ABSTRACT
This study focuses on determining the orientation and constraining the
magnitude of present-day stresses in the Dezful Embayment in Iran’s
Zagros Fold and Thrust Belt. Two
datasets are used: the first includes petrophysical data
from 25 wells (3 to 4 km deep),
and the second contains 108
earthquake focal mechanisms, mostly occurring in blind active basement
faults (5 to 20 km deep). Formal stress inversion analysis of the focal
mechanisms demonstrates that there is currently a compressional stress
state (\(A_{\varphi}=2.0-2.2)\) in the basement. The seismologically
determined SHmax direction is 37°±10°, nearly
perpendicular to the strike of most faults in the region. However,
borehole geomechanics analysis using rock strength and drilling evidence
leads to the counterintuitive result that the shallow state of stress is
a normal/strike-slip regime. These results are consistent with the low
seismicity level in the sedimentary cover in the Dezful Embayment, and
may be evidence of stress decoupling due to the existence of salt
layers. The stress state situation in the field was used to identify the
optimally oriented fault planes and the fault friction
coefficient. This finding also
aligns with the prediction Coulomb faulting theory in that the N-S
strike-slip basement Kazerun Fault System has an unfavourable
orientation for slip in a reverse fault regime with an average SW-NE
SHmax orientation. These results are useful for
determining the origin of seismic activity in the basin and better
assessing fault-associated seismic hazards in the area.
Keywords: Stress State, Borehole Breakout, Focal mechanisms, Fault
Mechanics, Seismicity, Dezful Embayment, Zagros Fold and Thrust Belt.