Fault/fracture-related Dolomitisation in the Thebes Formation, Hammam Faraun Fault Block, Suez Rift, Egypt: An Outcrop Study of Massive and Stratabound Dolomite Bodies in Remobilised Carbonates

Corlett, Hilary J.; Hollis, Cathy; Gawthorpe, Rob; Hirani, Jesal; Hodgetts, David; Rotevatn, Atle; Bastesen, Eivind

Fault/fracture controlled-dolomitisation influences porosity and permeability in many carbonate reservoirs. Subsurface datasets often lack the resolution necessary to create a reliable 3-D geocellular model to represent the geometry of depositional and diagenetic bodies. Few quantitative datasets document the spatial distribution of these bodies and consider their origin within a well-constrained tectono-stratigraphic framework. This study examines the footwall of the Hammam Faraun Fault (HFF), exposed on the Sinai margin of the Suez Rift, where pseudo-3-D exposure reveals that the Eocene Thebes Formation underwent extensive diagenesis and fracturing during rifting.Sedimentary logging, field mapping and microfacies analysis reveals three principal facies associations: 1) matrix-supported debris flows hosted in a backgound of slope packstones, 2) slumped grainstones in planktic foraminifera packstones, and 3) grainflows in planktic foraminiferal wackestones. The spatial distribution of remobilized deposits has been mapped and body geometry quantified using a laser range finder and photograph image analysis. The debris flows range between 0.5-20 m thick and 1-300 m in downflow distance. Grainflow deposits are 1-12 m thick and extend up to 1 km parallel to depositional dip. Plug-based core analysis of these facies has revealed higher porosity and permeability in grainflows than packstones, which in turn have better reservoir quality than debris flow facies. In the footwall of the HFF, there are two types of dolomite bodies: 1) stratabound dolomite, hosted and terminating primarily within debris flows up to 2 km away from the HFF, and 2) massive, non-fabric selective dolomite pods up to 250 m wide that cross-cut bedding in and parallel to the HFF. Overall, the porosity and permeability of dolomitised beds is lower than for the limestone. The measured permeability from plug-based core analysis for an equivalent porosity is higher in stratabound dolomite than for the massive dolomite; however, the massive dolomites are intensely fractured, which may have influenced the style of dolomitisation due to enhanced macro-scale permeability. A 3-D geocellular model of the HFF Block has been built in order to represent the distribution of these flow-controlling facies and illustrate the petrophysical properties of the dolomites. The results are of direct application to exploration and development of reservoirs that have undergone fault/fracture-controlled dolomitisation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013