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Abstract: Fractography Applied to Investigations of Cores, Outcrops, and Fractured Reservoirs

Byron Kulander

Fractography focuses investigations on the topography of fracture surfaces. This topography is composed of fractographic features produced by changing stress magnitudes and directions along the advancing crack tip. Fractographic features commonly useful in core and outcrop analysis include the origin, twist hackle, inclusion hackle, and rib marks. These structures develop during brittle failure by Mode I loading at the crack tip and act together to form a hackle plume. Fractographic components throughout the plume record the dynamic history of fracture development. Components show, to the limit of visual scale, the principal stress directions, as well as relative stress magnitudes and propagation velocities, that existed at the advancing fracture front. This information c ntributes to more meaningful conclusions in fracture investigations.

In core studies, fractography aids identification of induced and natural fractures. Induced fractures and fractographic features show distinct geometry with that of the core and reflect the effects of the core boundary, in-situ stresses, drilling stresses, and rock anisotropies. Certain drilling- and coring-induced fractures possess orientations and fractographic features that suggest the direction of minimum in-situ stress and that this direction may change abruptly within the drilled volume of rock. Cored natural fractures generally originated away from the bit and possess fractographic features that bear no geometrical relationship to core parameters. However, fractographic information from a small segment of the fracture may permit extrapolation of fracture characteristics away fr m the well. Abrupt changes of natural fracture strike and development of twist hackle suggest locally complex paleostress distributions. A combined knowledge of in-situ stress and natural fracture trends is useful in predicting reservoir permeability.

In outcrop, fractographic features, including abutting relationships between joints, more readily depict order of development, intrastratum distribution of fracturing stress, and size for joints in any set. Outcrop and core data may permit partitioning of joint patterns into domains separated by distinct domain boundaries. The unique joint signature within a domain reflects stresses responsible for joint development. In-situ stress and joint-derived paleostress trends may differ. Nevertheless, both joint and in-situ stress trends might be partitioned into domains by the same domain boundaries. This congruity suggests the long-standing effect of regional geological features on stress configurations.

AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers