fractured hydrothermal dolomite reservoirs in the devonian dundee formation of the central

AUTHORS

John A. Luczaj � Department of Natural and Ap-plied Sciences, University of Wisconsin–Green Bay,Green Bay, Wisconsin 54311; [email protected]

John Luczaj is an assistant professor of earth sciencein the Department of Natural and Applied Sciencesat the University of Wisconsin–Green Bay. He earnedhis B.S. degree in geology from the University ofWisconsin–Oshkosh. This was followed by an M.S.degree in geology from the University of Kansas. Heholds a Ph.D. in geology from Johns Hopkins Uni-versity in Baltimore, Maryland. His recent interestsinclude the investigation of water-rock interaction inPaleozoic sedimentary rocks in the Michigan Basinand eastern Wisconsin. Previous research activitiesinvolve mapping subsurface uranium distributions,reflux dolomitization, and U-Pb dating of PermianChase Group carbonates in southwestern Kansas.

William B. Harrison III � Michigan Basin CoreResearch Laboratory, Western Michigan University,Kalamazoo, Michigan 49008; [email protected]

William B. Harrison, III, is the director of the MichiganBasin Core Research Laboratory and is professoremeritus in the Department of Geosciences at West-ern Michigan University. He is also the director ofthe Michigan Center of the Midwest Region of thePetroleum Technology Transfer Council. He holdsa Ph.D. in paleontology and sedimentology from theUniversity of Cincinnati. His interests include pa-leontology and stratigraphy of Ordovician andSilurian carbonates in the central United States,oil and gas resources of the Michigan Basin, Devo-nian stratigraphy and depositional facies of theMichigan Basin, and methods of improved oil recov-ery from depleted or abandoned oil and gas fields.

Natalie Smith Williams � Department ofGeosciences, Western Michigan University, Kala-mazoo, Michigan 49008

Natalie Smith Williams holds an M.S. degree in earthscience from Western Michigan University and aB.A. degree in geology from DePauw University.

ACKNOWLEDGEMENTS

We acknowledge a grant from the U.S. Departmentof Energy (Project Number DE-AC26-00BC15122)awarded to J. R.Wood, T. J. Bornhorst,W. B. Harrison, III,and W. Quinlan that partially supported this project.Additional support was made available from theUniversity of Wisconsin–Green Bay. Drill cores andother materials were available through the MichiganBasin Core Research Laboratory. The authors alsothank James Wood, Robert Gillespie, and David Barnesfor ideas and discussion regarding this research andJames Duggan for reviewing the manuscript.

Fractured hydrothermaldolomite reservoirs inthe Devonian DundeeFormation of the centralMichigan BasinJohn A. Luczaj, William B. Harrison III, andNatalie Smith Williams

ABSTRACT

The Middle Devonian Dundee Formation is the most prolific oil-

producing unit in theMichigan Basin, withmore than 375million bbl

of oil produced to date. Reservoir types in the Dundee Formation

can be fracture controlled or facies controlled, and each type may

have been diagenetically modified. Although fracture-controlled res-

ervoirs produce more oil than facies-controlled reservoirs, little is

known about the process by which they were formed and diageneti-

cally modified.

In parts of the Dundee, preexisting sedimentary fabrics have

been strongly overprinted by medium- to coarse-grained dolomite.

Dolomitized intervals contain planar and saddle dolomite, with

minor calcite, anhydrite, pyrite, and uncommon fluorite. Fluid-

inclusion analyses of two-phase aqueous inclusions in dolomite

and calcite suggest that some water-rock interaction in these rocks

occurred at temperatures as high as 120–150jC in the presence of

dense Na-Ca-Mg-Cl brines. These data, in conjunction with pub-

lished organic maturity data and burial reconstructions, are not

easily explained by a long-term burial model and have important

implications for the thermal history of the Michigan Basin. The

data are best explained by a model involving short-duration trans-

port of fluids and heat from deeper parts of the basin along major

fault and fracture zones connected to structures in the Precam-

brian basement. These data give new insight into the hydrothermal

processes responsible for the formation of these reservoirs.

AAPG Bulletin, v. 90, no. 11 (November 2006), pp. 1787–1801 1787

Copyright #2006. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received May 5, 2005; provisional acceptance December 1, 2005; revised manuscriptreceived June 14, 2006; final acceptance June 27, 2006.

DOI:10.1306/06270605082

INTRODUCTION

The Michigan Basin is the classic example of an intra-

cratonic sedimentary basin. It contains as much as

5 km (3.1 mi) of Paleozoic and Mesozoic sediments

that include carbonate, siliciclastic, and evaporite sedi-

ments (Sleep et al., 1980). The Devonian Dundee For-

mation presently lies 3200–4000 ft (�975–1200 m)

