Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

• Published in: Journal of petroleum science & engineering 2021-05, Vol.200, p.108340, Article 108340
• Main Authors: Kassem, Ahmed A., Hussein, Walid S., Radwan, Ahmed E., Anani, Nasr, Abioui, Mohamed, Jain, Sreepat, Shehata, Amer A.
• Format: Article
• Language: English
• Subjects: Diagenesis; Lithofacies; Northeastern Egypt; Petrography; Reservoir quality
• ISSN: 0920-4105; 1873-4715
• DOI: 10.1016/j.petrol.2020.108340

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have