AAPG Middle East Region Geoscience Technology Workshop

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Structural Interpretation of the Ara Group Carbonates South Oman Salt Basin

Abstract

The oldest sediments in the Sultanate of Oman belong to the Neoproterozoic – Early Cambrian Huqf Supergroup. The lower part of the Huqf Supergroup comprises localized fault-bounded continental siliciclastics of the Abu Mahara Group. The overlying laterally extensive marine siliciclastics and ramp carbonates of the Nafun Group mark a period of relative tectonic quiescence with broad regional subsidence. The Nafun Group – Ara Group boundary indicates a shift from this regional subsidence to a tectonic style marked by the uplift of large basement blocks that segmented a broader basin into several fault-bounded sub-basins. These broadly became the South Oman Salt Basin (SOSB), the Ghaba Salt Basin, and the Fahud Salt Basin. Basin restriction during the latest Ediacaran caused Ara-salt sedimentation in very shallow waters. Periods of differential subsidence led to transgressive – highstand conditions that caused the growth of isolated carbonate platforms. Further segmentation formed uplifted blocks which continued to be sites of carbonate deposition. Downfaulted blocks were overlain with black shale and in places silica-rich shale. Part of the geologically oldest commercial hydrocarbon province in the world, the Ara play is Oman’s third largest in terms of yet to find volumes. Risking of the Ara play is heavily dependent on good quality seismic data. New Pre-Stack Depth Migration Wide-Azimuth seismic has reduced drilling surprises and reserves are now being updated. This talk will focus on several fields that have base case in place volumes totaling ~0.5 Bbbl of oil & 1.75 TCF of gas. The SOSB strikes NE – SW and has a lateral extent of c. 400 × 150 km. Its western margin is formed by the Western Deformation Front (WDF), and its Eastern Flank is a structural high where the thinning basin fill overlies Early Neoproterozoic crystalline basement. Approximately six carbonate to evaporite (rock salt, gypsum) sequences of the Ara Group are recognised straddling the Pre-Cambrian to Cambrian boundary over a relatively short 15 million year period (A0/A1-A6). There are obvious contrasting structural styles between the Neoproterozoic basal Ara carbonates (A0/A1-A2/A3) and the Palaeozoic carbonate (A3/A4-A6) stringers. The basal carbonates are well developed, laterally extensive reservoirs that coalesce on the block highs. The overlying carbonate ‘stringer’ reservoirs have a smaller footprint, are self-charging, over-pressured, and encased in the lower part of the Ara salt directly above the older carbonate platforms. This change in the Ara carbonates’ structural style and the continued segmentation of the SOSB is interpreted to have been caused by the emplacement of the WDF. Subsequent Cambro-Ordovician deposition of continental siliciclastics on the mobile Ara Salt then led to halokinesis. Differential loading formed 5 – 15 km wide clastic pods and salt diapirs with subsequent folding and fragmentation of some of the carbonate platforms and isolated stringers encased in the mobile passive salt. Given their age and subsequent history since deposition these ancient Cambrian stringers were assumed to be geologically structurally compartmentalized with the Ara salt acting as barriers to fluid flow. This model has now been challenged by historical pressure data that weren't originally available when the initial field models, based upon interpretation of the now vintage Narrow-Azimuth seismic, were first being constructed. A consistent interpretation workflow has since been adopted to capture the full range of remaining potential GRV & fault uncertainties. Having a better appreciation of the structure of these sour Ara carbonate fields allows for robust and clearer decisions regarding the number and location of their future development wells.