prairie Evaporite formations in southern Saskatchewan (modified

from Wardlaw and Reinson 1971; Jin and Bergman 2001)


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Dolomite petrography and distribution

In the study area, Ratner carbonate rocks vary from lime-

stone to dolomite. In the about 285-m length of Ratner

cores examined in this study, 87 m of cores is composed of

dolomites. Dolomite phases are estimated to account for

approximately two-thirds of the Ratner carbonates volu-

metrically. Three major types of dolomite are identified onthe basis of petrography and distribution: (1) microcrys-

talline to finely (micro-finely) crystalline dolomite (Fu

et al. 2006); (2) medium crystalline, planar-e to planar-s

dolomite (hereafter referred to as Ratner ME dolomite or

ME dolomite); and (3) medium to coarsely crystalline,

nonplanar-a dolomite (hereafter referred to as Ratner CA

dolomite or CA dolomite). This study is focused on the

latter two types of dolomite, which account for about one-

fifth of all dolomite phases.

Petrography

In core slabs, ME dolomite of the Ratner Formation occurs

as dolomite patches or thin layers in limestone (Fig. 3a, b)

or as relatively thick dolomite intervals (Fig. 3c, d). Vugs

may be abundant and are commonly concentrated along

laminations, forming channels (Fig. 3d). In thin section,

ME dolomite displays a planar-e to planar-s texture (cf.

Sibley and Gregg 1987; Fig. 4a–c) and is commonly fabric

destructive. In partially dolomitized areas, newly formed

dolomite crystals in limestone matrix show a range in

crystal sizes, from particles with diameters equivalent to

adjacent microcrystalline calcite up to 300 lm across

(Fig. 4a). Dolomite crystals locally display cloudy cores

and relatively clear rims and are more lipid than those in

the completely dolomitized areas. Some dolomite crystals

appear to be rounded off (Fig. 4b). Remnant calcite crys-

tals vary from very finely crystalline to medium crystallinein size. In completely dolomitized carbonates, dolomite

crystal sizes are relatively uniform, varying from about

80–250 lm (Fig. 4c).

In thin section, Ratner CA dolomite shows nonplanar-a

fabric (Fig. 4d) and varies from about 50–500 lm in

crystal size. Crystals of this type of dolomite generally

have compromised (curved, serrated or irregular) bound-

aries, display slightly undulatory extinction, and show a

turbid appearance because of abundant inclusions (Fig. 4d).

In places, interlocked dolomite crystals are closely packed

and may have poorly defined boundaries.

Stylolites are present in both ME and CA dolomites.Where Ratner limestone is partly replaced by ME dolo-

mite, dolomite crystals may concentrate along or overgrow

stylolites or be restricted at one side of the stylolites. In CA

dolomite, many stylolites are blurred and smoothly undu-

latory (nonsutured) with low amplitude, and seem to be

stylolite relicts. However, crystals of CA dolomite are also

observed to be truncated by later generation of sutured

stylolites with relatively high amplitude.

Fig. 3 Photographs of core slabs showing medium crystalline, planar-e

to planar-s dolomite (ME dolomite) in the Ratner Formation. Each large

division on the scale equals 1 cm. a Ratner ME dolomite occurs as

patches in the Ratner laminated limestone. Location: well 01-15-048-

17W2; depth: 516.2 m. b ME dolomite occurs within laminated

limestone. Relative abundant vugs (hollow arrow) are observed in

dolomite, but not inlimestone. A solution seam (ss) is present. Location:

well 01-15-048-17W2; depth: 516.6 m. c Massive ME dolomite with

ghost of precursor wavy laminations. Generally, vugs are concentrated

along precursor laminations. Location: 12-27-004-09W2; depth:

2,440.8 m. d ME dolomite with abundant vugs and channels occurring

along laminations. Location: well 01-15-048-17W2; depth: 518.7 m


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Distribution

Ratner ME dolomite is only observed in cores that contain

limestone, and its occurrence and abundance are highly

variable over the study area, varying from several centi-

meters to a few meters in thickness. In the northern part of

the study area, ME dolomite is commonly restricted to the

lower cycle of the Ratner Formation (e.g., wells 01-15-048-

17W2 and 02-15-032-01W3). In the southern study area,

ME dolomite commonly occurs in the lower and middle

intervals of examined cores, or locally occurs in the upper

parts of the formation (e.g., well 12-27-004-09W2). In one

core from well 10-33-006-13W2, four thin intervals(varying from 0.2 to 1.4 m in thickness) of ME dolomite

are observed to be interlayered with limestone. ME dolo-

mite has not been observed in the central study area.

Generally, abundance of ME dolomite decreases upward.

In some individual cores, Ratner carbonates vary from pure

ME dolomite (calcite\1% in volume) to dolomitic lime-

stone (partly replaced by ME dolomite) and grade upward

into pure limestone (e.g., well 01-15-048-17W2). How-

ever, relatively abrupt contacts between ME dolomite and

Ratner limestone are also observed. ME dolomite displayslimited lateral continuity and may change to limestone or

micro-finely crystalline dolomite in adjacent wells less than

4 km apart. Regionally, ME dolomite is less abundant in

the Ratner Formation than in the underlying Brightholme

and Lower Winnipegosis members of the Winnipegosis

Formation.

Ratner CA dolomite is generally present in the top part

of the Ratner Formation that was replaced by early-formed

micro-finely crystalline dolomite and overlain by Whitkow

anhydrite (e.g., well 01-29-032-01W3). Locally, CA

dolomite is observed in the middle to lower part of the

formation and may be associated with Ratner anhydritebeds. Contacts between CA dolomite and micro-finely

crystalline dolomite are commonly gradual.

Implications

Medium to coarsely crystalline dolomites commonly form

at elevated temperatures during burial and/or result from

recrystallization of precursor finer dolomites in burial

environments or near-surface meteoric to mixed meteoric-

Fig. 4 Photomicrographs showing textures of Ratner ME and CA

dolomites. a Ratner ME dolomite has abundant remnant calcite

crystals (stained by red color). The remnant calcite accounts for about

30% of the volume. Visible intercrystalline pores are very rare.