below the surface in the study area in the central part

of the Michigan Basin (Figures 1, 2). Although the

Dundee Formation is formally undifferentiated in the

subsurface (Catacosinos et al., 2001), it is correlative

to both the Rogers City and Dundee formations along

the outcrop belt. However, the Rogers City and Reed

City units have typically been used as informal mem-

ber names to describe parts of the Dundee Formation

(Figure 2) for subsurface investigations. It consists of

mudstones through grainstones deposited along a car-

bonate bank and open-marine environment in the cen-

tral and eastern parts of the basin, with lagoon and

sabkha-type environments dominant in the western

part of the basin. Regionally extensive dolomite in the

Dundee Formation is mainly present in the western

parts of the basin, although most oil-producing dolo-

mitized reservoirs of the Dundee Formation are lo-

cated in the central part of the basin (Gardner, 1974),

where the maximum production appears to be related

to fractured, vug-bearing intervals (Montgomery et al.,

1998). The fields in the central Michigan Basin are in-

terpreted as discrete structures with a similar style of

faulting present among various fields. Montgomery

et al. (1998) presented additional geologic, stratigraph-

ic, and production data on the Dundee Formation in

the Michigan Basin.

The Middle Devonian Dundee Formation is the

most prolific oil-producing unit in the Michigan Basin,

with more than 375 million bbl of oil produced to date

from 137 fields, with about half of that production

coming fromdolomite-hosted reservoirs (Gardner, 1974;

Curran and Hurley, 1992; Montgomery et al., 1998;

Wylie and Wood, 2005). Reservoir types can be frac-

ture controlled or facies controlled, and each type may

Figure 1. Map showing locations of cores examined in this study (1–9) and the distribution of fields productive from the DevonianDundee Formation, Michigan Basin. Structure contours are on top of the Dundee interval. Details of drill-core localities are describedin Table 1. Modified from Montgomery et al. (1998).

1788 Devonian Fractured Hydrothermal Dolomite Reservoirs

have been diagenetically modified. Although fracture-

controlled reservoirs produce more oil than facies-

controlled reservoirs, little is known about the process

bywhich theywere formed and diageneticallymodified.

The earliest known reference regarding Devonian

fractured dolomite reservoirs in the Michigan Basin

was a brief article on the Deep River field by Lundy

(1969, p. 62). Lundydescribed theRogersCity–Dundee

oil production as being from ‘‘anomalous secondary do-

lomites believed developed along a fractured and broken

zone in the Rogers City limestone.’’ The dolomitized

Deep River field is considered anomalous because it oc-

curs in the eastern region of the basin where the Dundee

is mainly limestone. The dolomite occurred along a

linear N60jW-trending zone approximately 5.5 mi

(8.8 km) long but less than 0.5 mi (0.8 km) wide.

Cuttings were described as fine-grained brown matrix

dolomite with medium-size white rhombic dolomite

crystals. Prouty (1983) also proposed fracturing in the

Dundee as a conduit for dolomitizing fluids in several

central basin structures. Examples of prolific oil fields

are also present in the limestone-dominated part of

the eastern Michigan Basin, for example, the South

Buckeye field in Gladwin County, where no evidence

of faulting has been identified.

Recent unpublishedwork (e.g.,Wood andHarrison,

2002) using production data and structural data derived

from geophysical logs has suggested that some central

basin Devonian Dundee reservoirs, such as the Vernon

field in Isabella County and the Crystal field in Mont-

calm County, might be analogous to the hydrothermal

dolomite reservoir facies (HTDRF) of the Albion-Scipio

and Stony Point fields in the Trenton–Black River

formations of southeasternMichigan (D. Barnes, 2002,

personal communication). Only one small segment

(3 ft; 0.9 m) of the original drill core remains avail-

able for the Vernon field, but three cores in and around

the Crystal field were examined as part of this study.

Whereas drilling records suggest as many as 200 cores

may have been taken from the Dundee Formation

throughout the basin, only about 50 are currently known

to exist (Montgomery et al., 1998; this study). Many of

these cores are from the South Buckeye field in Gladwin

County, Michigan, in which the Dundee Formation

has not been dolomitized. This leaves only a few tens

of cores available for study in which the Dundee For-

mation has been partly or completely dolomitized.

Previous studies have focused on Trenton–Black

River production in hydrothermal dolomite reservoirs,

such as the Albion-Scipio field. However, studies on the

Devonian carbonates in the region have been qualitative

in regard to the conditions of fracturing and mineraliza-

tion (Prouty, 1983;Montgomery et al., 1998). This study

presents the first quantitative data on the temperature

of dolomitization and the characteristics of fractures in

Devonian Dundee reservoirs of the Michigan Basin.

PURPOSE

Cores of the Dundee Formation from nine localities in

the central Michigan Basin were examined (Figure 1;

Table 1) to document the distribution and character-

istics of fracturing in the Dundee Formation, the style

of fracturing present in these reservoir rocks, and their

relationship to rock type, grain size, and epigenetic min-

eralization. Fluid-inclusion microthermometric meth-

ods were used to document the temperature of saddle

dolomite precipitation in an attempt to place constraints

on the conditions of mineralization and reservoir devel-

opment in the Devonian Dundee Formation of the cen-

tral Michigan Basin.

METHODS

Parts of the slabbed drill core from nine wells were

examined for the distribution of dolomite and the char-

acteristics of fractures. Alizarin red stain was used to

distinguish between calcite and dolomite in the cores.

