Abstract: Fracture and Breakout Analysis from Wellbores within the Monterey Formation, Santa Maria Basin, California
Borehole televiewer (BHTV) data from several wells within the on- and offshore Santa Maria Basin were utilized to investigate the relationship between fracture distribution, orientation, and variation with depth and in situ stress. Analysis of stress-induced wellbore breakouts shows a uniform NE-SW maximum horizonal stress (SHmax) orientation with depth in each well. This direction is consistent with the SHmax direction determined from wellbore breakouts in other wells in this region, earthquake focal plane mechanisms and the NW-SE trend of active fold axes. In marked contrast to this, analysis of fractures indicates that fracture orientation and frequency vary considerably within each of these wells. Discriminating between various planar structures uch as bedding, joints, veins and faults is difficult and strongly limited by data quality and resolution. In general, fractures in each well can be divided into distinct subsets on the basis of fracture frequency and predominant strike and dip directions. The fracture subsets correlate with changes of lithology and physical properties indicating that factors such as tectonic history, diagenesis and structural variations may have strongly influenced the initiation and propagation of fractures. Only rarely do steeply-dipping fractures strike parallel to SHmax, suggestive that they are Mode I extension fractures. More commonly, the predominant fracture strike is observed to be orthogonal to SHmax. This suggests that they may be distributed reverse faults, active in th NE-SW trending compressive stress field.
AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California