| ABSTRACT
We used borehole televiewer
(BHTV) data from four wells within the onshore and offshore Santa Maria
basin, California, to investigate the relationships among fracture distribution,
orientation, and variation with depth and in-situ stress. Our analysis
of stress-induced well-bore breakouts shows a uniform northeast maximum
horizontal stress (SH max)
orientation in each well. This direction is consistent with the SH
max
direction determined from well-bore breakouts in other wells in this region,
the northwest trend of active fold axes, and kinematic inversion of nearby
earthquake focal plane mechanisms.
In contrast to the uniformity of the stress
field, fracture orientation, dip, and frequency vary considerably from
well to well and within each well. With depth, fractures can be divided
into distinct subsets on the basis of fracture frequency and orientation,
which correlate with changes of lithology and physical properties. Although
factors such as tectonic history, diagenesis, and structural variations
obviously have influenced fracture distribution, integration of the in-situ
stress and fracture data sets indicates that many of the fractures, faults,
and bedding planes are active, small-scale strike-slip and reverse faults
in the current northeast-trending transpressive stress field. In fact,
we observed local breakout rotations in the wells, providing kinematic
evidence for recent shear motion along fracture
©Copyright
1997. The American Association of Petroleum Geologists. All rights reserved. 1Manuscript
received April 15, 1996; revised manuscript received December 27, 1996;
final acceptance July 15, 1997.
2Department
of Geophysics, Stanford University, Stanford, California 94305-2215.
We
wish to thank Tom Zalan of the Chevron U.S.A. Production Company for providing
the offshore well data, Unocal Corporation for providing the data on the
onshore well, and Marcia McLaren from Pacific Gas and Electric Company
for providing the earthquake focal mechanisms used in the stress inversion
analysis. The data used as background seismicity in Figure 1 were extracted
from the World Wide Web of the Southern California Seismic Network (SCSN)
catalog operated jointly by the Seismological Laboratory at Caltech and
the U.S. Geological Survey, both in Pasadena, California. We appreciate
the comments and helpful discussions from Daniel Moos, Steve Graham, and
Lev Vernik. |