Location: 10-33-006-13W2; depth: 2,312.7 m. b ME dolomite with

sparse calcite crystals (cl), which is interpreted as calcite remnants

after dolomite replacement. Visible intercrystalline pores (ip) account

for about 7%. Note that some dolomite crystals are rounded off

(hollow arrow). Location: well 10-33-006-13W2; depth: 2,313.8 m.

c ME dolomite shows planar-e texture. Visible intercrystalline pores

(ip) account for about 10%. Location: 12-27-004-09W2; depth:

2,440.8 m. d Medium crystalline, nonplanar-a dolomite (CA dolo-

mite). Visible pores are very rare. Location: 01-29-032-01W3; depth:

1,355.5 m


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marine environments (Gregg and Sibley 1984; Gregg and

Shelton 1990; Kupecz and Land 1991). Evidence of

meteoric diagenesis is absent in the Ratner Formation. ME

dolomite is interpreted as having formed by direct

replacement of limestone and probably has not been

significantly recrystallized with respect to petrography.

Two lines of evidence support this interpretation: (1) ME

dolomite may gradually change into limestone, and abun-dant remnant calcite is locally present in ME dolomite

(Fig. 4a, b), and (2) ME dolomite crystals show relatively

limpid, euhedral rhombs and uniform crystal size and

extinction, which is inconsistent with recrystallized dolo-

mite (cf. Mazzullo 1992).

Ratner CA dolomite commonly grades into micro-finely

crystalline dolomite, has no contacts with limestone, and

contains no calcite relicts. In addition, CA dolomite dis-

plays a turbid appearance because of abundant inclusions,

patchy distribution of crystal size, nonplanar-a texture, and

highly irregular or weakly defined intercrystalline bound-

aries (Fig. 4d), all of which are in accordance withrecrystallized products of precursor dolomite (cf. Mazzullo

1992). Therefore, CA dolomite is interpreted to result from

crystallization of precursor microcrystalline dolomite that

might have formed in near-surface, evaporitic environ-

ments. Dolomites formed close to the surface and from

evaporitic brines tend to recrystallize with time during

burial (Coniglio et al. 2003; Machel 2004).

Probably, two stages of stylolites are present in the

Ratner Formation (Fu et al. 2008). Early-stage stylolites

have relatively low amplitude and likely occurred in late

Paleozoic time. Late-stage stylolites display relatively high

amplitude, and they are interpreted to occur coevally with

the late Cretaceous–Eocene Laramide Orogeny. In places,

ME dolomite crystals are restricted to one side of, or

concentrate along, low-amplitude stylolites, suggesting that

ME dolomite occurred after or concurrently with early-

stage stylolitization.

Dolomite geochemistry

Isotopic results

Ratner limestone (lime-mudstone) has d13C values between

?1.2 and ?3.3% VPDB, with an average of ?2.3%; d18O

values between -3.6 and -7.8% VPDB, with an average

of -6.1%; and 87Sr/ 86Sr ratio averaging 0.7079 (Table 1).

The d13C values of Ratner micro-finely crystalline dolo-

mite show a variation from ?1.6 to ?3.7% VPDB (aver-

aging ?2.8%), d18O values lie in a narrow range between

-5.1 and -6.4% VPDB (averaging -5.7%), and the

average 87Sr/ 86Sr ratio is 0.7080.

Ratner ME dolomite has d13C values between ?0.7 and

?3.3% VPDB, with an average of ?1.9%; d18O values

between -5.3 and -8.8% VPDB, with an average of

?7.0%; and 87Sr/ 86Sr ratios range between 0.7083 and

0.7086, with an average of 0.7085, showing enrichment of

radiogenic Sr, compared with that of associated limestone

(Table 1; Fig. 5). Ratner CA dolomite has d13C values

ranging from 0.8 to 3.6% VPDB (averaging ?1.9%), d18Ovalues ranging from -5.2 to -7.7% VPDB (averaging

-6.8%), and 87Sr/ 86Sr ratios ranging from 0.7080 to

0.7081 (averaging 0.7081).

Interpretation of isotopic results

The d13C values of Ratner ME dolomite (?0.7 * ?3.3%)

and CA dolomite (?0.8 * ?3.6%) mostly overlap the

d13C values of Ratner limestone (?1.2 * ?3.3%) and

micro-finely crystalline dolomite (?1.6 * ?3.7%)

(Fig. 5a). The narrow range and low positive values of

d13C of Ratner ME dolomite and CA dolomite suggest thatorganic matter was not significantly involved during dia-

genesis and that influence of meteoric water on diagenetic

alteration was minimal (cf. Allan and Wiggins 1993; Qing

and Mountjoy 1989; Veizer 1983). The d18O values of ME

dolomite (-5.3 * -8.8%) are significantly more negative

than postulated d18O values (about -1 * -4% VPDB) of

Middle Devonian marine dolomite (cf. Fritz 1971; Qing

1998; Veizer et al. 1999), suggesting that ME dolomite

might have formed at elevated temperatures during burial

and/or that diagenetic fluids might be more depleted in 18O

than is Middle Devonian seawater. The d18O values of CA

dolomite (-5.2 * -7.7%) are comparable to those of

micro-finely crystalline dolomite (-5.1 * -6.4%) that

were interpreted to result from recrystallization at elevated

temperatures (Fu et al. 2006).

Sr isotopic compositions of carbonate rocks, unlike

stable C and O isotopes, are not fractionated by pressure,

temperature, or microbial processes and directly record

isotope compositions of parent fluids (Banner 1995; Faure

and Powell 1972). Ratner ME dolomite has 87Sr/ 86Sr ratios

(averaging 0.7085) significantly higher than those of

associated limestone (Fig. 5b), suggesting that ME dolo-

mite probably formed from basinal fluids enriched in

radiogenic Sr isotope (cf. Chaudhuri et al. 1987; Stueber

et al. 1984). This suggestion is consistent with petrographic

evidence that constrains the formation of ME dolomite in a

burial environment. Ratner CA dolomite has 87Sr/ 86Sr

ratios (averaging 0.7081) similar to those of Ratner micro-

finely crystalline dolomite and Middle Devonian seawater

(ranging between 0.7078 and 0.7082, cf. Veizer et al.

