Relationship Between In-Situ Stress and Fractures and Faults in the Monterey Formation, Santa Maria Basin, California

Thomas Finkbeiner, Colleen A. Barton, and Mark D. Zoback

Borehole televiewer (BHTV) data from four 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 in each well shows a uniform NE-SW maximum horizontal stress (S_Hmax) orientation with depth. This is consistent with the regional S_Hmax direction throughout the entire seismogenic part of the crust as determined from other wellbore breakout data, NW-SE trends of active fold axes and results of kinematic stress-inversion of earthquake focal plane mechanisms. In marked contrast to the uniform stress field, analysis of fractures indicates that their orientation, dip, and frequency va y considerably within each well and laterally among them. With depth, fractures can be generally divided into distinct subsets on the basis of fracture frequency and predominant strike and dip directions. These 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 in the onshore well do steeply-dipping fractures strike parallel to S_hmax and are interpreted as Mode I extension fractures. In the three offshore wells, the fracture strike is mainly oblique or orthogonal to S_Hmax suggesting that they may be distributed strike-slip or reverse faults.

We believe currently active faults in the NE-SW transpressional deformation regime play a crucial role for fluid migration by providing permeability enhanced flow paths. Drill stem tests conducted in two of the offshore wells indicate that formation permeability is greatly enhanced in sections of the well where fractures strike predominantly perpendicular to S_Hmax and dip at moderate to steep angles. This is generally consistent with expected orientations of reverse and strike-slip faults. Other evidence for the importance of faults to fluid flow comes from core analyses and mudlogs where a loss of drilling mud is recorded within distinct depth intervals of these wells.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California