Fracture patterns and their origin in the Upper Devonian Antrim Shale gas reservoir of the Michigan basin: A review
Robert T. Ryder
Open-File Report 96-23
The Antrim Shale crops out along the northern margin of the Michigan basin within 10 to 15 mi updip of the regional gas accumulation (Komar, 1981; Walter and others, 1993)(fig. 1). Locally, there is partial overlap between the outcrop belt and the gas accumulation. Given these conditions, a reasonable first approximation of fracture characteristics in the Antrim Shale gas reservoir can be obtained from fractures observed in Antrim Shale (~200 ft thick) outcrops and in underlying limestone outcrops of the Middle Devonian Traverse Group (~750 ft thick) and Middle and Upper Devonian Squaw Bay Limestone (~30 ft thick)(fig. 2). Nearly 5000 measurements by Holst and Foote (1981) and Holst (1982) at 43 localities across northern Michigan show that the dominant joint sets in Devonian strata are oriented northeast-southwest (average orientation 052°) and northwest-southeast (average orientation 314°). Two additional, but subordinate, joint sets are oriented north-south (average orientation 002°) and east-west (average orientation 272°). These four joint sets maintain consistent orientations across the northern margin of the basin irrespective of geographic and stratigraphic position (Holst and Foote, 1981; Holst, 1982). Approximately 95% of the measured joints have vertical or sub-vertical dips.
Fracture orientation, fracture spacing, and fracture aperature width were measured at the Paxton quarry (fig. 1) at the east end of the northern Antrim Shale outcrop belt in Michigan (Richards, Walter, and others, 1994). The same dominant northeast-southwest and northwest-southeast fracture sets recognized by Holst and Foote (1981) and Holst (1982) were identified at the quarry in the Norwood and Lachine (~15 and 60 ft thick, respectively)(black shale) Members of the Antrim Shale (fig. 2). Only the northeast-southwest fracture set was recognized in the Paxton (gray shale) (~15 ft thick) Member (fig. 2) of the Antrim Shale at the quarry. The observed fracture spacing in the northeast-southwest set is 0.4 m (±0.02) for the Norwood Member, 0.2 m (±0.1) for the Paxton Member, and 1.1 m (±0.2) for the Lachine Member (Richards, Walter, and others, 1994). In the northwest-southeast set, the fracture spacing is 0.9 m (±0.3) for the Norwood Member and 2.0 m (±0.5) for the Lachine Member. For each of these three members of the Antrim Shale, the fracture spacing is always greater in the northwest-southeast set. The aperture width of fractures measured in the quarry ranges from about 0.3 mm to about 8 mm (Richards, Walter, and others, 1994).
Natural fractures in the subsurface part of the Antrim Shale can be detected and characterized with oriented and open-hole logging tools (acoustic, resistivity, and televiewer) (Decker, 1992, Caramanica, 1993). In wells where both core and fracture identification logs are available, fractures identified by each method have comparable spacing, orientation, and openness; however, the number of fractures detected in the core is usually greater than the number of fractures detected from the well logs (Manger and Curtis, 1991; Decker, 1992; Decker and others, 1992). Probably, more fractures are identified in core than by well logs because, with well logs, horizontal fractures are difficult to distinguish from bedding planes (Manger and Curtis, 1991; Decker and others, 1992).
Richards, Walter, and others (1994) measured over 600 fractures from oriented core, fracture identification logs, and outcrop along the Antrim Shale gas-producing trend. Results of the outcrop measurements at the Paxton quarry are summarized in the previous section. Stratigraphic intervals involved in the study are the lower three members of the Antrim Shale (Norwood, Paxton, and Lachine), the overlying Upper Devonian Ellsworth Shale, and the underlying Middle Devonian Traverse Group and Middle and Upper Devonian Squaw Bay Limestone (fig. 2). Fracture and joint types measured by Richards, Walter, and others (1994) consist of major (dip >80°), intermediate (dip >80°), and small joints (dip >60°); bedding joints (dip <30°); and microfractures (dip variable). The joint surfaces are relatively smooth and commonly are lined with calcite cement. Large joints and fractures are rarely occluded by cement.