Doubly polished, epoxy-impregnated thin sections

were prepared for fluid-inclusion work; extreme care

was taken to avoid heating the sample at any time be-

fore microthermometric measurements were per-

formed using a method similar to that of Barker and

Reynolds (1984). No indication of other heating events

is available that might suggest that the cores were heated

after washing or other testing by the driller. Ultraviolet-

cured epoxywas used as themountingmedium to elimi-

nate heating during this part of thin-section prepara-

tion. Wet sandpaper and diamond-impregnated plastic

discs over plate glass were used under a constant stream

of cool water during the polishing procedure. This meth-

od was used to improve the final polish of the material

and to eliminate heating of the thin sections on a quickly

rotating lapidary wheel.

Standard fluid-inclusion measurement techniques

of Goldstein and Reynolds (1994) were performed using

Fluid Inc.-adaptedU.S.Geological Surveydesign gas-flow

heating and freezing stages at Western Michigan Uni-

versity and at the University of Wisconsin–Green Bay.

Observations of the homogenization temperatures (Th),

Luczaj et al. 1789

the final melting temperature of ice (Tm ice), the

temperature at which hydrohalite breaks down (Thh),

and the eutectic temperature (Te) were made. The Th

and Tm ice values for most inclusions were measured

to the nearest 0.1–0.5jC, when possible. The Th data

were collected before the sample was subjected to

freezing. Vapor bubbles were typically present at room

temperatures prior to the initial heating cycle. All tem-

perature data were recorded using cycling techniques

outlined by Goldstein and Reynolds (1994), and freez-

ing run data were collected with the presence of the

vapor phase in each inclusion.

Stable isotopic analyses of dolomite were performed

at Western Michigan University using the carbonate

reaction method of Krishnamurthy et al. (1997).

FRACTURE CHARACTERISTICS

The facies and stratigraphic patterns in the Dundee are

similar in cores examined from the central Michigan

Basin. Figure 2B shows the generalized lithology and

facies characteristics, along with petrophysical data

for the Thelma Rousseau 1–12 core in Mecosta Coun-

ty, Michigan (locality 1). An additional core log and

core photographs are presented by Montgomery et al.

(1998) for the Dundee Formation in the Cronus De-

velopment Tow 1–3 HD-1 well (locality 5).

Fractures are present in both the Rogers City and

Reed City/Dundee parts of the section and are present

in both limestone and dolostone lithofacies. However,

only thedolomite lithology contains high-density swarms

Figure 2. (A) Stratigraphic column, UpperSilurian–Devonian section, Michigan Basin. Modi-fied from Montgomery et al. (1998) and Cataco-sinos et al. (2001). (B) Generalized lithology andfacies characteristics for Midwest Thelma Rousseau1-12 core in Mecosta County, Michigan (locality 1).Conventional porosity (ruled) and permeability(solid black) measured from the core at everyfoot is graphically displayed on the right. Thisfacies and stratigraphic pattern is typical of allthe cores examined in this study from the centralMichigan Basin.


of fractures. Macroscopic fractures are almost exclu-

sively present in finer grained mudstones and wacke-

stones. Facies with grainstones, packstones, or abundant

interconnected porosity (vuggy and fenestral) have fewer

macroscopic fractures. Many fractures are isolated, ver-

tical, and short in length, although long fractures (up to

Figure 2. Continued.

Table 1. Locations of Drill Cores Used in This Study*

Core Number Core Name Operator County Permit Number Location

1 Thelma Rousseau 1-12 Midwest Mecosta 35426 Section 12, T16N, R8W

2 Stegman and Anderson 3-33 Newstar Isabella 51656 Section 33, T15N, R6W

3 Shuttleworth 1

Michigan Consolidated

Gas Company Gratiot 26779 Section 5, T10N, R4W

4 Bessie and Fernon Lee 1 Leonard Oil Montcalm 24011 Section 8, T11N, R5W

5 Tow 1-3 HD-1 Cronus Development Montcalm 50047 Section 3, T10N, R5W

6 McDonald 1-12 Peninsular Oil Newaygo 38437 Section 12, T11N, R11W

7


Gas Company LR 83-2


Gas Company Osceola 29261 Section 5, T17N, R10W

8 Paul Rieman 1


Gas Company Osceola 27191 Section 4, T17N, R9W

9 Hamming 1-22 Dart Missaukee 31448 Section 22, T21N, R7W

*Refer to Figure 1 for map locations.

Luczaj et al. 1791

1 m [3.3 ft]) and swarms of intersecting fractures are

abundant in several wells.

Fractures generally range in width from less than

1 mm (0.04 in.) to several millimeters across. Most

fractures are either totally filled with cement or have

a thin coating of crystals that holds the fracture to-

gether; only a small percentage of fractures are partially

cement filled.Cements aremainly saddle dolomitewith

lesser amounts of planar dolomite, calcite, anhydrite,

pyrite, and barite. One well in Oceana County con-

tained minor amounts of fluorite, which was also pre-

viously reported as cavity lining cement in wells from

the Vernon field in the central Michigan Basin (Fitz-

gerald and Thomas, 1932).

Fractures do not appear to be solution enlarged.

