Fractured hydrothermal AUTHORS John A. Luczaj Department of Natural and Ap- dolomite reservoirs in plied Sciences, University of Wisconsin–Green Bay, Green Bay, Wisconsin 54311; [email protected] the Devonian Dundee John Luczaj is an assistant professor of earth science in the Department of Natural and Applied Sciences at the University of Wisconsin–Green Bay. He earned Formation of the central his B.S. degree in geology from the University of Wisconsin–Oshkosh. This was followed by an M.S. Michigan Basin degree in geology from the University of Kansas. He holds a Ph.D. in geology from Johns Hopkins Uni- versity in Baltimore, Maryland. His recent interests John A. Luczaj, William B. Harrison III, and include the investigation of water-rock interaction in Natalie Smith Williams Paleozoic sedimentary rocks in the Michigan Basin and eastern Wisconsin. Previous research activities involve mapping subsurface uranium distributions, reflux dolomitization, and U-Pb dating of Permian ABSTRACT Chase Group carbonates in southwestern Kansas. The Middle Devonian Dundee Formation is the most prolific oil- William B. Harrison III Michigan Basin Core producing unit in the Michigan Basin, with more than 375 million bbl Research Laboratory, Western Michigan University, of oil produced to date. Reservoir types in the Dundee Formation Kalamazoo, Michigan 49008; [email protected] can be fracture controlled or facies controlled, and each type may William B. Harrison, III, is the director of the Michigan have been diagenetically modified. Although fracture-controlled res- Basin Core Research Laboratory and is professor emeritus in the Department of Geosciences at West- ervoirs produce more oil than facies-controlled reservoirs, little is ern Michigan University. He is also the director of known about the process by which they were formed and diageneti- the Michigan Center of the Midwest Region of the cally modified. Petroleum Technology Transfer Council. He holds In parts of the Dundee, preexisting sedimentary fabrics have a Ph.D. in paleontology and sedimentology from the been strongly overprinted by medium- to coarse-grained dolomite. University of Cincinnati. His interests include pa- Dolomitized intervals contain planar and saddle dolomite, with leontology and stratigraphy of Ordovician and Silurian carbonates in the central United States, minor calcite, anhydrite, pyrite, and uncommon fluorite. Fluid- oil and gas resources of the Michigan Basin, Devo- inclusion analyses of two-phase aqueous inclusions in dolomite nian stratigraphy and depositional facies of the and calcite suggest that some water-rock interaction in these rocks Michigan Basin, and methods of improved oil recov- occurred at temperatures as high as 120–150jC in the presence of ery from depleted or abandoned oil and gas fields. dense Na-Ca-Mg-Cl brines. These data, in conjunction with pub- Natalie Smith Williams Department of lished organic maturity data and burial reconstructions, are not Geosciences, Western Michigan University, Kala- easily explained by a long-term burial model and have important mazoo, Michigan 49008 implications for the thermal history of the Michigan Basin. The Natalie Smith Williams holds an M.S. degree in earth data are best explained by a model involving short-duration trans- science from Western Michigan University and a port of fluids and heat from deeper parts of the basin along major B.A. degree in geology from DePauw University. fault and fracture zones connected to structures in the Precam- brian basement. These data give new insight into the hydrothermal ACKNOWLEDGEMENTS processes responsible for the formation of these reservoirs. We acknowledge a grant from the U.S. Department of 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 the University of Wisconsin–Green Bay. Drill cores and other materials were available through the Michigan Basin Core Research Laboratory. The authors also Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. thank James Wood, Robert Gillespie, and David Barnes Manuscript received May 5, 2005; provisional acceptance December 1, 2005; revised manuscript for ideas and discussion regarding this research and received June 14, 2006; final acceptance June 27, 2006. James Duggan for reviewing the manuscript. DOI:10.1306/06270605082 AAPG Bulletin, v. 90, no. 11 (November 2006), pp. 1787–1801 1787 INTRODUCTION part of the basin. Regionally extensive dolomite in the Dundee Formation is mainly present in the western The Michigan Basin is the classic example of an intra- parts of the basin, although most oil-producing