1999), suggesting that either limited external fluid was

involved during the formation of CA dolomite or the


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T a b l e 1

C a r b o n , o x y g e n a n d s t r o n

d i u m i s o t o p e c o m p o s i t i o n s f o r t h e R a t n e r

c a r b o n a t e s

S a m p l e n u m b e r

W e l l l o c a t i o n

D e p t h ( m )

L i t h o l o g y

d 1 3 C

( % , V P D B )

d 1 8 O ( % , V P D B )

8 7 S r / 8 6 S r

( ± s t a n d a r d e r r o r )

T W A 0 6 a a

1 6 - 1 1 - 0 3 3 - 0 1 W 3

- 1 2 6 5 . 0

M i c r

o c r y s t a l l i n e l i m e s t o n e

2 . 4 2

- 3 . 5 7

0 . 7 0 7 8 6 3 ±

0 3

P M

7

1 2 - 2 7 - 0 0 4 - 0 9 W 2

- 2 4 4 5 . 6

M i c r


2 . 1 9

- 7 . 8 3

0 . 7 0 7 8 8 2 ±

0 7

C S 0 9 a

0 1 - 1 5 - 0 4 8 - 1 7 W 2

- 5 1 2 . 3

M i c r


2 . 1 8

- 6 . 2 9

0 . 7 0 7 9 0 8 ±

0 7

W r e 0 3 b a

0 7 - 0 2 - 0 3 8 - 0 1 W 3

- 1 1 7 5 . 6

M i c r


2 . 7 5

- 6 . 6 4

W r e 0 6 d a

0 7 - 0 2 - 0 3 8 - 0 1 W 3

- 1 1 8 0 . 0

M i c r


1 . 1 5

- 5 . 4 0

W R 0 2

0 6 - 1 6 - 0 3 8 - 0 1 W 3

- 1 1 6 5 . 3

M i c r


3 . 2 9

- 6 . 1 7

W R 0 5

0 6 - 1 6 - 0 3 8 - 0 1 W 3

- 1 1 6 8 . 0

M i c r


1 . 6 2

- 6 . 3 4

W R 0 8

0 6 - 1 6 - 0 3 8 - 0 1 W 3

- 1 1 7 2 . 5

M i c r


1 . 7 7

- 6 . 4 6

C G 2

1 3 - 0 3 - 0 4 2 - 2 6 W 2

- 9 8 6 . 2

M i c r


3 . 0 5

- 6 . 2 5

D S 0 2 a

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 3 4 . 3

M i c r

o - fi n e l y c r y s t a l l i n e d o l o m i t e

1 . 7 2

- 5 . 3 7

0 . 7 0 7 9 2 7 ±

0 3

W T 0 2 a

1 6 - 3 6 - 0 3 6 - 2 8 W 2

- 1 1 6 4 . 5

M i c r


3 . 6 1

- 6 . 2 7

0 . 7 0 7 9 3 7 ±

0 8

W T 1 0 a

1 6 - 3 6 - 0 3 6 - 2 8 W 2

- 1 1 6 9 . 2

M i c r


3 . 5 1

- 5 . 1 0

0 . 7 0 7 9 5 9 ±

0 8

S M

5 a

0 9 - 3 1 - 0 0 4 - 0 8 W 2

- 2 4 1 1 . 5

M i c r


2 . 8 7

- 5 . 6 4

0 . 7 0 8 1 4 2 ±

0 8

D S 0 1 a a

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 3 2 . 7

M i c r


2 . 4 7

- 6 . 2 6

W T 0 3

1 6 - 3 6 - 0 3 6 - 2 8 W 2

- 1 1 6 4 . 9

M i c r


3 . 7 4

- 5 . 8 7

W T 1 3 a

1 6 - 3 6 - 0 3 6 - 2 8 W 2

- 1 1 7 1 . 7

M i c r


1 . 6 4

- 5 . 0 9

S M

2

0 9 - 3 1 - 0 0 4 - 0 8 W 2

- 2 4 0 5 . 5

M i c r


3 . 2 5

- 5 . 7 4

S M

6

0 9 - 3 1 - 0 0 4 - 0 8 W 2

- 2 4 1 3 . 7

M i c r


2 . 8 9

- 6 . 4 1

S M

8

0 9 - 3 1 - 0 0 4 - 0 8 W 2

- 2 4 1 9 . 5

M i c r


2 . 1 9

- 5 . 6 4

C S 0 9

0 1 - 1 5 - 0 4 8 - 1 7 W 2

- 5 1 2 . 3

M E d o l o m i t e

3 . 3 2

- 7 . 0 6

0 . 7 0 8 6 2 1 ±

0 7

T W A 0 4 a

1 6 - 1 1 - 0 3 3 - 0 1 W 3

- 1 2 6 3 . 0

M E d o l o m i t e

2 . 6 3

- 7 . 4 1

0 . 7 0 8 3 1 9 ±

0 3

P M 5

1 2 - 2 7 - 0 0 4 - 0 9 W 2

- 2 4 4 0 . 9

M E d o l o m i t e

2 . 7 7

- 6 . 5 2

0 . 7 0 8 4 6 5 ±

1 0

C S R 8 ( 1 )

1 0 - 3 3 - 0 0 6 - 1 3 W 2

- 2 3 1 4 . 0

M E d o l o m i t e

1 . 2 9

- 6 . 1 9

0 . 7 0 8 5 0 2 ±

0 9

C S R 8 ( 2 )