They have fitted fabrics and angular edges on the brec-

cia fragments, suggesting a mechanical opening pro-

cess (Figure 3). Some, such as those in the Tow core

at 3200 ft (975 m), show classic brittle fracture char-

acteristics in three dimensions (Figure 3A, B). These

fractures exhibit patterns similar to those observed in

tectonically active settings and likely were an impor-

tant factor in moving the diagenetic fluids (Nelson,

2004). Some fractures emanate from or terminate at

stylolites. Fractures were considered natural, as op-

posed to drilling induced, if theywere coated by crystals

or if the core sample with an open fracture had just

enough cement present to hold it together. Most mac-

roscopic fractures that appeared to be drilling induced

occurred along bedding planes near stylolites or at

lithologic contacts.

On a regional scale, few outcrops ofMichigan Basin

rocks exist because they are covered by thick glacial

sediments. Large-scale fracturing, faulting, and a re-

gional joint system are along dominantly northwest-

southeast trends, which can be mapped in outcrop and

inferred from subsurface data (e.g., Prouty, 1983). Con-

jugate northeast-southwest–trending structures are also

evident. The locations and geometry of oil- and gas-

producing fields in the central part of the Michigan

Basin appear to be related to deep basement structural

trends related to the Precambrian Mid-Continent rift

system (Wood and Harrison, 2002). In south-central

Michigan, mineralized zones in the Mississippian Bay-

port Limestone may indicate a possible extension of

the northwest-trending Albion-Scipio oil field. Quarries

at Bellevue, Michigan, contain limestone-hosted Mis-

sissippi Valley–typemineralization such as pyrite,mar-

casite, and calcite (Blaske, 2003; this study).

Along the western margin of the basin, mineral-

ized faults and fractures with the same northwest-

southeast and northeast-southwest orientations occur

in Cambrian through Devonian age rocks throughout

eastern Wisconsin and the western upper peninsula re-

gion of Michigan. Here, fractured, massively dolomi-

tized carbonate host rocks reveal distinct hydrothermal

signatures and are genetically associated with Missis-

sippi Valley–type mineralization and K-silicate miner-

alization (Luczaj, 2000, 2006).

PETROGRAPHY AND FLUID-INCLUSIONMICROTHERMOMETRY

Petrography

Replacive dolomite and euhedral dolomite cements

are pervasive in nearly all of the Dundee cores ex-

amined in the western and central basin. Only a few

cores examined still contained areas of unaltered lime-

stone, which was restricted to mudstone facies and to

parts of the Bell Shale that overlies the Dundee For-

mation (Figure 2A).

The size and texture of dolomite crystals vary, and

many crystals have euhedral surfaces, even at the thin-

section scale. Fracture and vug-filling saddle dolomite

crystals several millimeters in length are abundant in

most cores and were observed in all cores. Saddle dolo-

mite fills primary fenestral porosity in some cores. Frac-

ture and void-filling dolomite formed before calcite and

anhydrite, which are volumetrically insignificant in

the reservoirs. One sample of late fracture-filling cal-

cite examined using epifluorenscence microscopy con-

tained petroleum fluid inclusions, indicating that oil

generation and migration most likely began during or

after the final stages of dolomitization, but before pre-

cipitation of fracture-filling calcite.

Fluid-inclusion assemblages (FIAs) are defined petro-

graphically. Primary inclusions are best identified by

their occurrence along growth zones in a crystal, and

these primary inclusions contain a sample of the fluid

present during the precipitation of the diagenetic phase

(Goldstein and Reynolds, 1994).

Typical saddle dolomite crystals analyzed contain

a cloudy (inclusion-rich) dolomite core surrounded by a

clear, inclusion-poor dolomite overgrowth (Figure 4A).

All fluid inclusions inside each cloudy, inclusion-rich

dolomite crystal core are treated as a discrete FIA

because that is the finest petrographically distinguish-

able assemblage that could be determined (see Gold-

stein and Reynolds, 1994). However, some crystals con-

tained twowell-defined FIAs thatwere bounded on each


Figure 3. Photographsof drill core illustratingfracture and dolomite char-acteristics in the DundeeFormation. (A) Locality 5(Tow 1-3 at 3200 ft[975.4 m]). Mechanicallyproduced fractures withopen pore spaces betweenangular breccia fragments(view looking upwardalong core axis). A verythin coating of crystals ispresent holding the brec-cia fragments together.(B) Side view of the samecore specimen as in(A) showing brittle frac-ture characteristics inthree dimensions. (C) Lo-cality 4 (Lee 1 at 3466.5 ft[1056.6 m]). Abundantbrecciation of fine-graineddolostone matrix accom-panied by fracture andvug-filling white saddle do-lomite cements. Vugs (V)are partially filled by sad-dle dolomite. (D) Locality 1(Thelma Rousseau 1-12at 3916 ft [1193.6 m]).Brittle fracturing similarto that shown in (A and B),but with coarse whitesaddle dolomite cementsfilling fractures. Brecciafragments in the lower halfof the specimen have afitted fabric of fractures inwhich the fragments canbe pieced back together withadjacent clasts. (E) Local-ity 3 (Shuttleworth 1 coresegment 1-10-2 at approxi-mately 3272 ft [�997 m]).This locality exhibits abun-dant fracturingwith fitted (f)and random (r) brecciafabrics and saddle dolo-mite cements in fine-grained dolomitized stro-matoporoid (s)-bearingfacies. Vugs (V) are par-tially filled with saddledolomite.

Luczaj et al. 1793

side by clear rim overgrowths (Figure 4B). Fluid inclu-

sions observed within these isolated primary growth

zones in the dolomite were treated as different FIAs.

Most of the fluid inclusions in the cloudy crystal cores

and overgrowths are irregularly shaped, range in size

from about 1 tomore than 20 mm, and have consistently

low vapor/liquid ratios.

Homogenization Temperature Data

Homogenization temperatures (Th) from primary

fluid inclusions provide an understanding of the pre-

cipitation temperatures of the saddle dolomites. Pri-

mary fluid inclusions were measured in saddle dolo-

mite crystals from two separate localities, the Stegman

andAnderson 3-33 core and the ThelmaRousseau 1-12

core (Figure 1). Two-phase fluid inclusions in the sad-

dle dolomites from the two drill coresmeasured had Th

values between 76.5 and 180.6jC, with most in the

range of 120–150jC (Figure 5; Table 2). The Th values

averaged 131.4jC for 53 two-phase aqueous fluid

inclusions from the Stegman and Anderson 3-33 core.

In one crystal from the Thelma Rousseau 1-12 core,

two clearly distinguishable FIAs were present. The

cloudy inner core of the crystal (FIA 1) had Th values

that averaged 124.6jC for 20 inclusions, whereas an

outer growth band of the crystal (FIA 2) had Th values

that averaged 145.3jC for 34 inclusions (Figure 5).

Possible evidence for necking down of inclusions

after a phase change was observed with only two in-

clusions in this study from the Rousseau core. Fluid

inclusions 32 and 41 in FIA 1 are adjacent to one an-

other and fall on opposite ends of a histogram plot

for FIA 1. Their values were included in the above

Th averages. If necking after a phase change occurred

in this case, then the Th for the original combined in-

clusion would have been somewhere between the two

measured Th values.

No correlation between inclusion size and the pres-

ence of a vapor bubble was observed. Inclusion 34

from FIA 1 in the Thelma Rousseau well appeared to

have leaked 1 day after Th measurement. If this in-

clusion is not counted (n = 19), the averageTh for FIA 1

in the Thelma Rousseau 1-12 sample is only slightly

lower at 123.3jC (Table 2; Figure 5). A second in-

clusion leaked in this FIA before a Th measurement

could be obtained.

Freezing Data

Observations of the final melting temperature of ice

(Tm ice), the temperature at which hydrohalite breaks

down (Thh), and the eutectic temperature (Te) were

made on inclusions from both localities (Figure 6;

Table 3). Fluid inclusions from both localities exhib-

ited similar behavior at low temperatures. Halite was

not observed in any inclusions from either locality.

For seven inclusions in dolomite from locality 1

(Thelma Rousseau 1-12), ice was the last phase ob-

served to break down in all inclusions. The Tm ice

values ranged from �25.3 to �37.5jC and averaged

�34.4jC (Table 3). With the exception of one inclu-

sion in the outer growth zone FIA, all measured in-

clusions had similar Tm ice values in both inclusions

(Table 3). Eutectic melting (Te) was observed between

Figure 4. Transmitted-light images illustrating typical primarytwo-phase aqueous fluid inclusions in saddle dolomite from thestudy area. (A) Dolomite crystal from locality 1 (Thelma Rousseau1-12 at 3941 ft [1201 m]) containing a distinct inclusion-rich coreand a distinct band of inclusions in the outer clear overgrowth.(B) Enlargement of the upper-right part of the same dolomitecrystal shown above. Each inclusion-rich part of the crystal wastreated as a separate fluid-inclusion assemblage (FIA).


�57 and �64jC, with melting occurring in most in-

clusions by �61jC.

For five inclusions in dolomite from locality 2

(Stegman and Anderson 3-33), ice was the last phase

observed to break down in all but one of the inclusions.

The Tm ice values ranged from �27.7 to �34.0jC and

averaged �30.8jC (Table 3). For inclusion 8, a solid

phase was observed to break down at +1.5jC after all

ice melted at �27.7jC. Eutectic melting (Te) in in-

clusions from locality 2was observed between �57 and

�66jC, with melting occurring in most inclusions by

�60jC.

STABLE ISOTOPE DATA

The d18O values of carbonates can be helpful in de-

termining whether the minerals formed at elevated tem-

peratures in burial or hydrothermal settings. Elevated

temperature drives the isotopic composition of diage-

netic carbonate to negative values (Hardie, 1987; Allan

and Wiggins, 1993). Although stable isotopic data have

been reported for Ordovician dolomites of the Michigan

Basin (Taylor and Sibley, 1986; Budai andWilson, 1991;

Allan andWiggins, 1993), data fromDevonian carbon-

ates in the basin are limited.

