1. Divergent fluid evolution and the formation of fracture-fill hydrothermal dolomite: examples from Devonian
and Mississippian carbonate reservoirs
IHSAN AL-AASM AND CAROLE MRAD
University of Windsor, Windsor, Ontario, Canada N9B 3P4
E-mail: , alaasm@uwindsor.ca, mrad@uwindsor.ca
Abstract
Integrated petrographic, geochemical and fluid inclusion study of fracture filling mineralization, including
saddle dolomite cement extending from Alberta to British Columbia, Canada aims at quantifying the
type and nature of fluids that precipitated this dolomite and whether these fluids represent a single or
multiple events. Fracture-filling saddle dolomite from three Devonian and two Mississippian carbonate
successions were investigated in this study. The Devonian formations include Slave Point and
Duvernay formations and the Mississippian Upper Debolt and Turner Valley formations.
Paragenetically, saddle dolomite occluded fractures and vugs, predated by calcite cement but
postdated by anhydrite and sometimes quartz.
The δ18O isotopic values for the Devonian saddle dolomite (-14.6 to -5.58 ‰ VPDB; average -12.2 ‰)
combined with enriched 87Sr/86Sr isotopic ratios (0.708626 to 0.713480) and fluid inclusion data (125-
191.78oC; average 160 oC, 9.28 to 24.7 wt.% NaCl) show significant differences from the Mississippian
dolomite, which is characterized by less depleted δ18O isotopic values (-10.8 to -7.8 ‰ VPDB; average
-9.05 ‰), less radiogenic 87Sr/86Sr isotopic ratios (0.708591 to 0.709975) and lower homogenization
temperatures (Th) and salinity values of fluid inclusions (87-214.5 oC; average 130 oC , 2.0 to 13.2 wt.%
NaCl). These results suggest a possibly two different hydrothermal pulses related to early and late
tectonic events that affected the Western Canada Sedimentary Basin. δ18O isotopic values along with
87Sr/86Sr isotopic ratios and fluid inclusion data show somewhat spatial variability existed within the
Mississippian and Devonian fields whereby more depleted δ18O values, higher salinity and higher
temperature are observed in saddle dolomite from the Devonian carbonates in the NE part of the basin
compared to Mississippian dolostones in the NW part. This suggest the effect of compartmentalization
of hydrothermal fluids in the basin.
Objectives
 Quantification of type and nature of fluids that precipitated fracture-filling saddle dolomite.
 Determination of the relative timing and evolution of fracture mineralization as related to tectonic
history of the basin.
Methodology
 Petrographic investigations including transmitted light and cathodoluminescence microscopy.
 Stable (δ18O and δ13C) and strontium isotopic analysis of saddle dolomite and host dolostone in the
studied carbonate reservoirs.
 Fluid inclusion analysis on saddle dolomites including homogenization temperature (Th) and salinity
(Tm) for primary fluid inclusions.
Geologic Setting
The studied fields included Sikanni and Quirk Creek from the Mississippian along with Hamburg,
Jedney and Duvernay from the Devonian.
Field Formation Age Location
Sikanni Upper Debolt Mississippian North East British Columbia
Hamburg Slave Point Middle Devonian North Western Alberta
Quirk Creek Turner Valley Upper Mississippian South Western Alberta
Jedney Slave Point Devonian North East British Columbia
-
Duvernay Upper Devonian Central Alberta
Map of WCSB showing the Devonian and Mississippian successions (modified from Richards 1989b)
Structural elements of the WCSB (modified from Geological Atlas of Western Canada Sedimentary Basin
1994)
A northwest-trending trough in front of the Cordilleran Fold and Thrust Belt termed the Alberta Basin and
the cratonic Williston Basin along with the eastern Canadian Cordillera constitute the Western Canada
Sedimentary Basin. The above sedimentary Basins are separated by the Bow Island Arch (Wright, 1984).
A major east-northeast trending basement structure called the Peace River Arch extended from the
Cordillera towards the craton across northeastern British Columbia and northwestern Alberta (Cant,
1988).The Peace River Arch in the Mississippian to Permian time became the site of a faulted basin
termed the Peace River Embayment. Prior to the Mississippian that Arch represented a topographic high
in Cambrian to late Devonian time.
Extensional tectonics produced the Liard Basin and the east-west oriented Peace River Embayment
during the late Devonian-Mississippian Antler Orogeny. During the late Mississippian (Pennsylvanian
period) a structural feature near the eastern part of the Peace River Embayment termed the Dunvegan
Fault was active. The Prophet Trough of Western Canada, which developed during the late Devonian to
early Carboniferous and persisted into late Cretaceous, contained the thickest Carboniferous sections
(Wright, 1984). Richards et al. (1994) suggested its extension from southeastern British Columbia to the
late Devonian and early Carboniferous Yukon Fold Belt as well as the Prophet Trough connection to the
Antler Foreland Basin (Western United States).
Paragenetic sequence of the Devonian Formations
Paragenetic sequence of the Mississippian Formations Petrographic Analysis of Devonian Saddle dolomite
Photomicrographs of Devonian saddle dolomite. (A-B) PPL and CL images
showing planar subhedral medium dolomite crystals followed by vug-filling
saddle dolomite and quartz infilling pore space, (C) CL image showing different
generations of pore filling saddle dolomite cement with multiple growth zones.
Under CL, SD displays oscillatory zonation of dull to bright red color with bright
red rims,(D) Anhydrite cement postdating saddle dolomite,(E) fracture filling
saddle dolomite cement crosscut by stylolite and postdating fine crystalline
matrix dolomite, (F) pore filling saddle dolomite cement postdating fine to
medium crystalline matrix anhedral to subhedral dolomite
Petrographic Analysis of Mississippian Saddle dolomite
Photomicrographs of Mississippian Saddle dolomite cement. (A) pore filling
calcite cement postdating anhydrite and pore filling saddle dolomite cement,
(B) pore filling saddle dolomite cement postdating planar subhedral to
euhedral medium grained dolomite, (C) saddle dolomite postdating fine
crystalline matrix dolomite, (D) saddle dolomite postdating early calcite
cement
Stable Isotope Geochemistry
-16 -14 -12 -10 -8 -6 -4 -2 2 4
-3
-2
-1
1
2
3
4
5
Mississippian Marine Dolomite
Middle Devonian Marine Dolomite