Two dominant fracture sets are noted in the study by Richards, Walter, and others (1994): the set with the highest relative frequency has a northeast-southwest orientation (average orientation 052° ±5°) and the second set has a northwest-southeast orientation (average orientation 312° ±5°). Although there are slight variations, the orientation of each dominant fracture set remains very consistent between stratigraphic intervals. These fracture orientations are nearly identical to those reported in outcrop by Holst and Foote (1981), Holst (1982), and Richards, Walter, and others (1994). Moreover, the spacing of subsurface fractures in the northeast-southwest (0.4-0.7 m ±0.2) and northwest-southeast (0.5 m ±0.2 to 2.3 m ±1.0) fracture sets in the Norwood, Paxton, and Lachine Members of the Antrim Shale are very similar to those measured in the Antrim Shale at the Paxton quarry (Richards, Walter, and others, 1994). Whereas subsurface fractures in the Antrim Shale, Squaw Bay Limestone, and Traverse Group have similar spacing, those in the Ellsworth Shale (2.9 m ±0.7 for the northeast-southwest set and 3.5 m ±0.8 for the northwest-southeast set) have a much greater spacing. For a given stratigraphic interval, fracture spacing measured in the subsurface--and in outcrop at the Paxton quarry as noted in the previous section--is always greatest in the northwest-southeast fracture set. The aperture width of the subsurface fractures reported by Richards, Walter, and others (1994) ranges from less than 0.001 mm to about 0.75 mm and the mean width is a function of stratigraphic interval.
Based on these fracture-spacing and aperture-width measurements, Richards, Budai, and others (1994) calculated that the Norwood Shale Member has the largest intrinsic permeability (2.0 x 10-11 m2 for the northeast-southwest set and 1.4 x 10-11 m2 for the northwest-southeast set) and the Ellsworth Shale has the lowest intrinsic permeability (4.0 x 10-15 m2 for the northeast-southwest set and 1.3 x 10-15 m2 for the northwest-southeast set). In all the stratigraphic intervals, the northeast-southwest fracture set is associated with the highest intrinsic fracture permeability.
Additional fractures were described and interpreted from six wells in Otsego County, Michigan (fig. 3) using core and formation microscanner logs (Cain, 1991; Decker and others, 1992; Caramanica, 1993). Fractures in one of the wells, the Latuszak B1-32, also were included in the analysis by Richards, Walter, and others (1994). Borehole maps of the Latuszak B1-32 well (Cain, 1991) and the Rachow B3-31 well (Caramanica, 1993) illustrate many of the complexities of the fracture patterns such as spacing, dip angle and direction, diversity of orientation, and fracture intersections (figs. 4, 5). Decker and others (1992) and Caramanica (1993) conclude that a northeast-southwest oriented fracture set is dominant in four of the six wells studied; in a fifth well, both northeast-southwest and northwest-southeast fracture sets are dominant and in a sixth well, the dominant fracture set is oriented east-west. The dip angle of the fractures range from subvertical to horizontal. An analysis of open fractures vs. total fractures indicates that the most abundant total fractures and open fractures are in the Lachine Member (Caramanica, 1993). Moreover, Decker and others (1992) conclude that the Lachine Member has the greatest diversity of fracture orientations.
Dellapenna (1991) and Dellapenna and Harrison (1993) report fracture characteristics in the cored MI-2 State Chester 18 well in southern Otsego County (fig. 3). Fractures are most common in the Norwood and Lachine Members where they show a dominant northeast-southwest set and a secondary northwest-southeast set. Most of the fractures are subvertical and several are lined with calcite. The small number of fractures in the Squaw Bay Limestone, Paxton Member, and Ellsworth Shale are characterized by a dominant east-west set and secondary northeast-southwest and northwest-southeast sets. Fracture frequency, expressed in fractures/ft, is 1.33 in the Norwood Member and 0.87 in the lower part of the Lachine Member. Fracture frequency is much lower in the Squaw Bay Limestone (0.29), Paxton Member (0.09), upper part of the Lachine Member (0.31), and the Ellsworth Shale (0.06 to 0.11).
Fractures in Antrim Shale cores from the Amoco No. 1-28 Swift and No. MI-1 Union wells, Otsego County (not located on fig. 3) have been recorded by Nelson (1985). The orientations of fourteen calcite-mineralized, vertical fractures in the No. MI-21 Union well show a dominant northeast-southwest set and a secondary northwest-southeast set. Nonvertical fractures in the No. 1-28 Swift well , plotted on a stereographic projection of poles to fracture planes, demonstrate a high diversity of fracture intersections in the well bore.
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