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Mar 1, 2013 - The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated ...
Diagenetic evolution and associated mineralization in Middle Devonian carbonates, southwestern Ontario, Canada

Omid Haeri-Ardakani Department of Earth and Environmental Sciences University of Windsor Windsor, ON N9B 3P4 [email protected]

Ihsan Al-Aasm

Abstract The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated field, petrographic, fluid inclusion and isotope geochemistry reveal a variety of diagenetic processes that include the formation of dolomite, calcite, celestine and fluorite under diverse geochemical conditions. The host carbonate strata display two types of replacive dolomite fabrics: 1) fine-crystalline matrix dolomite (5–20 µm); and 2) mediumcrystalline dolomite (50–150 µm) with an average size of 100 µm. Stable oxygen and carbon isotope results for both dolomite fabrics show a wide range of values (δ18O = –9.4‰ to –3.9‰ VPDB, δ13C = 0.4‰ to 3.4‰ VPBD). For late fracture-filling calcite, δ 18O and δ 13C values range from –9.4 to –5.9‰ VPDB and –7.7‰ to –0.04‰ VPDB, respectively. The 87Sr/86Sr ratios of selected dolomite and calcite samples range from 0.70798–0.70838, preserving Middle Devonian seawater values while some samples have slightly more radiogenic ratios than contemporaneous seawater. Field and petrographic evidence and Sr isotope ratios of dolomite suggest early dolomitization from evaporated seawater in a sabkha setting (shallow seepage reflux or evaporitive pumping). However, the depletion in δ18O (varying from –3.9 to –9.4‰ VPDB), and radiogenic Sr isotope values of the Lucas Formation dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures in a burial environment. The 18O-depleted values and slightly radiogenic 87Sr/86Sr ratios of calcite cements suggest precipitation from hot, saline basinal/hydrothermal brines. In addition, the 13C-depleted calcites indicate that carbon was possibly formed by oxidation of organic matter. Fluid inclusions in some of the late calcite cements fluoresce under UV light, indicating the presence of hydrocarbons. Homogenization temperatures of these inclusions vary from 87.1°C to 171.9°C with an average value of 126°C. Average salinity of fluid inclusions is 19.7 wt. % NaCl eq. Celestine samples have lower salinities relative to calcite, averaging 14.5 wt. % NaCl eq. with an average homogenization temperature of 219.5°C. Based on the geochemical and fluid inclusion results, hot, saline basinal/hydrocarbon bearing brines originated from central part of the basin and migrated to Devonian carbonates via fractures and faults. Celestine is a product of thermochemical sulfate reduction and formed due to mixing of saline, Sr and sulfate bearing brines and meteoric waters.

Department of Earth and Environmental Sciences University of Windsor Windsor, ON N9B 3P4 [email protected]

Mario Coniglio Department of Earth and Environmental Sciences University of Waterloo Waterloo, ON N2L 3G1 [email protected]

Iain Samson Department of Earth and Environmental Sciences University of Windsor Windsor, ON N9B 3P4 [email protected]

Résumé La Formation de Lucas du Dévonien moyen du sud-ouest de l’Ontario, au Canada, constitue la principale roche-réservoir d’hydrocarbures dans la région. Terrains intégrés, pétrographie, inclusion fluide et géochimie des isotopes révèlent une variété de processus diagénétique incluant la formation de dolomite, calcite, célestine et fluorite sous diverses conditions géochimiques. Les strates de carbonate hôte présentent deux types de dolomite de remplacement dans la fabrique : 1) matrice de dolomite cristalline fine (de 5 à 20 µm); et 2) matrice de dolomite cristalline moyenne (de 50 à 150 µm) pour une taille moyenne 100 µm. Les résultats des isotopes stables du carbone et de l’oxygène pour les deux fabriques de dolomite présentent un large éventail de valeur (δ180 = –9,4 % à

BULLETIN OF CANADIAN PETROLEUM GEOLOGY Volume 61, Number 1 March 2013 Pages 41–58 Page 41

