Sajaa Field was discovered in December, 1980 on Amoco's 100% working interest concession in Sharjah, United Arab Emirates. The Sajaa Field is significant as it is both Sharjah's largest producing field and Sharjah's first gas-condensate development. This paper will review the geologic and reservoir data gathered to date and outline the field's Phase One development. The thrusted, lower Cretaceous trend on which the Sajaa Field lies has become an important play as shown by the recent Margham Field discovery in Dubai and the substantial exploration activity in the area. The development cost per barrel for gas-condensate reservoirs is high compared to onshore Middle East oil projects and a firm development plan is required. In order for Amoco to have completed it's Sajaa Phase One development in 18 months, extensive planning and close cooperation with the government of Sharjah were necessary.
The Sajaa Field is a retrograde gas-condensate reservoir found in the thrusted Lower Cretaceous Thamama limestone of east central Sharjah. It is part of the frontal thrust sheet of the Oman Mountains and was moved to its current relative structural position in Late Cretaceous time. The Thamama position in Late Cretaceous time. The Thamama overlies a massive section of carbonates thought to be Jurassic to Permian in age. The upper Jurassic carbonates have been found containing both rich gas and water depending on its structural position and are considered to be part of the Sajaa Reservoir. Deeper in the Jurassic, a dry gas has been encountered. Development has been centered around the upper Jurassic and Thamama, since the Lower Jurassic dry gas is sour (5%, H2S, 14% CO2, 80% CH4) with a high water to gas ratio (150 BBL/MMSCF, 844 M3/MMSCM).
The Thamama in Sajaa Field is a massive section of open marine shelf limestones. The average thickness of the Thamama is 2764 ft. (842 m) and is fairly uniform across the field. The limestone section is described as consisting mostly of tight wackestone to packstone with occasional thin shale beds; packstone with occasional thin shale beds; however, within the limestone substantial pay is found in correlatable intervals in the higher structural positions of the field. Net Thamama pay varies from 90 to 760 feet (27 to 231 m). The porosity found in the Thamama is predominately intercrystalline and a large predominately intercrystalline and a large fracture system is suspected. Several hypotheses of the diagentic history of the Thamama have been proposed, but a completely satisfactory explanation has yet to be determined. The results of the diagentic study will be of major interest for the field's future development.
The Nahr Umr shales and limestones overly the Thamama and are thought to provide the seal for the reservoir. During thrusting, the Thamama underwent tectonic translation and is described structurally as a thrusted hanging wall anticline at Sajaa Field. Several minor faults have been mapped within the field and these should become better defined as development continues.
Tables One and Two summarize the basic reservoir and fluid data gathered to date. Porosity and permeability are fairly typical Porosity and permeability are fairly typical for Thamama reservoirs; however, fluid type, net pay, and water salinity are not. Water has been encountered in two wells, but control is not sufficient to place a definitive gas-water contact. Throughout the Thamama, consistent fluid types and pressures have been found. The large variation in temperature complicates any reservoir analysis, since isothermal conditions cannot be assumed.