1 0 - 3 3 - 0 0 6 - 1 3 W 2

- 2 3 1 4 . 0

M E d o l o m i t e

1 . 3 3

- 6 . 0 8

C S R 7

1 0 - 3 3 - 0 0 6 - 1 3 W 2

- 2 3 1 3 . 0

M E d o l o m i t e

1 . 8 9

- 6 . 4 3

C S R 6

1 0 - 3 3 - 0 0 6 - 1 3 W 2

- 2 3 1 3 . 2

M E d o l o m i t e

1 . 7 7

- 6 . 7 4

C S 1 3

0 1 - 1 5 - 0 4 8 - 1 7 W 2

- 5 1 6 . 6

M E d o l o m i t e

1 . 5 3

- 8 . 7 6

C S 1 6

0 1 - 1 5 - 0 4 8 - 1 7 W 2

- 5 1 7 . 4

M E d o l o m i t e

1 . 7 6

- 7 . 8 9

C S 1 8

0 1 - 1 5 - 0 4 8 - 1 7 W 2

- 5 1 8 . 6

M E d o l o m i t e

1 . 9 9

- 7 . 9 9

C S 1 9

0 1 - 1 5 - 0 4 8 - 1 7 W 2

- 5 1 9 . 1

M E d o l o m i t e

2 . 1 0

- 8 . 1 8

C K F 1

0 8 - 0 2 - 0 3 4 - 0 6 W 2

- 8 1 7 . 0

M E d o l o m i t e

0 . 7 0

- 5 . 2 9

D S 0 4 ( 1 )

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 3 6 . 7

C A d o l o m i t e

1 . 4 3

- 7 . 6 6

0 . 7 0 8 0 8 6 ±

1 0

D S 0 5

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 3 9 . 0

C A d o l o m i t e

1 . 4 3

- 7 . 5 3

0 . 7 0 8 1 0 3 ±

0 8

D S 0 6

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 4 0 . 5

C A d o l o m i t e

1 . 6 0

- 7 . 1 9

0 . 7 0 8 1 0 7 ±

1 8

D S 0 7 a

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 4 1 . 1

C A d o l o m i t e

0 . 8 0

- 7 . 4 8

0 . 7 0 8 0 4 3 ±

1 1


1 3


8/15

diagenetic fluid had 87Sr/ 86Sr ratios similar to those of Middle Devonian seawater.

Elemental results

Ratner ME dolomite varies from nearly stoichiometric to

slightly calcian (CaCO3 mol% values varying from 50.6 to

54.6, averaging Ca52.3M g47.7CO3), and CA dolomite has

CaCO3 mol% values varying from 50.8 to 52.6 (averaging

Ca51.4Mg48.6CO3). Strontium concentrations of ME and

CA dolomites vary from 31 to 56 (averaging 39 ppm) and

from 41 to 84 ppm (averaging 58), respectively (Table 2).

Na concentrations of ME and CA dolomites vary from 240

to 772 ppm (averaging 434) and from 690 to 1,020 ppm

(averaging 861), respectively. ME dolomite shows dra-

matic variation in Fe concentrations, ranging between 746

and 3,125 ppm, with an average of 2,260; and has Mn

concentrations ranging between 120 and 217 ppm, with an

average of 160. Fe and Mn concentrations of CA dolomite

vary from 161 to 286 ppm (averaging 250) and from 96 to

118 ppm (averaging 104), respectively (Table 2). Mn con-

centrations are positively correlated to Fe concentrations in

0.7078

0.7080

0.7082

0.7084

0.7086

0.7088

0.0

1.0

2.0

3.0

4.0

-9.0 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0

-10.0 -9.0 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0

1 3

C

) B D P

(

V ,

‰

6 8

7 8

r S

/ r S

18O ( PDB)‰, V

18O ( PDB)‰, V

ME dolomiteCA dolomiteMicro-finelycrystalline dolomite

Microcrystalline limestone

b

a

ME dolomiteCA dolomiteMicro-finelycrystalline dolomite

Microcrystalline limestone

Fig. 5 a d13C values versus d18O values for Ratner carbonates.

b 87Sr/ 86Sr ratios versus d18O values for Ratner carbonates

T a b l e 1

c o n t i n u e d

S a m p l e n u m b e r

W e l l l o c a t i o n

D e p t h ( m )

L i t h o l o g y

d 1 3 C

( % , V P D B )

d 1 8 O ( % , V P D B )

8 7 S r /

8 6 S r

( ± s t a n d a r d e r r o r )

A A 0 1 a

0 1 - 2 9 - 0 3 2 - 0 1 W 3

- 1 3 5 3 . 0

C A d o l o m i t e

3 . 5 9

- 6 . 9 5

A A 0 2

0 1 - 2 9 - 0 3 2 - 0 1 W 3

- 1 3 5 4 . 1

C A d o l o m i t e

2 . 2 6

- 6 . 4 4

S B 2

1 6 - 3 3 - 0 0 4 - 0 9 W 2

- 2 4 0 6 . 0

C A d o l o m i t e

3 . 2 6

- 5 . 5 1

D S 3 b

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 3 7 . 3

C A d o l o m i t e

1 . 3 4

- 7 . 4 5

D S 0 4 ( 2 )

1 6 - 1 3 - 0 4 2 - 1 9 W 2

- 8 3 6 . 7

C A d o l o m i t e

1 . 3 0

- 5 . 2 1

a

D a t a f r o m F u e t a l . ( 2 0 0 6 )


1 3


9/15

Ratner carbonates, and both Fe and Mn concentrations of

ME dolomite are significantly higher than those of CA

dolomite (Fig. 6a). In addition, elemental compositions of

Ratner limestone and micro-finely crystalline dolomite arepresented in Table 2.

Interpretation of elemental results

Generally, Ratner ME dolomite is slightly calcian (aver-

aging 52.3 mol% Ca). Slight nonstoichiometry of dolomite

that formed in burial environments has been documented

from many dolomite bodies elsewhere (e.g., Gao et al.

1992; Kupecz and Land 1991). Ratner CA dolomite is

nearly stoichiometric (averaging 51.4 mol% Ca, similar to

Ratner micro-finely crystalline dolomite in stoichiometry),

which is a weak indication of recrystallization becausesome dolomites originating in marine environments could

be stoichiometric initially (Mazzullo 1992).

