Controls on the Ara Carbonate Stringers Productivity, South Oman Salt Basin

Al-Abry, Nadia, Mia VanSteenwinkel, Petroleum Development of Oman, Muscat, Oman

Cambrian Ara intrasalt carbonate stringers in the South Oman Salt Basin have proved to be excellent and unique reservoirs with substantial hydrocarbon accumulations that PDO have started to produce from in the Harweel Field in March 2004 (STOIIP of 1.78 bln bbl and currently producing over 3000 b/d). To date more than 70 wells have penetrated the stringers which are deeply buried (up to 5km) and totally encased within salt. Hydrocarbons were encountered in many of these wells but despite favourable primary reservoir facies (dolomitised parallel/crinkly laminites shallowing to thrombolites and grainstones), a num­ber of these wells have failed to produce at significant rates.

Detailed examination of core and thin sections revealed extensive salt plugging through­out the reservoir interval, in addition to the prevalence of large quantities of bitumen (up to 10% volume). Preliminary studies conducted on the salt and bitumen plugging (Urai et al., 2004) indicated that the halite (diagenetically late) is highly deformed with rare primary fluid inclusions. Bitumen is found in pore space and within halite crystals and several generations of solid bitumen are thought to exist.

Salt plugging coupled with bitumen plugging seems to form a major barrier to hydrocar­bon flow by reducing the effective porosity and permeability values in the stringers. Models for the plugging are being constructed and show that the critical factors are: the burial his­tory, charging history, diagenetic history, salt tectonics, fracture and fault patterns and changes in the local paleo-contacts. Therefore producing from the Ara carbonate stringers of the SOSB will prove to be quite a challenge, requiring a holistic and complete integration of information.