Online Journal for E&P Geoscientists

Amthor, Joachim ^*1; Gaghman, Abdulghani ¹; Tull, Simon ¹; Ruf, Michael ²; Healey, Martin ¹
(1) PDO, Muscat, Oman. (2) Badley Ashton & Associates, Winceby, United Kingdom.

The Terminal Neoproterozoic to early Cambrian Ara Group of the South Oman Salt Basin comprises at least seven third-order evaporite/carbonate depositional sequences, which contain prolific hydrocarbon reservoirs. The salt basins formed (and were later deformed) during transtensional and transpressional local tectonics within a compressional plate setting. The main reservoirs occur in dolomitized carbonate platforms ('Ara Carbonate Stringers') which are fully encased in evaporites (mainly anhydrite and halite).

A recent oil discovery in a > 400m thick sequence of Ara carbonates without intervening evaporites has highlighted the range of stratigraphic play types and lateral facies variations to be encountered in the Ara Salt Basins.

Seismic data show the stacked sequence as a prominent transparent package that thins westward. Towards the east, the package is characterised by a steep transition into the deeper basin. This prominent carbonate escarpment strikes NE/SW, following the underlying basement trends, and is dissected by NW/SW trending faults which play an important sealing role. The available well and core data reveal depositional facies that indicate the presence of a highly productive microbial boundstone factory.

Characteristic facies include a variety of stromatolite and thrombolite boundstones and associated grainstones, oolitic and intraclast pack/grainstones, calcrete and pisod-dominated facies with tepee structures. In particular, isopachous stromatolites and pisolitic units can be regarded as indicator facies, as they have not been observed to date in typical stringer carbonates.

Diagenetic elements that are present and which have altered and/or obliterated the depositional facies are: pervasive dolomitisation, evidence for multiple episodes of dissolution and brecciation during periods of subaerial exposure, isopachous and fibrous early diagenetic (marine) cementation and fault-related dissolution and cementation. The depositional and early diagenetic matrix complexities are compounded by fault and fracture related processes which have generated locally enhanced matrix permeability.

The appraisal drilling has highlighted the potential and the challenges for reservoir of the platform margin escarpment. Particular uncertainties still exist in terms of detailed reservoir architecture, aquifer development and prediction of permeability and modelling the dynamic flow behaviour and fluid displacement / sweep in these heterogeneous carbonates.