–3,9 % VPDB, δ13C = 0,4 % à 3,4 % VPDB). Pour les fractures plus tardives remplies par la calcite, les valeurs δ18O et δ13C varient de –9,4 % à –5,9 ‰ VPDB et de –7,7 à -0,4 % VPDB, respectivement. Les ratios de 87Sr/86Sr des spécimens de dolomite et de calcite sélectionnées donnent des rapports de 0,70798 à 0,70838, les valeurs de l’eau marine du Dévonien moyen étant préservées, alors que d’autres spécimens reflètent des ratios légèrement plus radiogéniques que l’eau marine contemporaine. Les éléments probants pétrographiques et sur le terrain, de même que les ratios isotopiques en Sr de la dolomite, suggèrent une dolomitisation précoce de l’eau marine évaporée dans un environnement de sebkha (nappe d’eau peu profonde créée par suintement ou pompage par évaporation). Cependant, on considère que l’appauvrissement en δ18O (variant de –3,9 % à –9,4% VPDB) et les valeurs isotopiques en Sr radiogéniques de la dolomite de la Formation de Lucas sont le résultat de la recristallisation qui survient, selon l’interprétation, à de haute température dans des conditions d’enfouissement. Les valeurs appauvries en 18O et les ratios légèrement radiogéniques en 87Sr/ 86Sr en ciment calcitique suggèrent la précipitation de saumures chaudes de bassin ou hydrothermales. En outre, les calcites appauvries en 13C indiquent que la formation de carbone est peut-être due à l’oxydation de matières organiques. Devenant fluorescentes sous la lumière ultraviolette, les inclusions fluides dans certains ciments calcitiques indiquent la présence d’hydrocarbures. Les températures d’homogénéisation de ces inclusions fluides varient de 87,1°C à 171,9°C, pour une moyenne de 126°C. La salinité moyenne des inclusions fluides est de 19,7 % d’équivalent en poids de NaCl. Avec une moyenne de 14,5 % d’équivalent en poids de NaCl et une température d’homogénéisation de 219,5°C, la salinité des spécimens de célestine est inférieure à celle de la calcite. Selon les résultats géochimiques et de l’inclusion fluide, les saumures chaudes de bassin à hydrocarbures proviennent du centre du bassin et ont migré dans les carbonates du Dévonien par les fractures et failles. La célestine est un produit de la réduction du sulfate thermochimique et s’est formé par le mélange de saumures et d’eaux météoriques comportant du strontium et du sulfate. Michel Ory

Introduction Middle Devonian carbonate rocks near Oil Springs in southwestern Ontario were the first commercial oil reservoirs discovered in North America (Hamilton 1991, 2004). By the end of 2006, the cumulative oil production from Middle Devonian reservoirs totalled more than 7 million m3, and oil continues to be produced from six active pools (Lazorek and Carter, 2008). More than 50% of oil production in Ontario was recovered from dolomitized zones within the Middle Devonian Lucas and Dundee formations (Powell et al., 1984; Hamilton, 2004). Diagenetic processes, most importantly dolomitization, have played a major role in reservoir development and porosity enhancement in these carbonates. Thus, a better understanding of the origin and distribution of dolomite in Middle Devonian carbonates in southwestern Ontario is of interest to the petroleum industry. Page 42

The Middle Devonian Lucas Formation of the Michigan Basin was the subject of several studies (Hamilton, 1991; Birchard et al., 2004). Although dolomitization was responsible for the creation of the reservoir in dolomitized facies in these formations, few studies have dealt with dolomitization of these reservoirs (Hamilton, 1991). Stratigraphy and facies analysis of the Lucas and Dundee formations was the focus of most previous studies and dolomitization was not discussed in detail. Based on petrographic studies, facies analysis and limited geochemical data, Hamilton (1991) suggested reflux of evaporative brines to be responsible for forming fine crystalline dolomite, whereas the medium crystalline variety was postulated to have formed in a burial environment. The sabkha model was also suggested as a possible mechanism for dolomitization of the Lucas Formation (Fagerstrom, 1983). Using petrography, stable oxygen and carbon isotopes, strontium isotopes and fluid inclusion data, this paper presents a better understanding of the mechanism for dolomitization of the host rocks and later mineralized fractures in the Middle Devonian Lucas Formation. Late-stage mineralization is also examined in order to understand the possible timing of hydrocarbon migration and the nature of the fluids involved.

