Isabelle Moretti

Longitudinal evolution of the Suez rift structure (Egypt)

Abstract A three-dimensional study of the structure of the Suez rift has been carried out using field and subsurface data, in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift. As in most intracontinental rifts, the structure of the Gulf of Suez area is governed by normal faults and tilted blocks whose crests constitute the main target of exploratory wells. The fault pattern consists of two major sets of trends: 1. (1) longitudinal faults parallel to the rift axis and created in an extensional regime where σ3 was trending ENE-WSW; and 2. (2) transverse faults with a N-S to NNE-SSW dominant trend. The transverse faults are inherited passive discontinuities, while most of the longitudinal faults were created during Neogene times in a purely extensional regime. Both sets were simultaneously active, producing a zigzag pattern and rhombic-shaped blocks. The transverse faults can show horizontal strike-slip components and act as relays between major normal faults. Although the Suez rift appears as a simple narrow elongated trough dominated by two almost symmetrical shoulders, its internal structure is asymmetrical. Cross-profiles show that all the major blocks are tilted in the same direction. However, the tilt direction changes twice along the rift. To the north and to the south of the rift, the blocks are tilted eastward, while in the central part they are tilted westward. To the north the change of dip is accompan ed by a graben-type “twist zone” without transverse faulting, at least in the Neogene series. To the south the change of dip is accompanied by a more complex structure involving both a major transverse fault and a horst-type “twist zone”. In this latter case the transverse fault does not cut through the entire rift. Balanced cross-sections established from subsurface data show that the tilt angle and the amount of extension increase from north to south, while the width of the blocks decreases, indicating a pole of opening close to the northern end of the Gulf. Minimum values for the amount of opening range from 5 km in the north to about 20 km in the south.

Late cenozoic subduction complex of Sicily

Abstract Besides remnants of Hercynian deformations in the Peloritani nappe and of pre-Oligocene Alpine structures in the Troiani nappe, most compressive structures observed in the Sicilian accretionary wedge result from the late Cenozoic (Tortonian to Present) continental subduction of the Apulia (Iblei) block, and are thus synchronous with distensive structures related to the opening of the Tyrrhenian Sea. Syntectonic deposits fill southward-migrating foredeeps in a sequential fashion, and the dating of these deposits helps to constrain the timing of deformation. Similarly, Plio-Quaternary sediments, eroded from the accreted units, rest on top of the allochthon in either compressive piggy-back depressions or extensional basins. The age and configuration of these overlap deposits constrain our reconstructions of the subsurface geometry of the underlying peri-Tyrrhenian detachment faults or S-verging thrust-faults. Post-depositional erosion, normal faulting and syntectonic filling of basins contribute to maintaining the critical taper of the prism, whose geometry is continuously altered owing to frontal accretion, underplating and isostatic uplift.

Seismic study of the crust of the northern Red Sea and Gulf of Suez

We report the results of fifteen Expanding Spread Profiles (ESPs), and a seismic wide angle reflection-refraction Une, performed during March–April 1986, in the Gulf of Suez and the Egyptian part of the northern Red Sea area (north of 25°N). Four 16.4 air guns were used as a sound source on board R.V. “Le Suroit” and a 96-channel 2.4-km long streamer was towed by a supply vessel, the “Whity Tide”. Most of the profiles show good crustal reflection and refraction arrivals and often good Moho arrivals obtained for a distance of 80 to 100 km. We present the results of X-T and τ-p analysis, obtained by a velocity inversion performed in the τ-p plane and by ray-tracing modeling of both the τ-p and the X-T sections. The velocity models are computed for planar and linear gradient velocity layers. The northernmost part of the Red Sea appears to be characterized by a continental type crust, extremely thinned (β ≥ 4), lying at a mean depth of 7–8 km, whereas the Moho is at a mean 14–15 km depth. The southern part shows a seismic velocity structure of an oceanic type, except in the 40 km closest to the coastline. In both parts, seismic waves progressively get more attenuated with distance from the shore to the axial zone, which is presently tectonically active. The difference between the northern continental and southern oceanic zone is related to the termination of the Levant Fault. The northern continental area appears to represent the termination of the Levant Fault as a zone of distributed deformation.

