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January 01, 1990


The following sets of conditions indicate that associated fractures in core may have been induced naturally.

  • (1)

    Polished and slickenlined fracture faces in well-lithified rock usually are a clear indicator of natural fracture origin (Figure 6a). However, polished surfaces, perpendicular to the core axis and accompanied by curved slickenlines, can be induced during the coring process (Figure 6b).

  • (2)

    Marker bed offsets along faults that have not lost cohesion indicate a natural fracture even though the fault surface in this case is not visible (Figure 5). Such faults may have developed in soft sediments prior to lithification.

  • (3)

    Secondary mineral growths on the walls of fractures clearly indicate that the fractures were naturally induced (Figures 1 and 2).

  • (4)

    Scribe-grooved fractures and conchoidal chips that originate at the scribe knife and that curve (hook) abruptly into a nearby fracture indicate a natural origin. However, drilling-induced fractures that originate before the bit, or any fracture that developed before the scribe, can also show this scribe-chip relationship (Figure 24b).

  • (5)

    Natural fracture faces that are parallel or subparallel to the core axis may be cut by small conchoidal chips at the fracture-core margin intersection. These chips are located preferentially along the right-hand edge of the fracture face when the face is held toward the observer with the downcore direction pointing downward. The chips are formed by the plucking action of the drill bit rotating clockwise. Drilling-induced fractures originating before the bit may also show chipped right-hand margins (Figure 29).

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AAPG Methods in Exploration

Fractured Core Analysis: Interpretation, Logging, and Use of Natural and Induced Fractures in Core

B. R. Kulander
B. R. Kulander
Department of Geological Sciences Wright State University Dayton, Ohio, U.S.A.
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S. L. Dean
S. L. Dean
Department of Geology University of Toledo Toledo, Ohio, U.S.A.
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B. J. Ward, Jr.
B. J. Ward, Jr.
Amoco Production Company Research Center Tulsa, Oklahoma, U.S.A.
Currently with Associated Fracture Consultants, Broken Arrow, Oklahoma, U.S.A.
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American Association of Petroleum Geologists
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Publication date:
January 01, 1990




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