Figure 5. Two frequency histogramsof Th data from primary aqueous inclu-sions in single crystals of saddle dolomitefrom localities 1 and 2. The T h data aresimilar for both localities and suggestprecipitation of dolomite over a moderaterange of temperatures. This is well illus-trated by the data from locality 1, inwhich different crystal growth zones yieldTh values indicative of warming duringprecipitation of saddle dolomite. Datafrom two inclusions in the interior ofthe crystal at locality 1 were not usedbecause of leakage.

Luczaj et al. 1795

Table 2. Homogenization Temperature Data for Rousseau 1-12 and Stegman and Anderson 3-33 Saddle Dolomite

Thelma Rousseau 1-12 at 3941 ft (1201 m) Stegman and Anderson 3-33 at 3677.7 ft (1120.9 m)

Fluid Inclusion FIA** Th Fluid Inclusion FIA** Th

1 2 180.6 1 1 160.8

2a 2 150.2 2 1 154.9

2b 2 136.0 3 1 167.4

3 2 149.5 4a 1 111.4

4 2 149.9 4b 1 119.5

5 2 152.1 5 1 131.6

6 2 137.0 6 1 143.6

7 2 150.4 7 1 125.0

8a 2 126.0 8 1 122.8

8b 2 154.9 9a 1 137.5

9 2 150.5 9b 1 149.0

10 2 151.5 10 1 132.1

11a 2 162.5 11 1 136.0

11b 2 124.9 12a 1 76.5

12 2 137.5 12b 1 132.1

13 2 144.5 13 1 139.9

14 2 135.5 14 1 139.9

15 2 153.6 15 1 159.9

16 2 162.5 16 1 134.9

17 2 166.5 17 1 126.0

18 2 137.0 18 1 139.9

19 2 157.0 19 1 174.9

20 2 160.5 20 1 134.9

21 2 154.9 21 1 124.0

22 2 149.9 22 1 144.9

23 2 130.8 23 1 141.9

24 2 152.1 24 1 109.1

25 2 143.0 25 1 126.0

26 2 126.3 26 1 121.5

27 2 123.7 27 1 121.5

28a 1 126.0 28 1 157.1

28b 1 127.0 29 1 128.0

29 1 134.0 30 1 134.9

30 1 119.4 31 1 124.9

31 1 121.9 32 1 126.0

32 1 143.1 33 1 123.2

33 1 119.3 34 1 121.0

34* 1 150.2 35 1 110.8

35 1 136.7 36 1 121.1

36 1 119.2 37 1 123.5

37 1 117.6 38 1 122.3

38 1 114.4 39 1 142.1

39 1 128.7 40 1 126.1

40 1 126.8 41 1 112.5

41 1 109.2 42 1 124.9

42 1 107.5 43 1 124.0


Stable isotopic (d18O) compositions of saddle do-

lomite were determined for samples from the Devonian

Dundee Formation in three different oil fields in central

Michigan. Two samples of saddle dolomite cements

were analyzed from drill core from the Tow 1–3 well

in the Crystal field of Montcalm County, Michigan

(locality 5). The d18O (PDB) composition of saddle

dolomites from depths of 3196 and 3231 ft (974 and

985 m) were reported as �9.03 and �9.19x, respec-

tively. Two saddle dolomite crystals from theMichigan

Consolidated Gas Company LR83-2 well in Osceola

County (locality 7) from depths of 3577 and 3586 ft

(1091 and 1093 m) yielded d18O values of �8.18 and

�8.64x, respectively. Additional d18O data ranging

between �6.68 and �8.93xwere reported for sam-

ples of white dolomite fragments from cuttings of sev-

eral wells in the Deep River field in Arenac County,

Michigan. The d18O data are consistent with precipita-

tion of saddle dolomite at elevated temperatures (Allan

and Wiggins, 1993).

DISCUSSION

The common occurrence of saddle dolomite along frac-

tures suggests that the fracturing predated or was con-

temporaneous with the precipitation of saddle do-

lomite. Fractures observed at the core scale may be

related to reservoir-scale features because they com-

monly are in other fractured reservoirs (Nelson, 2004).

Core-scale and reservoir-scale fracturing and faulting

in the Dundee Formation are analogous to well-

documented dolomitization related to deep basement

features in the Ordovician Albion-Scipio trend of

the south-central Michigan Basin (Hurley and Budros,

1990).

The presence of two-phase fluid inclusions that

homogenize in the range of 120 to at least 155jC in-

dicates that rocks were exposed to temperatures of that

magnitude (Barker andGoldstein, 1990).A fewhomog-

enization temperatures approaching 175jC for dolo-

mite suggest that rocks were exposed to tempera-

tures above 155jC. However, because Th values above

155jC are only a small proportion of the data sets, un-

recognized necking down after a phase change or het-

erogeneous entrapment of a relatively small fraction of

the vapor phase cannot be ruled out.

43 2 146.2 44 1 139.9

44 1 117.0 45 1 129.9

45 2 132.0 46 1 124.9

46 2 132.0 47 1 124.1

47 2 119.9 48 1 128.6

48 1 118.6 49 1 120.8

49 1 135.4 50 1 135.1

50 1 120.7

FIA 1 (n = 19) Average Th 123.3 FIA 1 (n = 53) Average Th 131.4

FIA 2 (n = 34) Average Th 145.3

*Inclusion 34 leaked after heating and was not used to calculate the average T h.**FIA 1 corresponds to the crystal interior, whereas FIA 2 corresponds to an isolated growth zone in the clear outer rim of the saddle dolomite crystal.

