Stress Variation Around the Balarud Lineament in the Zagros Fold and Thrust Belt and its Implication for Reservoir Geomechanics

Abstract

ABSTRACT We investigate horizontal stress variation in the vicinity of the deep-seated Balarud Lineament in the northern part of Iran's Dezful Embayment in the Zagros Fold and Thrust Belt (ZFTB). Both petrophysical data from drilled oil and gas wells (3-4 km deep) and earthquake focal plane solutions (6 to 25 km deep) are used to constrain the orientation and relative magnitudes of the local and regional stresses. The stress orientations in the sedimentary rock strata and the basement are of two entirely distinct types. In the basement, constant regional NE-SW SHmax orientation is observed in the northern ZFTB. The seismologically determined local SHmax direction from 25 focal mechanisms around the Balarud Lineament is 29.3°±8.5°. However, observations of borehole breakouts and tensile-induced fractures indicate that the dominant SHmax orientation is N-S near the Balarud Lineament. The consistent stress direction in the basement indicates a high differential horizontal stress magnitude, whereas the principal stress orientation rotates 35° counter-clockwise in the sedimentary cover where the state of stress is extensional. The Lineament's second-order stress patterns are discussed in terms of wellbore placement and completion decisions. INTRODUCTION Anomalous relative stress magnitudes and orientations have been observed in the world's various uniform lithospheric stress fields. Stress deflection may be observed due to lateral density/strength contrasts, flexural stresses, or superimposed geological structures such as faults (Sonder, 1990) and salt diapirs (Dusseault et al., 2004). In the east-west-trending Transverse Ranges (California), the horizontal stress orientation is different by 25° from the reference stress state in the NW-SE San Andreas fault (Sonder, 1990). The regional NE-SW SHmax is reoriented to N-S in the area overlying the Peace River Arch in the Western Canadian Sedimentary Basin (Bell & McCallum, 1990). The same phenomenon is reported for the Amazon rift in central Brazil (Zoback & Richardson, 1996) and in the Swiss Alps and the northern Alpines foreland (Kastrup et al., 2004). Stress deflections can also be caused by reservoir depletion (Yale et al., 1994) and by earthquakes (Hauksson, 1994). Hauksson (1994) observed a 15° (±10°) rotation of local stress axes due to the 1992 Mw = 7.3 Landers's earthquake sequence. The scale at which second-order stress patterns occur depends on the degree of lateral density/strength contrasts and the size of geological structures and their orientation relative to regional stress fields (Sonder, 1990; Zoback, 1992).