dolo- cratonic sedimentary basin. It contains as much as mitized reservoirs of the Dundee Formation are lo- 5 km (3.1 mi) of Paleozoic and Mesozoic sediments cated in the central part of the basin (Gardner, 1974), that include carbonate, siliciclastic, and evaporite sedi- where the maximum production appears to be related ments (Sleep et al., 1980). The Devonian Dundee For- to fractured, vug-bearing intervals (Montgomery et al., mation presently lies 3200–4000 ft (975–1200 m) 1998). The fields in the central Michigan Basin are in- below the surface in the study area in the central part terpreted as discrete structures with a similar style of of the Michigan Basin (Figures 1, 2). Although the faulting present among various fields. Montgomery Dundee Formation is formally undifferentiated in the et al. (1998) presented additional geologic, stratigraph- subsurface (Catacosinos et al., 2001), it is correlative ic, and production data on the Dundee Formation in to both the Rogers City and Dundee formations along the Michigan Basin. the outcrop belt. However, the Rogers City and Reed The Middle Devonian Dundee Formation is the City units have typically been used as informal mem- most prolific oil-producing unit in the Michigan Basin, ber names to describe parts of the Dundee Formation with more than 375 million bbl of oil produced to date (Figure 2) for subsurface investigations. It consists of from 137 fields, with about half of that production mudstones through grainstones deposited along a car- coming from dolomite-hosted reservoirs (Gardner, 1974; bonate bank and open-marine environment in the cen- Curran and Hurley, 1992; Montgomery et al., 1998; tral and eastern parts of the basin, with lagoon and Wylie and Wood, 2005). Reservoir types can be frac- sabkha-type environments dominant in the western 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 Devonian Dundee Formation, Michigan Basin. Structure contours are on top of the Dundee interval. Details of drill-core localities are described in Table 1. Modified from Montgomery et al. (1998). 1788 Devonian Fractured Hydrothermal Dolomite Reservoirs have been diagenetically modified. Although fracture- in regard to the conditions of fracturing and mineraliza- controlled reservoirs produce more oil than facies- tion (Prouty, 1983; Montgomery et al., 1998). This study controlled reservoirs, little is known about the process presents the first quantitative data on the temperature by which they were formed and diagenetically modified. of dolomitization and the characteristics of fractures in The earliest known reference regarding Devonian Devonian Dundee reservoirs of the Michigan Basin. fractured dolomite reservoirs in the Michigan Basin was a brief article on the Deep River field by Lundy (1969, p. 62). Lundy described the Rogers City–Dundee PURPOSE oil production as being from ‘‘anomalous secondary do- lomites believed developed along a fractured and broken Cores of the Dundee Formation from nine localities in zone in the Rogers City limestone.’’ The dolomitized the central Michigan Basin were examined (Figure 1; Deep River field is considered anomalous because it oc- Table 1) to document the distribution and character- curs in the eastern region of the basin where the Dundee istics of fracturing in the Dundee Formation, the style is mainly limestone. The dolomite occurred along a of fracturing present in these reservoir rocks, and their linear N60jW-trending zone approximately 5.5 mi relationship to rock type, grain size, and epigenetic min- (8.8 km) long but less than 0.5 mi (0.8 km) wide. eralization. Fluid-inclusion microthermometric meth- Cuttings were described as fine-grained brown matrix ods were used to document the temperature of saddle dolomite with medium-size white rhombic dolomite dolomite precipitation in an attempt to place constraints crystals. Prouty (1983) also proposed fracturing in the on the conditions of mineralization and reservoir devel- Dundee as a conduit for dolomitizing fluids in several opment in the Devonian Dundee Formation of the cen- central basin structures. Examples of prolific oil fields tral Michigan Basin. 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 METHODS of faulting has been identified. Recent unpublished work (e.g., Wood and Harrison, Parts of the slabbed drill core from nine wells were 2002) using production data and structural data derived examined for the distribution of dolomite and the char- from geophysical logs has suggested that some central acteristics of fractures. Alizarin red stain was used to basin