Quantification and Mapping of the Regional State-of-Stress across the Offshore Nile Delta, Egypt

Tingay, Mark ¹; Bentham, Peter A.²
(1) Exploration, BP Egypt, Cairo, Egypt. (2) School of Earth & Environmental Sciences, University of Adelaide, Adelaide, SA, Australia.

The current state of stress in the offshore Nile Delta has been investigated by the analysis of 23.2 kilometers of image log and 67.8 kilometers of dip-meter log data from 57 wells distributed widely across the offshore area. Construction of this regional dataset was only possible through the close collaboration of BP Egypt and its partners, ENI and RWE Dea. A total of 588 breakouts and 68 drilling-induced fractures were observed in 50 wells.

The present-day maximum horizontal stress direction in the Nile Delta varies from NNE-SSW in the western offshore region, E-W in the central offshore and onshore parts and ESE-WNW in the eastern offshore area. Hence, the present-day stress orientation in the Nile Delta is predominately sub-parallel to the margin (coastline). Sequences in the eastern offshore Nile Delta that overlie Messinian evaporites exhibit variable, though predominately NE-SW, present-day maximum horizontal stress orientations.

The differing present-day stress orientations above the evaporites confirm that the Messinian evaporites are acting as a sub-regional detachment surface and highlight that two different tectonic zones presently exist in the Nile Delta. The margin-parallel stress orientation observed throughout most of the region suggests typical deltaic gravity sliding tectonics and is consistent with the most commonly observed structural styles. However, the NE-SW (margin-normal) maximum horizontal stress orientation in sequences overlying the Messinian evaporites in the Eastern Nile Delta is orthogonal to the structural grain and is inconsistent with that predicted by classic tectonic models of delta systems.

The maximum horizontal stress magnitude is estimated as being predominately below the vertical stress magnitude in the more normally-pressured Plio-Pleistocene sequences, suggesting a primarily normal faulting stress regime (Sv > SHmax > Shmin) consistent with other Tertiary deltas. However, horizontal stress magnitudes increase in deeper over-pressured pre-Messinian sequences and the stress regime trends towards transtension (Sv ≈ SHmax > Shmin) and even strike-slip (SHmax > Sv > Shmin) in formations showing pore pressure gradients greater than 14.0 MPa/km.

The resultant regional variations in stress orientations and stress magnitudes observed in this study have significant technical and economic implications for ongoing exploration, appraisal and development activities.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.