The evolution of the Suez rift: a combination of stretching and secondary convection

Abstract Different physical mechanisms have been put forward in order to explain the lithospheric evolution beneath a continental rift. Models of stretching of the lithosphere, though giving good values for the trough subsidence, could not satisfactorily account for other features of the rifts, such as the rapid uplift of the rift shoulders in some cases. However, the idea that the thinning of the lithosphere is also accomplished by secondary convection phenomena is now receiving growing attention. Using a thermomechanical model dealing with a non-Newtonian crust and mantle, Moretti and Froidevaux have suggested that the evolution of numerous continental rifts is the consequence of the combination of the upward propagation of an asthenospheric thermal anomaly and of an extensional regional stress. The Suez rift evolution, examined by tectonic subsidence studies, may be regarded as the result of an asthenospheric upwelling and of a short active initial lithospheric stretching phase.

Construction of coherent 3D geological blocks

Abstract This paper presents a new approach and efficient methods to build directly coherent geological models honoring the most reliable data set and the assumed local deformation mode. A first set of methods, based on the simple shear model, allows one to build geological structures such as similar folds, rollovers and listric faults. A second set of methods, based on the flexural slip deformation mode, enables one to build structures such as concentric folds and ramp anticlines. The principle of the proposed methods is to construct new horizons and faults coherent with the better known ones. Thanks to these methods, it is straightforward to build quickly coherent 3D blocks based on the most reliable data (well, 2D or 3D seismic image, surface data). Horizons poorly imaged on seismic data can be completed simply. Eroded parts of the upper horizons can be easily extrapolated as well. Finally this provides a useful methodology to check the coherency between the fault and the horizon geometry.

Thermal regime of fold and thrust belts—an application to the Bolivian sub Andean zone

A quantitative analysis of the various parameters influencing the thermal regime in orogenic belts and related foredeeps shows that (i) the increasing heat flow in internal zones is mainly due to the thickening of radiogenic layers, although there is no simple proportionality between crustal thickness and heat flow signal at large scale; (ii) in external zones, where the horizontal strain rate is large (such as in the Bolivian Andes), surface processes can be of first order within the first kilometers of the crust. Hence, they induce a large scatter in the thermal data which are acquired at shallow depths. The deep thermal regime can be restored only by a quantitative assessment of these parameters. Active erosion (respectively sedimentation) can increase (resp. reduce) the heat flow by a factor of 2 in the uppermost kilometers. The effects of fluid circulation percolating at depth can also generate significant local disturbances. Other processes such as heat advection during thrusting, surface morphology and climate change have a minor influence in most settings, compared to the aforesaid effects. In the Bolivian Sub Andean Zone, between 18S and 22S, the very active deformation enhances the surface thermal perturbations (particularly erosion and sedimentation) and disturb the thermal field. The analysis of these data accounting for the kinematics of the belt allows the lateral variations of the thermal regime at various scales to be assessed. A slight eastward increase in the thermal regime towards the Chaco plain is evidenced as well as towards the Boomerang area, as the Mesozoic and Cenozoic sedimentary cover gets thinner. D 2002 Elsevier Science B.V. All rights reserved.

The role of faults in hydrocarbon migration

It is well known that the hydrodynamic behaviour of faults may be rather different from that of their surrounding beds. These characteristics may not be constant through time for a single fault and especially change during rupture events. Various numerical tests were performed using the TEMISPACK software to calculate the quantity of hydrocarbons that can migrate through damaged zones with high permeability in and near to a fault. The paper examines the influence on fluid flow of (1) the thickness of the fault zone, (2) the connectivity between fault and carrier beds, and (3) the opening time when permeability increases sporadically. The results confirm the importance of fault zones on hydrocarbon migration. Even when very narrow (2 meters), temporarily open (<100000 years), and moderately permeable (<10 mD), faults focus the migration of hydrocarbons. The flow is stronger in narrow, temporary faults but the quantities in circulation remain essentially the same. The nodes of the migration paths are the connections between faults and carrier beds (and/or source rock). This connection has a greater influence on the quantity of migrating hydrocarbons than does the intrinsic permeability of the fault. Given major hydrocarbon losses in the rocks porosity, thick drains are less efficient than are narrow zones for bringing the hydrocarbons to the reservoirs.