Parent fluid chemistry, precursor carbonate mineralogy

and remnant inclusions, and distribution coefficients of Sr

(Dsr 1) between dolomite and diagenetic fluids are

major factors controlling Sr concentrations in dolomite

(Budd 1997; Warren 2000). Ratner limestone is interpreted

to be calcite-dominated lime mud initially, since aragonite

relicts and pseudomorphs have not been observed in Ratner

micrite (cf. Lasemi and Sandberg 1984) and the Middle

Devonian sea is considered a ‘‘calcite sea’’ (cf. James and

Choquette 1990). Sr concentrations of the Ratner micro-

crystalline limestone probably represent initial Sr concen-trations of lime muds deposited in the Middle Devonian

Saskatchewan sub-basin. Therefore, low Sr concentrations

in ME dolomite relative to Ratner limestone (Table 1;

Fig. 6c) indicate considerable Sr removal during dolomi-

tization. Furthermore, low Sr concentrations in Ratner ME

dolomite compared with those of many ancient and

Cenozoic dolomites may reflect dolomitization of Ratner

limestone by fluids with a low Sr/Ca ratio and/or at a low

precipitation rate (cf. Banner 1995; Budd 1997; Vahrenk-

amp and Swart 1990). Decrease in Sr concentrations of CA

dolomite relative to those of micro-finely crystalline

dolomite can be attributed to the stabilization process of dissolution–precipitation, which favors Sr depletion (cf.

Land 1980; Mazzullo 1992).

Although whether Na concentrations are indicative of

diagenetic fluids’ salinity is debatable; the levels of Na, as

with Sr values, in a dolomite body can be used to distin-

guish dolomite types and perhaps give information

about parent waters (Warren 2000). Note that ME dolomite

has Na concentrations (averaging 431 ppm) similar to

those of Ratner limestone (averaging 434 ppm), and Na

Table 2 Elemental concentrations for the Ratner carbonates (all in ppm except Ca also given in mol%)

Sample no. Well location Depth (m) Lithology Ca (mol%) Ca Mg Fe Mn Sr Na

WR02 06-16-038-01W3 -1165.3 Microcrystalline limestone 98.3 381,000 3,900 128 28 230 320

Wre05 07-02-038-01W3 -1177.6 Microcrystalline limestone 98.1 370,000 4,350 74 20 305 145

Wre07C 07-02-038-01W3 -1182.7 Microcrystalline limestone 98.4 373,000 3,620 269 45 161 659

RN4 08-14-004-07W2 -2417.5 Microcrystalline limestone 97.8 368,300 5,100 391 54 205 313

WT10 16-36-036-28W2 -1169.2 Micro-finely crystalline dolomite 51.8 214,700 120,000 180 47 101 853

SM1 09-31-004-08W2 -2403.8 Micro-finely crystalline dolomite 54.0 220,800 114,000 325 55 216 593

SM5 09-31-004-08W2 -2411.5 Micro-finely crystalline dolomite 51.1 205,100 119,000 483 52 71 627

SM7 09-31-004-08W2 -2417.1 Micro-finely crystalline dolomite 50.6 204,400 121,000 563 56 47 1,000

BB02 05-29-047-03W3 -740.2 Micro-finely crystalline dolomite 50.6 222,000 131,200 347 79 36 486

BB06 05-29-047-03W3 -746.3 Micro-finely crystalline dolomite 50.6 221,000 130,700 382 79 37 623

CS16 01-15-048-17W2 -517.4 ME dolomite 51.2 228,000 131,500 2,878 126 39 240

CS18 01-15-048-17W2 -518.6 ME dolomite 50.6 207,800 122,800 1,060 120 37 328

CS19 01-15-048-17W2 -519.1 ME dolomite 51.3 229,700 132,000 746 124 44 346

PM5 12-27-004-09W2 -2440.9 ME dolomite 52.7 230,000 124,900 3,006 161 56 545

CSR6 10-33-006-13W2 -2313.2 ME dolomite 54.6 222,000 111,800 3,125 196 31 393

CSR7 10-33-006-13W2 -2313.0 ME dolomite 53.8 222,200 118,500 2,130 217 32 772

CSR8 10-33-006-13W2 -2314.0 ME dolomite 51.8 224,000 125,400 2,877 174 32 415

DS03b 16-13-042-19W2 -837.3 CA dolomite 50.8 216,200 126,800 245 105 75 1,020

DS04 16-13-042-19W2 -836.7 CA dolomite 51.2 223,500 129,000 161 101 84 1,020

DS05 16-13-042-19W2 -839.0 CA dolomite 51.4 209,000 119,700 282 96 48 740

DS06 16-13-042-19W2 -840.5 CA dolomite 51.1 213,000 123,600 278 118 41 835

DS07a 16-13-042-19W2 -841.1 CA dolomite 52.6 212,000 116,000 286 98 44 690


1 3


10/15

concentrations of CA dolomite are (averaging 861 ppm)

significantly higher than those of ME dolomite (Fig. 6b, d),

but comparable to those of micro-finely crystalline dolo-

mite (averaging 697 ppm).

Fe and Mn concentrations of ME dolomite (averaging

2,260 ppm and 160, respectively) are much higher than

those of Ratner limestone (averaging 263 ppm and 42,

respectively). CA dolomite has Fe concentrations (aver-

aging 250 ppm) that are lower and Mn concentrations

(averaging 104 ppm) that are higher than, but still rela-

tively comparable to, micro-finely crystalline dolomite

(averaging 380 ppm Fe and 61 ppm Mn). In contrast to Srand Na, Fe and Mn concentrations of dolomite tend to

increase during burial diagenesis and are particularly

related to the redox potential of pore fluids, and to Fe and

Mn concentrations in precursor sediments and diagenetic

fluids (Budd 1997; Vahrenkamp and Swart 1994). In

addition, Fe and Mn concentrations better reflect the redox

state of the diagenetic environment than the ionic strength

of diagenetic fluids (Azmy et al. 2001). Both elements have

distribution coefficients greater than one ([1) that is

enhanced at higher temperatures or slower precipitation

rates (Veizer 1983). High Fe and Mn concentrations in ME

dolomite are indicative of pore fluids in reduced conditions

and fluid–rock interactions sufficient to deliver the allochth-

onous elements during the formation of ME dolomite (cf.

