Middle Miocene syn-rift sequence on the central Gulf of Suez, Egypt: Depositional environment, diagenesis, and their roles in reservoir quality

The middle Miocene sediments, a key oil reservoir within the syn-rift structures of the Gulf of Suez, have received limited research attention despite their geological importance. To bridge this gap, we analyzed well cores and petrophysical data from the middle Miocene sandstone-shale sequence (Sidri Member) in the Belayim field, one of Egypt’s major oil fields, to assess the depositional and diagenetic characteristics and evaluate reservoir quality. It is subdivided into three sandstone units separated by calcareous shale interbeds. The units show distinct lateral and vertical distribution, reflecting an upward change in depositional environment, transitioning from shoreface, deltaic condition, to embayment settings, interrupted by shale deposition during transgression episodes. The sediments originated from uplifted and tilted rift margins along NW-SE trending faults. During the syn- to post-rift burial to depths exceeding 2.5 km, diagenetic processes, including compaction, dissolution, and cementation, altered the sandstones’ primary properties, resulting in significant petrophysical variations. Porosity reaches up to 30% in well-sorted, medium-grained sandstones with dissolution, corresponding to bulk density as low as 1.83 g/cm3 and permeability up to 1309 mD, while cementation reduced porosity in certain intervals, creating heterogeneity in grain density and permeability. Reservoir quality of the sandstones ranges from poor to excellent with variable oil saturation. Notably, the lower sandstone unit, located within an anticline structure, displays good to excellent quality and up to 77.6% oil saturation. These findings highlight the influences of syn-rift dynamics in shaping the middle Miocene sequence and the potential for optimizing hydrocarbon exploration and production strategies across the region.

Highlights

• The middle Miocene Sidri sequence shows distinct variations of depositional system.
• The Sidri sandstones are a key hydrocarbon reservoir in the Gulf of Suez, Egypt.
• Syn-rift dynamics and diagenetic processes controlled sandstone reservoir quality.
• The lower-middle units show a good to excellent quality with high oil saturation.
• The upper unit shows a poor reservoir quality and may serve as a minor caprock.

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