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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 48 (1964)

Issue: 4. (April)

First Page: 540

Last Page: 540

Title: Fissility in Argillaceous Rocks: ABSTRACT

Author(s): Neil R. O'Brien

Article Type: Meeting abstract


A study was undertaken to determine the cause of fissility and non-fissility in argillaceous rocks such as shale and underclay. The orientation of clay flakes in samples of Pennsylvanian shales and underclays was studied by using an X-ray diffractometer. It was found that clay mineral orientation is a factor in determining the degree of fissility of an argillaceous rock.

Sedimentation experiments were run in the laboratory to study the influence of various factors upon the development of fissility. Clay materials of various concentration were sedimented in different cation concentrations. Regardless of clay or cation concentration, the clay material flocculated. Various amounts of silt were sedimented on top of the flocculated clay masses. Preferred clay mineral orientation was produced in clay material on which silt was rapidly sedimented. Random orientation was preserved in clay material which had dewatered before a small amount of silt was applied.

Samples of clay were also compressed to study the influence of overburden pressure and water content upon the development of fissility. It was found that clay with a lower water content showed less reorientation due to compression than clay with a higher content. This fact suggested that the amount of liquid water relative to rigid water in the clay-water system at the time of application of overburden pressure may be an important factor in facilitating the orientation of clay flakes.

It is concluded that preferred orientation of clay flakes commonly found in fissile shales may have resulted from compression of flocculated clay mud under conditions of rapid sedimentation. The random clay mineral orientation present in underclay may have resulted as a flocculated clay accumulated under overburden pressure. Thus the presence of absence of fissility in argillaceous rocks may depend upon the amount of overburden pressure applied to the newly deposited clay material and the amount of liquid and rigid water in the clay at the time of application of pressure.

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Copyright 1997 American Association of Petroleum Geologists