Soreghan et al. 2000; Tucker and Wright 1990). The average

Fe/Mn ratio of ME dolomite is about 14. Barnaby and Read

(1992) suggested that high Fe/Mn ratios ([5) implied insig-

nificant sulfate reduction and pyrite precipitation during

dolomitization. This is consistent with authors’ petrographic

observation that pyrite is rarely present in ME dolomite. Fe

concentrations of CA dolomite are slightly lower than those of micro-finely crystalline dolomite, and the average Fe/Mn ratio

of CA dolomite is about 2.5, suggesting that recrystallization

occurred in a relatively closed system and/or that Fe was

depleted in diagenetic fluids because pyrite was rarely

observed in CA dolomite.

Significant differences in trace-element concentrations

between ME dolomite and CA dolomite support the

authors’ petrographic observations and further suggest

different origins for these two types of dolomite.

0

50

100

150

200

250

Fe (ppm)

M n ( p p m )

0

50

100

150

200

250

M n ( p p m )

Na (ppm)

ME dolomiteCA dolomiteMicro-finelycrystalline dolomiteMicrocrystalline limestone

d

a

0

50

100

150

200

250

300

350

Na (ppm)

b

0

50

100

150

200

250

300

350

0 500 1000 1500 2000 2500 3000 3500

0 200 400 600 800 1000 12000 200 400 600 800 1000 1200

0 50 100 150 200 250

S r

( p p m

)

Mn (ppm)

c

S

r ( p p m )

Fig. 6 a Cross plot of Mn2?–Fe2? concentrations for Ratner

carbonates. b Cross plot of Na?–Sr2? concentrations for Ratner

carbonates. Legend is the same as for a. c Cross plot of Sr2?–Mn2?

concentrations for Ratner carbonates showing a trend (arrow) that Sr

concentrations progressively decrease and Mn concentrations

progressively increase from microcrystalline limestone, micro-finely

crystalline dolomite, ME dolomite to CA dolomite. Legend is the

same as for a. d Cross plot of Na?–Mn2? concentrations for Ratner

carbonates. Legend is the same as for a


1 3


11/15

Origin of medium and coarsely crystalline dolomites

Dolomitization

Petrographic observations suggest that formation of ME

dolomite postdated or was synchronous with early styloli-

tization, which was interpreted to occur during late

Devonian and Carboniferous time (cf. Fu et al. 2008).In addition, d18O values of Ratner ME dolomite from

a northernmost core (well 01-15-048-17W2, Ratner

ME dolomite currently being about 497–520 m deep) and

a southern well (10-33-006-13W2, currently about

2,310–2,315 m deep) in the study area show no significant

variations (Table 1) with increasing burial depth southward

across about 410 km. This suggests that Ratner ME dolo-

mite might have formed prior to significant basin tilting,

which is thought to have occurred from the Pennsylvanian

onward. Thus, ME dolomite is interpreted to have formed

in a burial environment during late Devonian to Carbon-

iferous time.Geochemical evidence (i.e., higher 87Sr/ 86Sr ratios, and

higher Fe and Mn concentrations relative to Ratner

microcrystalline limestone) suggests that Ratner ME

dolomite might have formed from basinal fluids because

extensive water–rock interactions in sedimentary basins

commonly result in basinal brines enriched in 87Sr, Fe, and

Mn (cf. Banner 1995; Land and Prezbindowski 1981;

Stueber et al. 1984). Many basinal fluids are capable of

becoming dolomitizing fluids at temperatures above

60–70C (Hardie 1987). Upward decrease in the abundance

of ME dolomite suggests that dolomitizing fluids most

likely migrated upward.

The Western Canada Sedimentary Basin probably

experienced a large-scale heat-flow anomaly and fluid

event during late Paleozoic time (Al-Aasm et al. 2002;

Osadetz et al. 2002; Wendte et al. 1998), and elevated heat

flux may cause thermal convection (Morrow 1998). The

thermal convection model has been invoked to explain

dolomitization of Devonian carbonates in western Canada

(Morrow 1998; Wendte et al. 1998). The inferred timing of

ME dolomite coincided with the Antler Orogeny and a late

Paleozoic thermal anomaly, suggesting that the formation

of ME dolomite was possibly related to tectonic com-

pression and/or thermal convection of basinal fluids. Pre-

vious studies have shown that fault and fracture conduit

systems were important in delivering dolomitizing fluids to

the Devonian strata in the Western Canada Sedimentary

Basin, causing extensive dolomitization (Duggan et al.

2001; Green and Mountjoy 2005; Kaufman et al. 1991).

After the fluids reached the Ratner Formation, lateral

migration of dolomitizing fluids along laminations and

relatively permeable layers was likely and could explain

the locally layered and interlayered distribution of ME

dolomite. Probably owing to the relatively low Mg/Ca ratio

and/or limited volumes of basinal dolomitizing fluids

delivered, ME dolomite has incompletely replaced lime-

stone in places, and it displays planar-e to planar-s texture.

Recrystallization

Ratner CA dolomite is mainly distributed in the top parts of the Ratner Formation that is directly overlain by the

Whitkow anhydrite, or locally associated with Ratner

anhydrite beds, suggesting that the formation of CA dolo-

mite is likely related to the presence of anhydrite beds. In

this study, CA dolomite is interpreted to result from

recrystallization of early-formed microcrystalline dolomite

by gypsum dehydration water mixing with connate water.

A similar suggestion has been proposed to explain the

formation of finely to very coarsely crystalline limestone in

the Ratner Formation (Fu and Qing 2007).

Strontium data of Ratner CA dolomite support the

suggestion that gypsum dehydration water might have beenan important component of recrystallizing fluids. The87Sr/ 86Sr ratios (0.7080–0.7081) of CA dolomite are

comparable to those of micro-finely crystalline dolomite

(Fig. 5b), reflecting a rock buffering system, or the

recrystallizing fluids have 87Sr/ 86Sr ratios similar to those

of Middle Devonian seawater (cf. Veizer et al. 1999).

Gypsum dehydration water from the Whitkow Member is

thought to have 87Sr/ 86Sr ratios similar to those of Middle

Devonian seawater because Sr isotopes are not appreciably

fractionated by sulfate crystallization (cf. Schreiber and El

Tabakh 2000).

