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
REGIONAL AND LOCAL STRUCTURAL DOMAINS ACCOMPANYING THE ANTRIM SHALE GAS ACCUMULATION
Regional and local tectonic events in the vicinity of the Michigan basin, and their resultant structural styles, must be carefully evaluated as possible controls on fractures in the Antrim Shale. Very likely, one or more of these events have had an impact on the nature and origin of the Antrim Shale fractures. First, pre-Michigan basin Proterozoic tectonics may have left diagnostic structures in basement rocks that became reactived by later tectonic events. Secondly, Paleozoic compressional stresses imposed on the sedimentary sequence of the Michigan basin by distant orogenic events, such as the Alleghenian orogeny, may have reactivated the pre-basin structures and(or) introduced new structural fabrics. Thirdly, post-basin uplift and cooling of the crust in Mesozoic and post-Pleistocene times and modern intraplate compression may have accentuated pre-existing fracture patterns or, by modifying existing stress trajectories, may have created additional sets of fractures. These suggested tectonic events and terranes that may be linked to fractures in the Antrim Shale are summarized briefly in this section.
Proterozoic Basement Terranes and Tectonic Trends
The Paleozoic sedimentary cover of the Michigan basin is underlain by four Proterozoic terranes of the Laurentian continent that represent a complex history of orogenic, anorogenic, and extensional events (fig. 6). Because few holes have been drilled into basement rocks of the Michigan basin, the lithologic character and structural grain of the provinces, and the nature of the tectonic contacts between them, depend largely on outcrops in nearby States and on geophysical evidence (Hinze and others, 1975; Brown and others, 1982; Fisher and others, 1988; Green and others, 1988).
The Penokean Orogen is the oldest of the four terranes. This orogen is an Early Proterozoic (~1.85 Ga) magmatic belt that borders the southern margin of the Late Archean Superior Province in northern Wisconsin and the adjoining upper peninsula of Michigan (Van Schmus, 1992). Presumably, the Penokean Orogen extends eastward from central Wisconsin into northern Michigan and underlies most of the northern half of the Michigan basin (Budai and Wilson, 1991; Van Schmus, 1992). Anorogenic granitic plutons and felsic volcanic rocks of the Middle Proterozoic Eastern Granite-Rhyolite Province (~1.48 Ga) occupy most of the southern half of the Michigan basin (Budai and Wilson, 1991; Van Schmus, 1992). The transitional contact between the Eastern Granite-Rhyolite Province and the Penokean Orogen probably trends approximately east-west across the center of the Michigan basin (Budai and Wilson, 1991; Van Schmus, 1991). A north- to northwest-trending rift complex of Middle Proterozoic (~1.1 Ga) red beds and basalt cuts diagonally across the Eastern Granite-Rhyolite Province and the Penokean Orogen (Budai and Wilson, 1991; Van Schmus, 1991). The rift, considered to be a southeastern arm of the Midcontinent Rift System, was first identified by regional gravity surveys (Hinze and others, 1975) and later confirmed by a 17,466 ft-deep basement test (fig. 6)drilled in the center of the Michigan basin (Fowler and Kuenzi, 1978; Fisher and others, 1988). The extensional event that formed the Midcontinent Rift System was closely followed by a major collisional event that created the Middle Proterozoic (~1.0 Ga) Grenville orogenic belt. The leading edge of the thrust-faulted Grenville Province trends north to northeasterly across the eastern edge of the Michigan basin and thereby truncates or overrides adjoining parts of the three older provinces (fig. 6). Cannon (1994) proposes that the northwest-directed Grenvillian compression reactivated border faults of the Midcontinent Rift System; thrust faulting and tectonic inversion occurred along the main southwest-trending arm of the rift, to the west of the Michigan basin, whereas mostly strike-slip faulting occurred along the southeast-trending arm.
Structural Trends in Devonian Strata
Structure contour maps drawn on top of the Traverse Group and the Antrim Shale (Fisher, 1980; Fisher and others, 1988) show that Devonian strata in the Michigan basin have been folded into prominant anticlines with northwest-southeast trending axes (fig. 7). Many of the anticlines can be traced for over 50 mi and closely spaced contour lines along their flanks suggest that some of them have faulted limbs (Decker and others, 1992). One of these anticlines, a southeast-plunging anticlinal nose with a steeply dipping or faulted western limb, runs through the center of the Antrim gas-producing trend in western Otsego County and adjoining Crawford County (Decker, 1992). Fisher and others (1988), based on their east-west oriented diagrammatic cross section across the southern Michigan basin, suggest that the anticlines are underlain by horst blocks in the Proterozoic basement rocks. Although many of the large anticlines, anticlinal noses, and monoclines on the structure contour map (fig. 7) probably are underlain by basement fault blocks, their dominant northwest-southeast trend is not strongly aligned with any of the recognized tectonic trends of the Proterozoic basement, but most closely matches the north- and northwest-trending eastern arm of the Midcontinent rift system. Perhaps the anticlinal structures and the probable fracture sets associated with them were inherited from reactivated fault zones that originally were created or modified along the margins of the Midcontinent rift by later Grenvillian compression (Cannon, 1994).
