Fault/Fracture Related Dolomitisation of the Eocene Thebes Formation, Hammam Fauran Fault Block, Gulf of Suez

It is recognised that post-depositional processes, such as faulting, dolomitisation, dissolution and cementation, usually strongly influence the architecture of carbonate reservoirs. It is often difficult, however, to reconstruct patterns of diagenetic modification from subsurface datasets, and as such confidence in permeability models may be compromised. Nevertheless, interpretations can be informed by quantitative, spatial data extracted from robust, high quality outcrop studies. In this context, this study focuses upon reconstruction of fault-controlled dolomitisation within the Suez Rift in order to map the spatial distribution of dolomite geobodies and determine the tectono-stratigraphic controls on their distribution.

The Hammam Fauran Fault Block, NW Sinai, provides excellent pseudo-3D exposure of differentially dolomitised, pre-rift Cretaceous-Eocene carbonates, deposited in a deep marine slope environment. Field mapping reveals massive, non-fabric selective dolomitisation, with common zebra dolomite, associated with two main structural trends a) NW-SE, parallel to the Suez Rift and b) NE-SW, parallel to Aqaba transform zone. Baryte, gypsum/anhydrite, goethite and haematite are found within these fault zones. Stratabound dolomitisation is also observed within debris flows, grain flows that intersect the faults, and at the top of upward-coarsening beds, continuing laterally for several hundred metres in some cases. A third, subordinate, fault/fracture system trends north-south and is not associated with dolomitisation; these fractures are either open or quartz cemented.

These preliminary data indicate that fluid flux on the platform was controlled by the geometry of the normal faults and fratures associated with the development of the Suez rift. Flow away from the faults appears to have focused upon the coarsest, cleanest facies. Zebra dolomite fabrics within the fault-related dolomite bodies imply high temperature/high pressure fluid emplacement, whilst the mineralogical assemblage suggests fluid interaction with both syn-rift volcanic and evaporite beds. It is clear, therefore, that a relationship exists between facies architecture, tectonic evolution and dolomitisation. Future data collection will evolve this model and move towards a quantitative framework by which the length scales of key flow-controlling layers, for calibration of subsurface models, can be derived.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California