Whitkow and Ratner anhydrite was interpreted to have

been originally deposited as gypsum (Debout and Maiklem

1973). Gypsum might have been converted to anhydrite

because of increasing burial depth during late Devonian

and Carboniferous time (Fu and Qing 2007). The water

released through dehydration of gypsum is undersaturated

with respect to carbonates, and diagenetic alteration may

occur as dehydration water flows through adjacent car-

bonate rocks (cf. Warren 1989). In the study area, dehy-

dration water from the Whitkow and Ratner sulfate beds

could have been squeezed into adjacent Ratner carbonate

rocks, resulting in recrystallization of the early-formed

microcrystalline dolomite.

Reservoir characterization

Oil stain is observed in many cores of Ratner dolomites

from the study area. Residual bitumen is rarely observed in

Ratner CA dolomite, but is common in ME dolomite. ME

dolomite is thinner in thickness and less predictable in

occurrence than micro-finely crystalline dolomite.


1 3


12/15

Porosity and permeability

Core analyses and logs indicate that the Ratner dolomites

have porosity of 10–22% and permeability of 1.8–45 md

(Kissling and Slingsby 1999; Nimegeers 2005; Yurkow-

ski 1995). These dolomites are likely micro-finely crys-

talline, as they are from the upper part of the Ratner

Formation (cf. Fu et al. 2006). Intercrystalline pores areoverwhelmingly dominant in the micro-finely crystalline

dolomite.

Ratner ME dolomite has the most abundant visible pores

in the Ratner Formation. Intercrystalline pores are domi-

nant (Fig. 4b, c), and vugs and horizontal to sub-horizontal

channels are abundant in places (Fig. 3b–d). Importantly,

most vugs and channels can be classified as touching vugs

(cf. Lucia 1995) and are connected with intercrystalline

pores. Visible intercrystalline pores are less abundant in

ME dolomite with planar-s texture than in that with planar-

e texture, and they are rarely observed in calcareous ME

dolomite (dolomite\80% in volume). In slightly calcare-ous dolomite (dolomite [80%), visible intercrystalline

pores are common, varying from about 6.3 to 10.7% based

on point counting of thin sections. Generally, visible

intercrystalline porosities increase with increasing dolo-

mite percentage. In carbonate samples that are completely

replaced by ME dolomite, distribution and size of inter-

crystalline pores are relatively uniform (Fig. 4c), and vis-

ible intercrystalline porosities vary from about 8.7 to

19.3%.

CA dolomite is generally tight and has very rare visible

intercrystalline pores (Fig. 4d). Separate and touching vugs

are locally observed. Total vuggy porosity accounts for less

than 2% in CA dolomite.

In carbonate rocks, permeability is highly related to

rock fabric (Lucia 1983, 1995). The size and distribution

of pore space, or pore size distribution, is important

along with porosity in estimating permeability. Lucia

(1983) demonstrated that three permeability fields can be

defined using particle-size boundaries of 100 and 20 lm,

a relationship that appears to be limited to particle sizes

less than 500 lm. Ratner ME dolomite is classified as

Class 1 of Lucia (1995), and the permeability values can

be calculated using the rock-fabric approach (Lucia

2007):

k ¼ 45:35 108

u8:537ip

where k is permeability in md, and uip is fractional inter-

particle/intercrystalline porosity.

The permeability of ME dolomite calculated by this

rock-fabric approach varies from 0.3 to 3069 md. The

actual permeability of ME dolomite is probably higher

because touching vugs can significantly contribute to per-

meability (cf. Lucia 2007).

Discussion

There has long been discussion concerning the role of

dolomitization in porosity development and destruction

(Moore 2001; Lucia 2004). For a long time, it had been

claimed that dolomitization creates about 13% porosity

based on the mole-for-mole replacement equation and most

dolomites are more porous and more permeable thanlimestones (cf. Machel 2004). However, Schmoker et al.

(1985) compared numerous limestone reservoirs and

dolomite reservoirs located in the USA and found that

dolomites commonly have lower matrix porosities and

permeabilities, but higher fracture porosities and perme-

abilities, than do limestones. Some studies demonstrated

that dolomites become more porous than limestones at

depths greater than approximately 2,000 m (e.g., Amthor

et al. 1994; Schmoker and Halley 1982). Lucia (2004,

2007) argued that porosity in dolomites inherited from the

precursor limestones rather than created by a mole-for-

mole replacement mechanism, and overdolomitization mayocclude pores or reduce porosity. He further suggested that

Paleozoic dolomites are commonly more porous than jux-

taposed limestones, and most of younger dolomites are less

porous than or have similar porosity range to adjacent

limestones. Now, it is widely accepted that dolomitization

may generate, preserve, or destroy pores, depending on the

fabrics and textures being replaced and the rate, nature, and

volume of dolomitizing fluids passing through carbonate

sediments (Purser et al. 1994; Warren 2000).

It is almost impossible to know the porosity of the

precursor limestone by the time of burial dolomitization.

However, it is reasonable to infer that the porosity of

limestone being replaced by ME dolomite was low because

lime mud could readily compact in marine pore waters at

shallow depths (Shinn and Robbin 1983). In the Ratner

Formation, ME dolomite is more porous and permeable

than adjacent lime-mudstone. The improved petrophysical

quality is probably related to a significant increase in

crystal size in dolomitized products (crystal size changing

from \20 lm in Ratner lime-mudstone to 80–250 lm in

ME dolomite) and the ability of dolomite to retain porosity.

Slightly calcareous ME dolomite (dolomite[80) is less

porous than pure ME dolomite (dolomite [99%). Where

dolomitization is only partial, mole-per-mole replacement,

if it takes place, will generate porosity (Machel 2004).

However, porosity would not have been preserved unless

dolomitization is intensive enough to form a rigid cemen-

ted framework, thus preventing compaction. Murray (1960)

observed a marked increase in porosity with increasing

dolomite content in the Mississippian Midale Beds in

Saskatchewan where dolomite is more than approximately

50% in volume. Powers (1962) found that porosity begins

to increase at slightly above 75% dolomitization, reaches a


1 3


13/15

maximum development at 80% dolomitization, and then

decreases to very low porosity at 100% dolomitization (cf.

