Microscopic Examination of Mechanically Deformed Oil Shale
Authors: Budd C.H. , McLamore R.T. , Gray K.E. 
Published: December 2007

American Institute of Mining, Metallurgical and Petroleum Engineers, Inc.

This paper was prepared for the 42nd Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Tex., Oct. 1–4, 1967. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

A microscopic examination of two grades of mechanically deformed oil shale (one rich in kerogen and the other lean in kerogen) was accomplished with petrographic and electron microscopes. Cylindrical specimens were deformed by axial loading over a confining pressure range of 1,000 to 25,000 psi, at atmospheric pore pressure and room temperature. The orientation of the plane of anisotropy (bedding plane) was varied between 0 and 90 degrees relative to the axial load.

The results indicate that the three factors which determine the types of deformation structures which form are:

orientation of anisotropy to the compression axis, type of anisotropy, and confining pressure.

Three types of shear faulting are found in the deformed oil shale samples:

oil shales that failed in a brittle manner have fault surfaces which are slicken sided and contain fine powdered carbonate and quartz minerals, rocks deformed in a transition stage between brittle and ductile behavior exhibit a gouge zone, and oil shales that failed in a ductile manner have "plastic slip zones."

The occurrence of systematic shear fractures or joints in Green River Shales is found to be dependent on the amount of kerogen. In rocks lean in kerogen, most of the joints occur parallel or at right angles to the bedding planes except when both the confining pressure is greater than 15,000 psi and orientation of anisotropy to compression axis is greater than 45 degrees. The rocks rich in kerogen contain microscopic joints, termed "microjoints" which are only found in layers lean in kerogen within the rock. Microjoints are more related to the direction of axial load and confining pressure than bedding plane anisotropy. It has been found that jointing is one of the first deformation structures to form and as compression is continued, jointing in layers lean in kerogen is concomitant with plastic deformation in adjacent layers rich in kerogen.

Kink bands are found only in the oil shale rich in kerogen when the angle of anisotropy to compression axis is 20 degrees or less. Examination of the boundary between the kinked and unkinked rock with the electron microscope reveals that the rock behaved in a plastic manner. Kink bands of microscopic size are found and termed "micro-kink bands." They occur in (1) layers rich in kerogen, and (2) within larger kink bands.

Plastic slip zones are found in deformed oil shales rich in kerogen when the angle of anisotropy to compression axis is 30 degrees or greater and the confining pressure is above 10,000 psi. These zones are small and confined to rich, thin, layers of kerogen.