18
O vs. 
13
C for saddle dolomite by Age
Mississippian
Devonian

13
C(VPDB)

18
O (VPDB)
-16 -14 -12 -10 -8 -6 -4 -2 2 4
-3
-2
-1
1
2
3
4
5
Mississippian Marine Dolomite
Middle Devonian Marine Dolomite

18
O vs. 
13
C for saddle and matrix dolomite by Age
SD(Mississippian)
SD(Devonian)
MD(Mississippian)
MD (Devonian)

13
C(VPDB)

18
O (VPDB)
 Saddle dolomite (SD) cement shows highly depleted values
indicating a burial environment with a hydrothermal fluid source
 Devonian saddle dolomite (SD) δ18O isotopic values are more
depleted relative to the Mississippian SD indicating a different
pulse of hydrothermal fluid source
 Devonian matrix dolomite(MD) δ18O isotopic values are more
depleted relative to the Mississippian MD suggesting
recrystallization during burial by a later (hydrothermal) fluids
Strontium Isotope Geochemistry
 87Sr/86Sr isotopic ratios of the Devonian saddle
dolomite(SD), matrix dolomite (MD) and
pervasive dolomite (PD) are more enriched
relative to the Mississippian SD,MD and PD
indicating a more radiogenic source for Sr in
the Devonian system related to hydrothermal
fluids sourced from basement rocks
 87Sr/86Sr isotopic ratios of the Devonian saddle
dolomite(SD) are significantly more radiogenic
than the postulated values for Middle Devonian
marine carbonates
 87Sr/86Sr isotopic ratios of the Mississippian
saddle dolomite(SD) are more enriched than
the Middle Mississippian marine values
indicating precipitation from more radiogenic
fluid source.
Primary two-phase fluid inclusions in Saddle Dolomite
A
Fluid inclusions :( A) Fluid inclusion assemblage in saddle dolomite under 40x, (B): shows six two-phase
primary fluid inclusions (liquid rich with vapor bubble) in saddle dolomite ranging in shape from elongate to
sub circular and in size from 2 to 6 µm under 100x
Fluid Inclusion Results
B
 Saddle dolomite fluid inclusion results from both age groups indicate precipitation in a burial environment
by a hot, slightly to highly saline fluid source related to hydrothermal activity
 Saddle dolomites from the Devonian are characterized by a higher homogenization temperatures and
salinities relative to the Mississippian indicating two pulses of hydrothermal activity related to early and
late tectonic events
 Highly saline values (20-25 wt. % NaCl eq.) of Saddle dolomite from the Devonian Slave Point Formation
(Hamburg) suggests its association with the Antlers Orogeny contrary to lower salinity values from the
Devonian Salve Point Formation (Jedney) that is related to hydrothermal fluid flow that occurred during
the Laramide Orogeny
Conclusions
 Saddle dolomite formed in a shallow to intermediate burial depth from hydrothermal fluids
 Isotopic and fluid inclusion data indicates a possible two different hydrothermal pulses related to early (Antler) and late (Laramide) tectonic events characterized saddle dolomite formation in the Western Canada Sedimentary Basin
 Spatial variability existed within the Mississippian and Devonian fields whereby more depleted δ18O values, higher salinity and higher temperature are observed in saddle dolomite from the Devonian carbonates in the NE part of the basin compared to Mississippian saddle dolomite in the NW part as indicated by the isotopic
