Fractured hydrothermal dolomite reservoirs in the Devonian Dundee Formation of the central
Transcript of 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.
1790 Devonian Fractured Hydrothermal Dolomite Reservoirs
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
Michigan Consolidated
Gas Company LR 83-2
Michigan Consolidated
Gas Company Osceola 29261 Section 5, T17N, R10W
8 Paul Rieman 1
Michigan Consolidated
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
1792 Devonian Fractured Hydrothermal Dolomite Reservoirs
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).
1794 Devonian Fractured Hydrothermal Dolomite Reservoirs
�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
1796 Devonian Fractured Hydrothermal Dolomite Reservoirs
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.
1798 Devonian Fractured Hydrothermal Dolomite Reservoirs
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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