--> --> Abstract: Stress Orientation Changes Around Evaporites in the Nile Delta, by Mark Tingay, Peter Bentham, Arnoud De Feyter, and Axel Kellner; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Stress Orientation Changes Around Evaporites in the Nile Delta

Mark Tingay1; Peter Bentham2; Arnoud De Feyter3; Axel Kellner4

(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.