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Field Study of Fracture Characteristics as Function of Bed Curvature in Folded Dolomites

Jill A. Quillan, D. W. Stearns

In exploring for fractured reservoirs, it is a common practice to construct second-derivative curves (rate of change of dip) from cross sections or seismic lines and imply that high values correspond to the location of a high fracture frequency on a fold. Another common, but more direct, method for projecting fracture frequency into the subsurface is to use whole-core analysis. In order to assign both confidence and caution limits to these practices, fractures which should respond to curvature were studied in the field on very well exposed folds.

Results indicate that though subtle changes in curvature may not result in significant increases in the number of fractures, large changes do. Further, the ratio of extension fractures to shear fractures may increase with increased curvature. Both of these observations may be lithologically dependent.

Other results from this study indicate that specific numbers from core analysis may be poor estimates of true fracture frequency. Field data indicate that even in highly fractured dolomites, 9-ft line lengths are required to stabilize standard deviations of fracture-frequency measurements. However, even though small volumes of rock, such as cores, may not give a good measure of the fracture frequency in the larger volume of rock surrounding the borehole, core data may faithfully depict local fracture orientations.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.