and fluid inclusion data
Calculated oxygen isotopic composition of the dolomitization fluid
for saddle dolomite (SD) and matrix dolomite (MD) (expressed in
VSMOW). Fractionation equation that is used is from Land (1983).
Matrix dolomite formed at lower temperatures compared to saddle
dolomite (SD), which is characterized by enriched δ18OSMOW values
forming at higher temperatures. A clear divergent fluid source is
demonstrated where the Devonian SD formed by a warmer brines
at higher temperature compared to the Mississippian SD.
& Hanson, 1990).
80 100 120 140 160 180 200 220
1
2
3
4
5
6
7
8
Frequency
Th
(°C)
Mississippian
Devonian
Histogram showing the frequency distribution of Th for fluid inclusions from saddle dolomite
5 10 15 20 25 30
1
2
3
4
5
6
7
8
9
10
Frequency
Salinity wt.%NaCl
Mississippian
Devonian
Histogram showing the frequency distribution of salinity for fluid inclusions from saddle dolomite.
80 90 100 110 120 130 140 150 160 170 180 190 200 210
2
4
6
8
10
12
14
16
18
20
22
24
26 Mississippian
Devonian
Salinitywt.%NaCl
TH
(°C)
Th vs. salinity of saddle dolomite by age.
-9 -5
-1
Mississippian
Dolomite
Dolomite types in the Devonian and Mississippian
Successions
SikanniJedney Hamburg
Quirk
Creek
Duvernay
E
50µm
SD
SD
50µm
F
A
50µm
B
50µm
C
50µm
D
50µm
50µm
B
SD
A
50µm
SD
50µm
C
SD
D
50µm
SD
Type of Dolomite Typical Petrographic
Characteristics
Fine Crystalline
Matrix Dolomite
(FCMD)
Size:4 to 15 µm
Shape: euhedral to
subhedral and anhedral
Dark red color under CL
Medium Crystalline
Matrix Dolomite
(MCMD)
Size: 20 to 150 µm
Shape: euhedral to
subhedral and anhedral
Dark red color under CL
Coarse Crystalline
Dolomite (CCD)
Size: from 200 to 500 µm
Shape: subhedral to
anhedral crystals
Dull red luminescent cores
and bright red rims under
CL
Pervasive Dolomite
(PD)
fabric destructive
Size:50 to 250 µm
Dull brownish red color with
bright red rims under CL
Saddle Dolomite
(SD)
Sweeping extinction and
curved crystal faces
Pore filling size :20 to 150
µm Fracture filling size: 50
to 500 µm
Oscillatory zonation of dull
to bright red colors with
dark red rims under CL
Diagenetic Process Early Late
Fine crystalline matrix dolomite
Silicification
Medium crystalline matrix
dolomite
Coarse crystalline dolomite
Early pore filling equant calcite
cement
Early fracture filling calcite cement
Mechanical compaction
Chemical Compaction
Pervasive dolomite
Pore filling saddle dolomite
Fracture filling saddle dolomite
Late fracture filling calcite cement
Late pore filling blocky calcite
cement
Anhydrite
Sikanni
Hamburg
Jedney
Duvernay
Quirk Creek
Diagenetic Process Early Late
Fine crystalline matrix dolomite
Medium crystalline matrix
dolomite
Early pore filling equant calcite
cement
Early fracture filling calcite cement
Mechanical compaction
Chemical Compaction
Pervasive dolomite
Pore filling saddle dolomite
Fracture filling saddle dolomite
Late pore and fracture filling
blocky calcite cement
Silicification
Anhydrite