Strain-Based Prediction of Subseismic Faults in Rocks, 20-R9677

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Principal Investigators
Alan P. Morris
David A. Ferrill
Ronald N. McGinnis

Inclusive Dates:  01/01/07 – Current

Background -  A major challenge facing oil and gas exploration is the characterization of faults in a prospective hydrocarbon reservoir. Current techniques can typically detect faults that have vertical displacement of about 10 m or more. Small faults with displacements below this limit, however, influence the movement of oil, natural gas, water, and carbon dioxide. These "subseismic" faults are therefore of great economic importance to energy companies. Studies of exposed rock reveal a large number of faults with small displacements. Efficient prediction of subseismic fault populations from observable geologic data is a major technical challenge for oil production geoscientists.

Approach - SwRI has developed the basis of a new approach to subseismic fault prediction predicated on the observation that the number of faults present in a rock increases with the total strain that the rock has experienced. Highly detailed fault surveys suggest a predictive relationship between strain magnitude and faulting at all scales. This project will develop and test a new objective approach to subseismic fault prediction. This method characterizes subseismic faults in the context of the observation that rock strain and fault frequency are correlated. The approach differs from others in that it utilizes field-derived fault size relationships and explicitly incorporates a strain context determined for the individual case being considered. Field areas have been selected based on their excellent exposure, variety of rock types, and easy access.

Accomplishments - The project team has conducted fieldwork in the Balcones Fault Zone of Texas and in the Moab area of Utah. Methodologies used included photographing the exposure and mapping and collecting seismic-scale data such as layer geometry and large (approximately 10 m) displacement fault geometry, as well as subseismic-scale data in the form of orientation, displacement, and location of small-displacement (0.01 to 10 m) faults. In addition, bed thickness and Schmidt hammer measurements were recorded throughout the exposures. Preliminary calculations of bulk strain for the new datasets range from 7 percent to 13 percent. Combined with previous results, the new data extend the range of calculated strains based on measured fault displacement and frequency, and include clastic sedimentary rocks not previously analyzed in this way.

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