Facies analysis and sequence-stratigraphic control on reservoir architecture: Example from mixed carbonate/siliciclastic sediments of Raha Formation, Gulf of Suez, Egypt

Abstract

Mixed carbonate-siliciclastic systems are complex, and their development can sometimes be difficult to understand, given the various responses to each sub-environment to specific forcing, such as variation in sediment supply, climatic conditions, or available accommodation. This work integrates a detailed facies, associated with a depositional environment reconstruction, all of which is placed in a sequence stratigraphy framework in order identify the different controls on the reservoir architecture of the Upper Cretaceous (Cenomanian) Raha Formation, Gulf of Suez. The study utilized outcrop data from Gabal Ekma and Gabal Nezzazat, as well as subsurface data from five wells drilled through the October oil field.

Our findings revealed that the Raha Formation succession is subdivided into eleven facies associations (FA) deposited as one depositional sequence. The Abu Had Member of the Raha formation consists of mixed siliciclastic/carbonate packages, arranged in a retrogradational stacking pattern, which were deposited during a period of pulsative shoreline regression. As a result of the transgression, the siliciclastic input in the system increased, potentially due to increased precipitation and weathering in the hinterland of the study area. The upper part of the Raha Formation belongs to the Mellaha Sand Member, which is mostly composed of bioturbated quartz arenite with bioturbation, and is the main reservoir in the October Field. It was deposited during a period of shoreline progradation as testified by its progradational stacking pattern.

The reservoirs of the Raha Formation can be divided into five distinctive types (G, C, Q, P, and M), based on the factors controlling the permeability (Diagenetic criteria and compositional framework). The outcrops-subsurface correlations indicated that these reservoirs in the Mellaha Sand Member are laterally extensive, and thicken from 25 m in the SE to 91 m northwestwards. It is predicted that these reservoir intervals thicken toward the unexplored areas northern of October Field. Pore-filling cement and detrital matrix decreased the permeability and the reservoir quality to minimum while, the grain coating prevented the collapse of the reservoir quality. Pore-filling cement and detrital matrix decreased the permeability values, consequently lowering the reservoir quality to minimum, whereas the grain coating prevented the collapse of the reservoir quality, retaining the highest permeability values in the overall pore system. The aforementioned study provides significant insight into the understanding of depositional facies and sequence-stratigraphic control on reservoir architecture. Additionally, it provides insights for further exploration activities in these mixed carbonate/siliciclastic intervals along the Gulf of Suez and adjacent basin, and the improved understanding of these reservoirs units could also be applied to future Carbon Capture and Storage projects.