Table 2. Continued

Thelma Rousseau 1-12 at 3941 ft (1201 m) Stegman and Anderson 3-33 at 3677.7 ft (1120.9 m)

Fluid Inclusion FIA** Th Fluid Inclusion FIA** Th

Figure 6. Frequency histogram of Tm ice data from primaryaqueous inclusions in saddle dolomite from localities 1 and 2.Tm ice data are similar for both localities and suggest precip-itation of dolomite in the presence of a dense brine.

Luczaj et al. 1797

Inclusions in the FIAs measured have most of their

Th values spread over an interval of 25–40jC (Figure 5).

This suggests that there was either reequilibration of

existing fluid inclusions or original variability in con-

ditions of entrapment of the fluid inclusions (Goldstein

and Reynolds, 1994). The Th values are considered min-

imum estimates of the temperature of entrapment, and

no pressure corrections have been applied because the

burial pressures at the time of entrapment are un-

known. Any pressure correction applied to the data

would be added to the observed Th values (Goldstein

and Reynolds, 1994).

Entrapment of fluid inclusions over a small range

in temperature (10–30jC) is the favored mechanism

to explain the range of homogenization temperatures

measured in the saddle dolomites. No obvious corre-

lation was observed between the size of an inclusion

and its homogenization temperature, which would be

expected if reequilibration of existing inclusions caused

by stretching had occurred (Goldstein and Reynolds,

1994).

Evidence for entrapment of inclusions over a range

of temperatures is especially obviouswhen separate FIAs

within the same crystal are considered (see Figure 5). In

the case of the Thelma Rousseau core (locality 1), the

outer clear dolomite overgrowth contains inclusions

with higher Th values than the cloudy, inclusion-rich

core of the crystal. In this crystal, the inclusions can be

segregated petrographically into different FIAs with dif-

ferent Th ranges. This also suggests that saddle dolo-

mite likely precipitated over a range of temperatures

as temperatures rose to at least 145–150jC. Precipi-

tation of dolomite over a range of temperatures is

consistent with the idea that dolomite precipitation

is favored as temperature increases (Carpenter, 1980;

Land, 1985). Therefore, it would make sense that as

the temperatures rose when the Dundee rocks were

heated during and shortly after regional fracturing

events, both replacement of preexisting carbonatemin-

erals by dolomite and dolomite precipitation would be

favored.

Bulk salinity (weight percent, NaCl equivalent)

was calculated using the equation of Bodnar (1992). For

inclusions in dolomite from locality 1 (Thelma Rous-

seau 1-12), calculated salinities ranged from 25.8 to

34.0 wt.% (NaCl equivalent) and averaged 31.6 wt.%

(NaCl equivalent) for all seven inclusions (Table 3). The

observed eutectics are consistent with a complex sys-

tem similar to a model NaCl-CaCl2-MgCl2-H2O sys-

tem. ThemodelNaCl-CaCl2-MgCl2-H2O systemhas a

stable Te of �57jC, and the initial melting in the in-

clusions that occurs several degrees below the stable

eutectic was likely the result of metastable behavior

(Goldstein and Reynolds, 1994).

For inclusions in dolomite from locality 2 (Steg-

man and Anderson 3-33), calculated salinities range

from 27.2 to 31.3 wt.% (NaCl equivalent) and averaged

29.2wt.% (NaCl equivalent) for all five inclusions. The

observed eutectics for inclusions from locality 2 are

also consistent with a complex system similar to the

Table 3. Fluid-Inclusion Freezing Data for Rousseau 1-12 and Stegman and Anderson 3-33 Saddle Dolomite

Freezing Data

Locality Inclusion Number* FIA Type Tm ice (jC)Salinity (wt.%

NaCl Equivalent)

Eutectic

Temperature (jC)

Thelma Rousseau 1-12 11a Outer growth zone �34.3 31.5 �62

12 Outer growth zone �36.2 32.9 �65 to �61

16 Outer growth zone �33.3 30.8 –

25 Outer growth zone �37.3 33.8 �60 to �57

26 Outer growth zone �25.3 25.8 �64

28a Crystal interior �37.5 34.0 �64 to �57

29 Crystal interior �37.0 33.6 about �66 to �65

Stegman and Anderson 3-33 1 Crystal interior �29.5 28.3 �66 to �63

3 Crystal interior �29.3 28.2 �63 to �57

8 Crystal interior �27.7 27.2 �65 to �57

17 Crystal interior �33.7 31.1 –

51 Crystal interior �34.0 31.3 about �63

*T h data were not collected for inclusion 51.


model NaCl-CaCl2-MgCl2-H2O system interpreted

for locality 1.

Together, the Th, Tm ice, and Thh data indicate that

the fluid present during precipitation of the saddle do-

lomite in the hydrocarbon reservoirs was a very dense

Na-Ca-Mg-Cl brine at temperatures as high as 120–

150jC. This is similar to the bulk composition of mod-

ern Dundee Formation reservoir fluids in the Michigan

Basin (White et al., 1963; Dollar et al., 1991).

