Geology and Development of Thamama Zone 4, Zakum Field

Abstract

Summary Thamama Zone 4 of the Zakum field, one of the six oil-bearing zones of the Thamama Group, consists of three well-defined, very porous limestone reservoir units. Production from Zone 4 started in 1967, and by the end of 1978 cumulative production was 647 million bbl (102.8 × 106 m3) of oil. To arrest declining reservoir pressure, initially crestal water injection and later peripheral water injection were undertaken. Introduction The Zakum field is in the Arabian Gulf (Fig. 1) about 44 miles (70 km) northwest of Abu Dhabi. Water depths range from 20 to 80 ft (6 to 24.4 m). The field covers an area of about 256 sq miles (656 km2). The field was discovered in 1963 when Well ZK-1 tested the crest of a large, seismically defined, east-west trending structural feature. Oil was found to occur in all six zones of the Lower Cretaceous Thamama Group.Thamama Zone 4, currently the most important producer in Zakum field, occurs approximately in the middle of the Thamama Group. It consists of three discrete porous limestone units, each of which is underlain by dense argillaceous limestone. Appraisal, Development, Production, and Reserves Following the drilling of ZK-1, three appraisal wells were drilled that confirmed the existence of a large field. Development drilling followed immediately, and the field was brought on stream in Oct. 1967, with production coming mainly from the Thamama Zone 4. By the end of 1978, Zone 4 produced 647 million bbl (102.8 × 106 m3) of oil, representing about 69% of the total production from the Thamama zones.The estimated original oil in place in Zone 4 reservoirs is computed at 12.25 × 10 STB (1.95 × 109 stock-tank m3). The original reserves are estimated at 4.288 × 109 STB (0.682 × 109 stock-tank m3) assuming a 35% recovery factor. The oils in the three reservoirs are similar in composition and have API gravity of 40 deg. with sulfur content of about 1.0%. Regional Stratigraphy and Stratigraphic Position of Thamama Zone 4 Details of the stratigraphic sequence penetrated by drilling in offshore Abu Dhabi are given elsewhere. Basically, the succession (Fig. 2) from the Quaternary down to the Lower Jurassic Izhara formation comprises predominantly carbonates (limestone and subordinate dolomite) with minor evaporites (mainly anhydrite) and very minor shales.The Thamama Group is a thick sequence of limestone and subordinate dolomite of Lower Cretaceous age. In Zakum, the Thamama Group averages 2,250 ft (686 m) thick. The Thamama has a dual nomenclature (Fig. 2).1. A reservoir-oriented classification subdivides the group into six zones, given numerical notation from top to bottom, while the porous intervals within the zones are assigned alphabetical notation.2. A stratigraphic classification, applicable regionally, subdivides the Thamama into four formations - Shuaiba (top), Kharaib, Lekhwair, and Habshan.Thamama Zone 4, therefore, forms the lower part of the Lekhwair formation. It is a distinctive lithological unit which is developed regionally in the southern Arabian Gulf. In Zakum, Zone 4 has an average thickness of 250 ft (76.2 m). JPT P. 1327＾