Depositional Environments, Architecture and Diagenesis of Fahud Field, Oman

Volker C. Vahrenkamp¹, Alia Al Bahry¹, Talal Al Musallami¹, Abhijit Mookerjee¹, Wadie Mansour¹, Richard Singleton¹, Cathy Hollis², Peter Homewood³, Henk Droste³, Carine Grelaud³, Philippe Razin⁴, Quintin Davies⁵, Boris Kostic⁵, Conxita Taberner⁶, and Mateu Esteban^7
1 Petroleum Development Oman, Muscat, Oman
² Shell International Exploration and Production, Risjswijk, Netherlands
³ Sultan Qaboos University, Al Khod, Oman
⁴ Bordeaux University, Bordeaux, France
⁵ Badley Ashton and Associates, Lincolnshire, England
⁶ Institute of Earth Sciences, CSIC, Barcelona, Spain
⁷ Carbonates International Iberia, S.L, Caimari, Mallorca, Spain

Fahud Field is a giant fractured carbonate reservoir in the Sultanate of Oman. It is contained in the Natih Formation, a 450m thick section of Albian to Turonian high porosity, low permeability shallow water carbonates. Deposition occurred over a period of three regionally correlatable third order cycles in environments ranging from source rocks deposited in intrashelf basins, mud-dominated wacke/packstones of slope environments, coarse rudist sand shoals, tidal flats to back-shoal lagoons. On a large scale reservoir architecture is layer-cake and well constrained by some 400 wells. However, a close look at seismic, outcrop analogues and performance indicated the likelihood of significant internal heterogeneity.

In support of further development activities some 1800m of new cores were taken to investigate:

• Depositional environments & architecture (lateral facies variations & cycle stacking patterns)
• Diagenesis (paragenetic sequence, impact of subaerial exposure including lateral structurally related changes and subsequent burial)
• Distribution of petrophysical properties (standard & SCAL).

Core observations are anchored by extensive outcrop analogue studies, detailed mapping of 3D seismic, a wide variety of logs and the analysis of production behaviour. They suggest that reservoir properties and behaviour are directly related to a complex interplay between depositional heterogeneities related to aggrading and prograding carbonate sequences (clinoforms), cycle top alterations during several episodes of early exposure and porosity creation during burial diagenesis.

Findings from the core analysis are key elements to constrain ranges of uncertainties incoporated into 3D static and dynamic reservoir models, which were built to assess production forecasts and economics of various development options.