Predicting reservoir quality in the Ara stringer reservoirs, South Oman Salt Basin

Abstract

Hydrocarbon production from exploratory wells penetrating the Neoproterozoic-Cambrian Ara stringers in the South Oman Salt Basin (SOSB) have had variable results due to a heterogeneous pore system, multiple evaporitic cements and bitumen plugging. Ongoing sedimentary and diagenetic work, integrated with geochemistry and the structural evolution of the SOSB, showed that porosity enhancement is controlled by multiple dissolution events. The present-day porosity distribution is predominantly secondary, formed by dissolution in the deep burial environment. Pore-reducing processes started with the formation of reservoir bitumen, followed by anhydrite and then halite cementation. Two types of immobile non-pyrolisable bitumen can be distinguished petrographically and are thought to have precipitated from migrating or trapped hydrocarbons. The first bitumen phase occurred as thin rims, lining and partially filling secondary inter-crystalline and styloliterelated pores. It was observed in the earliest developed inter-crystalline secondary pores, suggesting migration soon after early burial replacement dolomitisation and a first dissolution event. The second phase of bitumen was observed as droplets in large dissolution vugs and is associated with microfractures, saddle dolomite and late diagenetic calcite. Mobile, pyrolisable bitumen has not been observed in thin section; its presence is indicated by geochemistry data. Other important pore-occluding phases are halite and anhydrite cementation. Preliminary results indicated that although cementation is abundant, its extent at reservoir scale can be laterally limited, and locally, trends can be mapped. Bitumen plugging is difficult to predict due to its multiple origins, this requires a better understanding of the relative timing of generation, expulsion and charge events.