Basement rocks of the Miocene Gulf of Suez rift were first deformed during the Precambrian African Orogeny producing fabrics that include foliation, fractures, and dikes. ⁴⁰Ar/³⁹Ar age dates of fracture-filling material (dikes) yield a range from 600 to 460 Ma. The fabrics cluster around N-S, N60°W, and N60°E orientations and were reactivated as previously described by Younes and Altamura (1998). Miocene rift faults trend N30°W, perpendicular to the Miocene extension direction and approximately 30° clockwise to the Precambrian NW fabric.

Mesoscale quartz veins and silicified faults from Gebel Zeit were sampled for fluid inclusion analysis. Dominant quartz veins cluster around N60°W followed by N60°E, N30°E, and N-S. Fluid inclusions in quartz from the N60°W set vary in both the number of phases and composition. Two-phase aqueous, 3-phase CO₂-H₂O, and 2-phase hydrocarbon (liquid) -natural gas (vapor) inclusions were found. The variation in composition and type of inclusions suggests that the N60°W fractures were reactivated over a time interval during which migrating fluids evolved or changed. For example, two of the veins are characterized by only 2-phase aqueous inclusions. While primary fluid inclusions in quartz from one vien yielded a mean minimum homogenization temperature (Th) of 412°C, a mean Th of 192°C was obtained for the second. Another N60°W quartz vein contains 2-phase aqueous as well as 3-phase CO₂-H₂O and oil-natural gas inclusions. Therefore, quartz emplacement occurred at various depths and from evolving fluids associated with multiple reactivation of the N60°W-trending fabrics.

Hydrothermal solutions that deposited the quartz vein containing the hydrocarbon inclusions must have time migrated through the fault, mixing with oil/gas phases in any rift or pre-rift section above the basement. While early quartz veins were emplaced at temperatures greater than the oil-maturation window (OMW, ~150°C), fluid temperature of the latter vein didn’t exceed OMW for any length of time. A minimum emplacement depth of 3.4 km is estimated using a geothermal gradient of 44°C/km. The depth to the top of the rift section is 2-3 km whereas that of the pre-rift section is 3-4 km. Although the solutions must not have departed from P & T conditions that characterize OMW, the blue fluorescence of these inclusions suggests a relatively high degree of oil maturity.