AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Stress Orientation Changes Around Evaporites in the Nile Delta
(1) Australian School of Petroleum, University of Adelaide, Adelaide, SA, Australia.
(2) BP Egypt, Cairo, Egypt.
(3) International Egypt Oil Company, Cairo, Egypt.
(4) RWE-Dea Egypt, Cairo, Egypt.
Evaporitic horizons are routinely interpreted to act as mechanical detachment sequences and thus significantly influence the structural evolution of sedimentary basins and fold-thrust belts. However, over 30 years of global in-situ stress analysis has provided only poor evidence to support this widespread assumption. This study examines present-day stress orientations inferred from borehole breakout and drilling-induced fractures in 57 boreholes in the offshore Nile Delta. A total of 588 breakouts and 68 drilling-induced fractures from 50 wells reveal sharply contrasting present-day maximum horizontal stress (SHmax) orientations in sequences above and below the extensive Messinian evaporites of the eastern Nile Delta. A typical deltaic margin-parallel SHmax is observed in parts of the Nile Delta that are below or do not contain evaporites (NNE-SSW in the western Nile, E-W in the central Nile and ESE-WNW in the eastern Nile). However, a scattered but largely margin-normal (NNE-SSW) SHmax is observed in sequences underlain by evaporites in the eastern Nile Delta. The approximately 90 degree variation in present-day SHmax orientation above and below the Messinian salts provides the first convincing evidence from in-situ stress analysis that evaporite sequences act as major mechanical detachment horizons. In addition, the margin-normal supra-salt SHmax orientation is sub-perpendicular to the strike of nearby active extensional faults, rather than being parallel to the faults as predicted by classic Andersonian criteria. We propose that the margin-normal non-Andersonian SHmax orientation observed in supra-salt sequences is related to localized gravity gliding tectonics and is the result of basal drag forces imparted on rafted blocks sliding down the basin-ward dipping evaporites. Finally, the switch in stress orientations above and below the Messinian evaporates has key implications for wellbore stability and sand production issues in the Nile Delta.