Jodry 1992). Powers’ (1962) work agrees fairly closely

with work done by Jodry (1955). However, this study

suggested that there were almost no visible intercrystalline

pores in Ratner calcareous dolomite (dolomite\80%), and

the carbonate samples that were completely replaced by

ME dolomite had the highest porosities.Touching vugs are locally abundant in Ratner ME

dolomite, and more common in the completely dolomitized

samples, suggesting that there may be a genetic relation-

ship between dissolution and dolomitization. In this study,

relatively abundant vuggy pores are interpreted to result

from dissolution during advanced stages of dolomitization

that appears to be an integral part of the replacement

process (cf. Machel 2004). This interpretation is supported

by the authors’ petrographic observation that Ratner lime-

mudstone does not contain secondary pores, whereas vugs

and channels occur in areas where the percentage of

dolomite exceeds about 80% volumetrically. Vugs andchannels are commonly concentrated along laminations,

suggesting that they are structurally focused bathyphreatic

sites, where there was an excess of dissolution over pre-

cipitation during replacement of limestone by dolomite (cf.

Warren 2000). No evidence of significant dissolution of

dolomite crystals facing vugs and channels was observed,

implying that the formation of vugs and channels did not

result from post-dolomitization dissolution.

Ratner ME dolomite is much more permeable than are

adjacent limestones. Permeability of carbonate is related to

rock fabric (Lucia 2004), and dolomitization can change

the rock fabric significantly. When a coarse dolomite spar

([100 lm) replaces lime mud, any porosity residing

between the crystal rhombs will have better flow properties

than its finer-grained mud counterpart because of an

increase in pore size and pore throat size during dolomi-

tization of mudstone precursors (Lucia 2004). In the Ratner

Formation, the carbonate rocks that are completely

replaced by planar-e dolomite have the highest perme-

ability. Woody et al. (1996) showed that as porosity

increases in a dolomite reservoir, permeability increases at

a greater rate in planar-e dolomite than in planar-s dolo-

mite. Because vugs and channels preferentially occur along

laminations of ME dolomite (Fig. 3b–d), it is inferred that

horizontal permeability is higher than vertical permeability

in ME dolomite.

There are almost no visible intercrystalline pores in

Ratner CA dolomite. Recrystallization usually reduces

porosity (Ahr 2008). The process forms a crystalline

mosaic with abundant compromise boundaries and virtu-

ally no intercrystalline porosity. During recrystallization,

porous parent early-formed dolomite composed of micro-

crystalline crystals with a high surface area to volume ratio

may have undergone coalescive transformation with

attendant loss of interparticle porosity (cf. Ahr 2008).

Conclusion

Two types of medium and coarsely crystalline dolomite aredifferentiated in the Ratner Formation on the basis of

petrographic characteristics coupled with geochemical

evidence. (1) Medium crystalline, planar-e to planar-s

(ME) dolomite is present mainly in the lower and middle

parts of the Ratner Formation that have not undergone

early near-surface dolomitization. Generally, abundance of

Ratner ME dolomite decreases upward. (2) Medium to

coarsely crystalline, nonplanar-a (CA) dolomite is com-

monly present in the top part of the Ratner Formation that

is overlain by Whitkow anhydrite, and occasionally occurs

below the top part of the formation.

Compared with associated microcrystalline lime-mudstone, ME dolomite is significantly richer in radiogenic

Sr (87Sr/ 86Sr ratios averaging 0.7085), Fe (averaging

2,260 ppm), and Mn (averaging 160 ppm). CA dolomite has

comparable 87Sr/ 86Sr ratios (averaging 0.7081), Fe concen-

trations (averaging 250 ppm), and Mn concentrations

(averaging 104), with micro-finely crystalline dolomite.

Petrographic, stratigraphic, and geochemical character-

istics constrain Ratner ME dolomite to have formed by

ascending basinal fluids in a burial environment during late

Devonian to Carboniferous time. Ratner CA dolomite is

interpreted to result from recrystallization of precursor

early-formed microcrystalline dolomite. Recrystallizingfluids were probably a mixture of gypsum dehydration

waters and connate fluids.

Ratner ME dolomite displays abundant visible inter-

crystalline pores and has highest reservoir quality with

respect to porosity, pore size distribution and permeability

in the Ratner Formation. The high petrophysical quality

of ME dolomite is considered to be mainly related to: (1)

an increase in crystal sizes from microcrystalline (\20 lm)

in the precursor lime-mudstone to medium crystalline

(80–250 lm) in ME dolomite, and (2) the ability of dolo-

mite to retain porosity. In intensively dolomitized rocks

(dolomite[ 80% in volume), porosity increases as MEdolomite percentage increases. Vuggy and channel pores

preferentially occur along laminations and are interpreted

as resulting from dissolution taking place during dolomi-

tization. CA dolomite has almost no visible intercrystalline

pores, but limited vugs (vuggy porosity\ 2%), and the very

low porosity is interpreted to result from recrystallization.

Acknowledgments This project was funded by an NSERC

(National Sciences and Engineering Research Council of Canada)

IOR grant to K. Bergman and J. Jin, with matching funds from the


1 3


14/15

Potash Corporation of Saskatchewan, and an NSERC Discovery grant

to H. Qing, as well as by the Bureau of Economic Geology (GAAC

fund), The University of Texas at Austin. The Subsurface Geological

Laboratory of Saskatchewan Industry and Resources provided free

access to cores and facilities. Initial work (such as data collection)

was carried out at the University of Regina. This manuscript benefited

by comments from Nancy Chow and Ian Hunter. We appreciate an

anonymous reviewer who helped to improve the manuscript signifi-

cantly. Amanda R. Masterson and Lana Dieterich are thanked for

their detailed editing. The publication was authorized by the Director,

Bureau of Economic Geology.

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Medium and Coarsely Crystalline Dolomites in the Middle

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