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

SUMMARY AND DISCUSSION OF FRACTURING MECHANISMS

The consistent northwest-southeast and northeast-southwest orientations of fracture sets in the Antrim Shale and adjacent Devonian strata across the northern margin of the Michigan basin probably originated from continent-scale compressional stress fields (Holst, 1982; Decker and others, 1992). The most plausible of the continent-scale stress fields are: 1) a northwest-southeast oriented far-field compressional stress caused by late Paleozoic Alleghanian plate collision (Craddock and van der Pluijm, 1989) and 2) a northeast-southwest oriented post-early Mesozoic compressional stress caused by lithosphere-asthenosphere interactions (Holst, 1982). The latter stress orientation is expressed by the modern in situ state of stress in the midcontinent region (Zoback and Zoback, 1980). Additional factors contributing to the origin or accentuation of the fracture sets may be post-Paleozoic uplift of the Michigan basin and abnormally high formation pressures created by hydrocarbon generation in the organic-rich Antrim Shale during maximum burial (Apotria and others, 1993, 1994; Richards, Walter, and others, 1994).

According to Apotria and others (1993, 1994), the northwest-southeast fracture set is the oldest as shown by its continuity and abutting relationships. They further suggest that fractures in this set formed as natural hydraulic fractures during northwest-southeast oriented Alleghanian compressive stress near peak burial and thermal maturation. Fracturing occurred preferentially in the black shales because of their low Poisson's ratio and their probable high fluid pressure owing to gas generation (Apotria and others, 1993, 1994). The northeast-southwest fracture set, according to Apotria and others (1993, 1994), formed during post-Alleghanian uplift of the Michigan basin under a state of northeast-southwest oriented maximum horizontal compression. Cooling and unloading reduced the minimum horizontal stress to form extension fractures (Apotria and others, 1993, 1994). This thermoelastic contraction mechanism is strongly influenced by Young's modulus (Apotria and others, 1993, 1994).

Northwest-directed Alleghanian compression, suggested by Apotria and others (1993, 1994) as a primary cause of the regional northwest-southeast fracture set, is supported by regional petrofabric studies of Craddock and van der Pluijm (1989). Moreover, northwest-directed Alleghanian compression may have created the dominant, basement-cored, northwest-trending anticlines. One possible mechanism is that an unrecognized Proterozoic basement fabric--such as one formed during Grenvillian compression of Midcontinent rift structures (Cannon, 1994)--was reactivated by Alleghanian tectonics.

Fractures imprinted on the regional northwest-southeast and northeast-southwest fracture sets by local flexures and anticlinal noses increase the diversity of fractures along the Antrim Shale gas-producing trend (Decker and others, 1992). Fracture diversity is improved by the addition of north-south and(or) east-west fracture sets and dip angles that range from 60° to horizontal. The locally derived fractures may be caused by a variety of mechanisms such as basement faulting, differential compaction, and local bedding plane detachment. An additional fracturing mechanism--whereby horizontal microfractures are caused by regional compressional stresses and overpressured black shale source beds (Vernick, 1994)--may also apply to the Antrim Shale.

Fracture frequency and aperature widths are greatest in the black shale members (Richards, Walter, and others, 1994), and consistently, the northeast-southwest fracture set has a higher fracture frequency than the northwest-southeast fracture set for a given stratigraphic interval. The importance of the northeast-southwest fracture set for gas productivity in the Antrim Shale is demonstrated by intrinsic permeability calculations, distribution of gas production "sweet spots", and the distribution of chemical ions in produced formation water. Therefore, gas production may be maximized by drilling directional, subvertical wells oriented normal to the northeast-southwest fracture set. Completions in the Norwood and Lachine Members are preferred because of the higher frequency and greater aperture width of their fractures. Well bore maps such as those shown by Cain(1991) and Caramanica (1993) help to identify zones of high fracture frequency and diversity prior to completion. Although fracture diversity seems to be important for high gas production in the Antrim Shale it is difficult to predict prior to drilling even where a detailed structure map is available.

Because fractures, undoubtedly, play a major role in the control of "sweet spots" in the Antrim Shale gas-producing trend it is imperative that we learn more about their character and distribution. The more that is known about the fractured Antrim Shale reservoir, the more efficient is the exploration, development, and assessment of its gas resource. A major need is to predict "sweet spots" ahead of the drill, particularly, in parts of the Michigan basin where the Antrim Shale is presently unproductive. In addition, gas productivity measurements are needed in wells where a variety of fracture characteristics and patterns are carefully documented. Especially needed are gas wells with long-term production. These data are necessary to test existing fracture-generation models and to formulate new ones. Moreover, these data show the degree of continuity in fracture trends and associated gas production across a given part of the accumulation.

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