Fracture distribution in faulted basement blocks: Gulf of Suez, Egypt

This paper is part of the special publication Structural geology in reservoir characterization (eds M.P. Coward, T.S. Daltaban and H. Johnson). Fractures in basement rocks of Gebel El Zeit and Esh El Mallaha fault blocks, Egypt, occur in three nearly orthogonal sets: a SE-dipping, cross-rift set that parallels ENE faults and dykes; a SW-dipping set sub-parallel to bedding in overlying clastic rocks; and a NE-dipping, rift-parallel set that parallels the rift-border faults. Restoration to pre-rift orientations puts these sets into vertical, horizontal and vertical attitudes respectively. All fracture sets are present throughout the fault blocks, except for the horizontal set which is absent on the NE side of Gebel El Zeit and in the Nubia Sandstone. Fracture density increases near the top of basement where sheet fracturing is common, near faults, and in and around dykes. The distribution of similar fracture sets is recorded from sub-surface Formation Micro Scanner and Formation Micro Imager logs in the basement of the Ashrafi Oil-field, southern Gulf of Suez. The similarity of the fracture pattern between Gebel El Zeit and the Ashrafi Field suggests that surface fault blocks can be adequate analogues to the sub-surface blocks. The intersection of sheet and fault-parallel fractures localizes the maximum fracture density at the edges and near the top of the fault blocks. Block faulting and subsequent depth of erosion govern the distribution of fractures within the blocks. Block rotation during rifting relocated the maximum fracture density at an upper corner or structural crest, whereas block faulting without rotation maintained the maximum fracture density at block edges. In both styles, erosion of an exhumed block removed rocks of high fracture density by eroding the crest in the first case, and by stripping off the sheet fractures in the second. The first style is an analogue for half- graben reservoirs, e.g. the southern Gulf of Suez, while the second style is an analogue for full-graben reservoirs, e.g. the Mekong Basin, southern Vietnam.