Geological Setting During the Palaeozoic Era, southwestern Ontario was located at tropical latitudes (van der Voo, 1988) and intermittently covered by inland seas that deposited a succession of carbonates, siliciclastics and evaporites. The Palaeozoic succession in the region consists of marine sediments ranging in age from Cambrian to Mississippian (Armstrong and Carter, 2006). The succession contains erosional and/or non-depositional gaps produced during regression, resulting in an incomplete stratigraphic record (Johnson et al., 1992). Southwestern Ontario is located between two major Palaeozoic sedimentary basins (Fig. 1). The Michigan Basin to the west is an intracratonic basin, and is carbonate-dominated with some evaporite successions. The Appalachian Basin, to the southeast, which is dominated by siliciclastics, is an elongate foreland basin that developed as a result of collisional tectonics along the eastern margin of the North American continent (Armstrong and Carter, 2006). During late Lower to early Middle Devonian time, the Michigan and Appalachian basins formed two large epicontinental seas, separated by the Algonquin Arch. A widespread marine transgression occurred during the early Middle Devonian (Emsian–Eifelian). At this time, southwestern Ontario was covered by a shallow sea that was characterized by restricted marine conditions, as indicated by absence of marine fauna, evaporative facies, supratidal, mud flat, shallow intertidal and subtidal lithofacies of the Lucas Formation (Gardner, 1974; Hamilton, 1991; Johnson et al., 1992; Birchard et al., 2004). Throughout most of the region, the Lucas Formation sharply and conformably overlies the Amherstburg Formation (Fig. 2). The Lucas Formation in the central part of the Michigan Basin consists of limestone and dolostone alternating with anhydrite and halite. The salt and anhydrite units that dominate in the central part of the basin wedge out over the Algonquin Arch eastward toward the O. Haeri-Ardakani, I. Al-Aasm, M. Coniglio and I. Samson

Figure 1. Generalized Paleozoic bedrock geology map of southern Ontario (modified after Armstrong and Carter, 2006). Inset shows generalized basement structural contours (metres above sea level datum) and location of structural arches and basins (modified after Johnson et al., 1992). Core (*) and quarry (×) locations are shown on the map.

basin margin, and are replaced by fine-crystalline dolostone with interbeds of anhydrite (Johnson et al., 1992; Sanford, 1993). Interlayers of anhydrite and pore spaces resulting from dissolution of evaporite minerals occur in the core samples in the studied area (Figs. 3A and B). Stromatolitic domes and anhydrite interbeds are commonly associated with this facies, which has been interpreted to indicate deposition in a shallow evaporitic environment similar to the supratidal environment of the Trucial coast in the Persian Gulf (Gardner, 1974; Hamilton, 1991; Birchard et al., 2004). Mineralization A belt of Mississippi Valley-Type (MVT) mineralization, hosted by Middle Silurian to Middle Devonian carbonate rocks extends from northeastern Indiana, through southeastern Michigan and southwestern Ontario. The main minerals in the mineralized areas are calcite, celestine, dolomite, fluorite, pyrite and sphalerite with minor amounts of barite, galena, quartz and native sulphur (Carlson, 1983). In the McGregor and Amherstburg quarries in southwestern Ontario, minor mineralization is present, and consists of celestine, Diagenesis Middle Devonian Ontario