In the study area, the Dundee Formation lies 3200–

4000 ft (�975–1200 m) below the surface. The ther-

mal evolution of the basin is controversial regarding

the thickness of missing strata, timing of thermal matu-

ration, and magnitudes of past geothermal gradients

(e.g.,Cercone, 1984;Nunnet al., 1984; Illich andGrizzle,

1985; Velbel and Brandt, 1989; Howell and van der

Pluijm, 1990;Cercone andPollack, 1991;Crowley, 1991).

Results vary widely, but some authors have concluded

that less than about 1200 m (4000 ft) of sediments

were eroded from the basin before the Jurassic (e.g.,

Hayba, 2004).

We assume that the thickness of the eroded sedi-

ments across theMichigan Basin was probably less than

approximately 1000m (3300 ft) based on the following:

(1) the Pennsylvanian and Devonian sediments in the

basin show relatively low thermal maturities (Sleep

et al., 1980; Landing andWardlaw, 1981;Moyer, 1982;

Dellapenna, 1991; Martini et al., 1998); (2) the pre-

dicted thickness of the eroded late Paleozoic strati-

graphic section in the basin is about 300 m (1000 ft)

(Beaumont et al., 1987); and (3) relatively noncom-

pacted Mississippian shales suggest a maximum addi-

tional thickness of 850 m (2800 ft) in the center of the

basin (Vugrinovich, 1988).

Late Jurassic sediments that overlie Pennsylvanian

sediments in the basin also place an important con-

straint on the time available for sediment accumula-

tion and complete erosion (less than approximately

150 m.y.). Deposition and erosion rate calculations

for this missing interval predict a maximum thick-

ness of eroded sediments of about 1125 m (3690 ft).

This estimate was calculated using a period of 75 m.y.

of deposition immediately followed by a period of

75 m.y. of erosion, with deposition and erosion rates

of 15 m/m.y. (50 ft/m.y.), which is close to the aver-

age net sediment accumulation rate for the whole ba-

sin during the early and middle Paleozoic. During the

Mississippian through the early Mesozoic, net rates of

deposition were probably much lower, as suggested

by significant unconformities present within the Mis-

sissippian and Pennsylvanian sections, which would

lead to considerably lower thicknesses of eroded sedi-

ments (see Luczaj, 2000, for details).

The term ‘‘hydrothermal dolomite’’ is applied to

these fractured dolomite reservoirs because of the rela-

tively high temperature of saddle dolomite precipita-

tion relative to what is expected from heating by burial

alone. Assuming a 20jC mean annual surface temper-

ature and a 20–25jC/km geothermal gradient, a mini-

mum of 3000 m (9800 ft) of sediments would need to

have been deposited and then eroded between the late

Pennsylvanian and the Late Jurassic to satisfy the tem-

peratures measured in this study, if burial heating alone

were the mechanism responsible. These burial con-

straints suggesting 3 km (1.8 mi) or more of missing

sediments are inconsistent with the known stratigraph-

ic history for the Michigan Basin. Therefore, the tem-

peratures measured are not representative of heating

because of burial alone, but instead are caused by in-

creased local or regional geothermal gradient related

to fluid flow. The precipitation of saddle dolomite at

temperatures above ambient temperature is consistent

with moving hot brines from greater depths upward

into the Dundee Formation along faults and fractures.

Strikingly similar fracture characteristics, mineral-

ogic associations, dolomitization fabric, dolomite for-

mation temperatures, and stable isotopic values exist

throughout the Dundee of the central Michigan Basin.

Therefore, some or most of the dolomitized Dundee

fields of the central Michigan Basin have the same ori-

gin and are genetically related to deep-seated fault and

fracture systems.

SUMMARY

Fractured dolomite reservoirs appear to account for a

substantial part of the oil production from Devonian

rocks in theMichigan Basin. The formation of fractures

and the precipitation of saddle dolomite in the Devo-

nian Dundee Formation in the central Michigan Basin

were integral parts of reservoir development because

they both apparently predate oil migration and reser-

voir filling.

The saddle dolomite was formed during hydro-

thermal fluid circulation of dense Na-Ca-Mg-Cl brines

along faults and fractures over a range of temperatures

as high as at least 135–145jC. Although it appears that

a wide area of the basin was affected by hot dolomi-

tizing fluids, heating and water-rock interaction were

likely focused along localized fractured and faulted zones

with which the reservoirs are genetically associated.

Luczaj et al. 1799

Some or most of the dolomitized Dundee fields of the

central Michigan Basin likely have the same hydrother-

mal signature, here interpreted to be related to deep-

seated fault and fracture systems.

Exploration and production models for dolomi-

tized reservoirs in the Devonian Dundee Formation, as

well as other carbonate units in the basin, need to in-

corporate the concepts of faulted, fractured hydro-

thermal dolomite reservoir facies models to achieve a

complete understanding of reservoir properties and

production potential.

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fractured hydrothermal dolomite reservoirs in the devonian dundee formation of the central

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