Structure contours (C.I.=100 ft) on the Traverse Group across a 28-township area of the gas-producing trend in Otsego County and adjoining Antrim and Montmorency Counties (Decker and others, 1992)(fig. 8) show numerous small-scale flexures in addition to the larger southeast-plunging anticlinal nose recognized on the 1:1,000,000 scale maps of Fisher (1980). Decker (1992) and Decker and others (1992) suggest that these small-scale structures may be a primary cause of Antrim Shale fractures. An even more detailed structure contour map (Manger and others, 1990), drawn on top of the Lachine Member (C.I.=25 ft) for a four-township area in southern Otsego County near the center of the previous map, shows a broad southward-plunging anticlinal nose whose general trend is interupted by numerous small-scale structural terraces and flexures (fig. 9). Five of the six wells used for fracture studies by Decker and others (1992) and Caramanica (1993) are located on the anticlinal nose. Variations in gas production across the 28- and 4-township structural contour maps and their implications for the distribution of open fractures in the Antrim Shale are presented in a subsequent section.
Effects of Alleghanian Compression
Numerical rheological models of eastern North America by Quinlan and Beaumont(1984) suggest that lithospheric downwarping and flexure of the Appalachian foreland basin is caused by loading of the crust by Alleghanian overthrusts. Flexural interactions between the Appalachian basin and the craton very likely influenced sedimentation patterns and structural features in the Michigan basin. Craddock and van der Pluijm (1989) have measured subhorizontal shortening fabrics in Paleozoic strata as much as 500 mi away from the Appalachian-Ouachita orogenic front. They suggest that these fabrics were caused by late Paleozoic compressional stresses due to the Alleghanian orogeny rather than by stresses due to lithospheric loading of the craton. Thus, far-field Alleghanian compressional stress and associated strains may have had an important effect on Michigan basin tectonics and, consequently, on fractures in the Antrim Shale (van der Pluijm and Craddock, 1993).
Modern In Situ Stresses of the Northern Midcontinent
Modern in situ maximum horizontal compressional stress in the crust of the midcontinent region is oriented northeast-southwest (Haimson, 1978; Zoback and Zoback, 1980). Holst (1982) concludes that the present-day stress orientation is certainly younger than early Mesozoic (Sbar and Sykes, 1973) and is best explained by lithosphere-asthenosphere drag beneath the North American plate or by a ridge-push force from the Mid-Atlantic Ridge (Voight, 1969; Zoback and Zoback, 1980). Although the present in situ stress is oriented normal to fold axes in the Devonian strata (fig. 7), it seems to be clearly too young to have caused the folds. However, the modern in situ stress field may be the cause of the prominent northeast-southwest fracture set observed in the outcrop and subsurface. The orientation of the modern stress field and the northeast-southwest fracture set are closely aligned and the timing, although less constrained, is certainly plausible. Possibly, the northeast-southwest fracture set is very young (Holst, 1982).
Post-Paleozoic Uplift of the Michigan Basin
Maturation indices (0.54 to 0.60% Ro) of the Pennsylvanian Saginaw coal and Lopatin modeling of the burial and thermal history of the Michigan basin suggest as much as to 3,280 ft of Carboniferous and(or) Permian strata were removed prior to the Late Jurassic (Cercone, 1984). Using similar vitrinite reflectance data, Apotria and others (1993) suggest that a net uplift of over 4,000 ft has occurred in the northern part of the Michigan basin since the Permian. This amount of uplift may significantly alter the magnitude and possibly the orientation of pre-existing principal horizontal stresses and associated fractures in the basin.
Post-Pleistocene isostatic adjustment of the crust following glacial retreat accounts for as much as 200 ft of uplift in the northern Michigan basin (Farrand, 1962). Goldthwait (1908) explained raised beach terraces of earlier glacial lakes according to a model that involves a single southern outlet and a west-northwest oriented hinge line that extended across the center of the Michigan basin. The hinge line separates the northern part of the Michigan basin having active post-glacial rebound from the southern part of the basin having little or no rebound. However, in a recent study, Larson (1987) interprets the array of raised beach ridges in terms of multiple lake outlets that preclude the need for a glacial hinge zone. Larson's (1987) study places in question the hypothesis by Matthews and Jones (1994) that preferred areas of fracturing in the Antrim Shale are coincident with one or more glacial hinge lines. Unloading of stored elastic strain in the Antrim Shale caused by glacial rebound may create horizontal (sheeting) fractures and open and(or) enlarge pre-existing sub-vertical fractures.
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