fluorite, and calcite filling vugs and fractures (Fig. 4A, B). Minor amounts of sphalerite, pyrite and native sulfur (Fig.  4C) also occur, however there is no evidence of mineralization in Saint Mary’s quarry in the eastern part of the study area. Burial History A number of investigations of the thermal history of the Michigan Basin (Legall et al., 1981; Cercone, 1984; Vugrinovich, 1988; Cercone and Pollack, 1991; Coniglio et al., 1994; Luczaj et al., 2006) based on a variety of proxies (e.g. organic maturity data, stable isotopes, fluid inclusions) point to the occurrence of high temperatures (up to 200°C) in the past (Ma et al., 2009). Uncertainties in the thickness of eroded sediments, the timing of thermal maturation, and the magnitude of the past geothermal gradient make the burial history and thermal evolution of the Michigan Basin a controversial issue (e.g. Legall et al., 1981; Cercone, 1984; Nunn et al., 1984; Vugrinovich, 1988; Cercone and Pollack, 1991). A wide range of paleo-geothermal gradients (20–65°C/km) have been proposed to satisfy maturation of Devonian age source rocks and Pennsylvanian coals in the Michigan Basin (e.g. Legall et al., 1981; Cercone, 1984; Page 43

Vugrinovich, 1988). Based on burial curve reconstruction, Middle Devonian rocks in the northern and southern Michigan Basin (i.e. margins of basin) reached a maximum depth of 1500 m (Fig. 5), assuming erosion of 1000 m of Carboniferous strata (Cercone, 1984). Conodont and acritarch colour alteration studies indicate a maximum paleotemperature of 60°C for the top of the Middle Devonian interval in southwestern Ontario, which corresponds to burial depths of 1300 m for a geothermal gradient of 30°C/km and a surface temperature of 20°C (Legall et al., 1981). The organic maturity of Devonian source rocks (i.e. Marcellus and Kettle Point formations) in southwestern Ontario indicates that organic matter in these formations is immature to marginally mature and did not produce oil (Powell et al., 1984; Obermajer et al., 1997), which indicates that the maximum paleotemperature for Middle

Figure 2. Middle Devonian stratigraphy of southwestern Ontario (modified after Birchard et al. 2004).

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Devonian rocks in southwestern Ontario was not higher than 60°C, which is at the lower boundary of the thermal window for generation of oil (60 to 90°C; Tissot and Welte, 1984).

Materials and Methods Surface samples were collected from the McGregor and Amherstburg quarries near Windsor, and the Saint Mary’s quarry north of London, Ontario (Fig. 1). Core samples were obtained from the Ontario Oil, Gas and Salt Library, in London, Ontario. One hundred thin sections stained with Alizarin red S and potassium ferricyanide were examined using optical, ultra-violet and cathodoluminescence microscopy. For oxygen and carbon stable isotope analysis, approximately 5 mg of selected micro-drilled host rocks and their diagenetic phases were reacted with 100% pure phosphoric acid for four hours at 25°C for calcite and 50°C for dolomite, respectively. Mixed calcite and dolomite samples were treated using a sequential extraction technique (Al-Aasm et al., 1990). The resultant CO2 was measured for its oxygen and carbon isotopic ratios on a Delta plus mass spectrometer at the University of Windsor. Isotopic values are presented in δ-notation and reported relative to the

Figure 3. Core photograph of Middle Devonian rocks, southwestern Ontario. A) Anhydrite interlayer in dolomite, Domtar Goderich, St. 1. B) Needle-shaped dissolution molds of evaporate minerals in a groundmass of dolomite.