Fault-block tilting: the Gebel Zeit example, Gulf of Suez

Abstract The Gebel Zeit consists of the eroded crest of a tilted block in the southern part of the Suez rift. The Zeit block displays a typical asymmetrical geometry: it is bordered to the east by a 35–45° E-dipping normal fault with kilometric throw and has a 30° SW-dipping homoclinal flank. Part of the pre-rift sedimentary sequence has been preserved on this flank and is unconformably covered by Upper Burdigalian (NN4) Globigerina marls. In the southern part of the block crest, the complete pre-rift series has been eroded and evaporites of Langhian age (NN5) rest directly on the Precambrian basement. Field evidence indicates alternations from erosion to sedimentation at the crest of the Zeit block. In an attempt to characterize its tilting, subsidence curves were computed along a cross-section with the backstripping method. Results indicate three stages in the evolution of the block: (1) a rapid subsidence between 22 and 16 My; (2) a pause between 16 and 10 My; and (3) a slow subsidence until present time. During the tectonic quiescence, the sedimentary loading effect alone produced an increase of 8° in the tilt angle. A simple kinematic model of tilting along a circular fault is proposed to quantify the Zeit rotation. Depth of the brittle-ductile transition is estimated at 10 km to explain the tilting. Strong driving of the tilting by the listric fault induces conjugate movements between the crest and trough of the block and explains the discrepancy between the Zeit and regional tectonic subsidence.

Theoretical model of block rotation along circular faults

Abstract The overall structure of rifts is often controlled by normal faults and tilted blocks. In several rifts the crests of the blocks have been strongly eroded while the associated half-graben has undergone large subsidence. The relative movement of the crest of a block can be analysed assuming simple circular geometry of its bounding fault. In such a geometric model, subsidence of the half-graben and relative uplift of the crest are connected and this coupling can be quantified as a function of the fault shape and of the block width. Knowing the geometry of a fault, we can calculate the movements of a rigid block rotating along this fault. Inversely, by knowing the tilting of a block, it should be possible to estimate the geometry of the guiding fault, i.e. its detachment depth, if we assume the fault to be listric. Numerical results show that a large block can undergo an absolute uplift. The direction of the movement depends on the initial width of the block and is quite independent of the tilt angle, except for very low-angle faults.

Subsidence, maturation and migration history of the Tampen Spur area

Abstract A complete study of the petroleum system of the Tampen Spur area has been carried out. The approach used was a two-dimensional TEMISPACK analysis of a seismic line crossing the area where various hypotheses could be tested and later applied in a three-dimensional sense. The profile crosses from the deepest part of the ‘Tampen Graben’ across the Visund Ridge and the Marflo Ridge to the Sogn Graben in the east. Four source rocks have been taken into account, two marine [the Draupne (the most prolific) and the Heather] and two deltaic [the Brent and the Dunlin]. Two different prospect types can be defined: the Brent at the top of the tilted blocks and Heather sand bodies at the foot of the eroded tilted blocks. Various hypotheses about the geometry of these sand bodies and about the contact between the rich Draupne source rocks and the sands have been tested. Maturation started in the middle Cretaceous, between 100 Ma in the deepest eastern part of the section (Sogn Graben) and 80 Ma for the Draupne on the Tampen Spur west graben. At this time, the structures were already sealed by the Cretaceous shales. Apart from the Visund Ridge, all the prospects are currently in the condensate or gas window. Overpressures are compatible with large closed faults. More data are required about the possible sealing of minor faults before any conclusions can be drawn. There is no easy migration path from the rich Draupne rocks to the brent reservoir and various hypotheses are discussed to explain the regional petroleum potential. One possible explanation for the regional charge is the contribution of the other source rocks, the Heather, the Brent and the Dunlin.