O. Haeri-Ardakani, I. Al-Aasm, M. Coniglio and I. Samson

VPDB standard. Reproducibility of isotopic measurements is better than ±0.05‰. Six doubly-polished wafers of calcite and celestine were prepared for fluid inclusion studies. To help avoid re-equilibration of fluid inclusions, a liquid-cooled diamond rotary saw was used for cutting of samples (Goldstein, 2003). Detailed fluid inclusion petrography was used to determine fluid inclusion origin (i.e. primary or secondary/pseudosecondary) using an Olympus BX51 microscope. To assess the presence of gases and hydrocarbons in inclusions, and characterize solid phases, Raman specroscopy was carried out using a 514 nm Ar ion laser with an ×50 objective at the University of Windsor. Fluid inclusion microthermometry measurements were carried out using a Linkam THMG 600 heating-freezing stage at the University of Windsor. Calibration with precision of ±1°C at 387°C and ± 0.1°C, at –56.6°C was conducted using synthetic H2O and CO2 fluid inclusion standards. To reduce the risk of stretching during freezing of inclusions, fluid inclusion heating experiments were conducted prior to cooling experiments. The 87Sr/86Sr isotope ratios were analyzed for a few selected calcite cements and matrix dolomites using an autmoated Finnigan 261 mass spectrometer equipped with nine Faraday collectors. Correction for isotopic fractionation was made by normalization to 86Sr/88Sr = 0.1194. The mean standard error of mass spectrometer performance was ± 0.00003 for standard NBS-987. Rare Earth Element (REE) concentrations were measured using an X series II ThermoFisher ICP-MS, subsequent to a general leach and digestion procedure. Each sample powder was weighed, reacted with 1% HNO3 acid, and then diluted with approximately 50 mg laboratory internal standards. Precision is in the range of 3.26% to 17%, and accuracy is in the range of 1.0% to 6.38%. The REE and strontium isotope analysis were carried out by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia. Measured values were normalized to Post-Archean Australian average Shale (PAAS).

Results Petrography Matrix Carbonates and Fossils The carbonate host rock consists of mudstone, skeletal grainstone, and peloidal grainstone. Mudstone generally is composed of non-ferroan microspar. Grainstones contain grains of stromatoporoids, brachiopods, echinoderms and rugose corals. Skeletal particles are generally preserved, and were in some cases replaced by chalcedony.

Figure 4. Cavities in host rock filled with diagenetic minerals, Amherstburg quarry. A) Cavity in the host rock dolomite filled with bladed crystals of celestine. B) Cavity in host rock dolomite filled with bladed crystals of celestine and fine-crystalline fluorite. C) Celestine and native sulfur in a smaller cavity. Diagenesis Middle Devonian Ontario

Calcite Cement Three types of calcite cement were identified (Fig. 6): 1) syntaxial calcite; 2) equant calcite cement with crystals ranging in size from 200–500 µm (Fig. 7A); and 3) blocky calcite, mainly filling fractures ranging in size from 500 µm to 5 mm. All calcite cements are non-ferroan. Calcite cements, with the exception of fracture-filling calcite, commonly fill interparticle, intra-skeletal and biomoldic pore spaces, and exhibit a uniform Page 45

dull orange luminescence. Blocky, fracture-filling calcite (type 3 above) is the most ubiquitous form of calcite cement in the studied samples. Calcite cement in fractures shows growth zones of bright and dark orange under CL (Fig. 7B). Dolomite Two main types of replacive dolomite have been distinguished based on petrographic observations. The first, occurring throughout the study area is a fine-crystalline, matrix dolomite with non-planar to planar-s crystals ranging in size from 5–20 μm that has mainly replaced mudstone facies, and in some samples replaced peloids (Fig. 7C and D). The extent of finecrystalline dolomitization varies from 5–100% of rock volume. The second type of dolomite is a medium-crystalline (50– 150 μm, average size of 100 μm) planar-e dolomite with cloudy cores and clear rims that has limited distribution (Fig.7E and F). Medium-crystalline dolomite replaced bioclasts and calcite cement and in some samples is associated with solution seams

(Fig. 7F). Both dolomite types have homogeneous dull-red cathodoluminescence. Celestine and Fluorite Bladed crystals of celestine mainly occur in vugs of variable size, which range from 100 μm–50 mm. In addition, in some samples, celestine fills micro-fractures. Small crystals of fluorite mainly are found in solution vugs and breccias with celestine and calcite. Fluorite crystals generally show pronounced growth zonation and range in size from 20–40 μm. Fluid Inclusions Fracture-filling calcite cement and celestine samples were selected for fluid inclusion study. The medium-crystalline dolomite contains few small inclusions (