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Development Strategy in the Bzz Field and the Importance of Detailed Depositional Model Studies in the Reservoir Characterization of Talang Akar Channel Sandstones
The BZZ Field is located approximately 56 miles (90 km) northeast of Jakarta, in the Ardjuna subbasin of the ARII ONWJ contract area. The field was discovered in the early 1980's with the drilling of the BZZ-1 and BZZ-2 discovery wells, the later of which tested a total of 5208 BOPD from the Baturaja limestones and Talang Akar sandstones. After delineation drilling, it became apparent that the main recoverable reserves lay within the Talang Akar channel sandstones. In order to help predict the subsurface distribution of these channel deposits, a 3D seismic survey was acquired and paleocurrent trends were determined from the dipmeter (SHDT) and Formation Micro Scanner (FMS) logs which were run in all subsequent development wells. Using this new data, a revised development plan was initiated to produce the 39 MMBO reserves the field contains.
The BZZ structure is a four-way-dip closure bounded to the west and south by normal faults. The Talang Akar interval comprises up to 1200 ft (365 m) of interbedded sand, shale, coal, and limestone interpreted to be of deltaic origin. At least four cycles of deltaic depositions are recognized, each of which is associated with a major distributary channel complex. The BZZ-69B sandstone, the main reservoir interval, represents the channel complex associated with the final deltaic cycle.
Despite a detailed knowledge of the BZZ-69B distribution, the second platform development well illustrated the complex nature sandstones of this type can possess. In this well, the reservoir was 25 ft (7.5 m) thinner than expected, and the fluid contact came in 17 ft (5 m) high. To investigate these discrepancies, a detailed depositional model of the 69B sandstone was constructed. Detailed correlations showed that although the sand was one correlatable unit, it actually consists of at least six, partially separated, sand packages interpreted to represent individual channel bar deposits. In most cases, these packages are isolated from one another by 2–4 ft (0.6–1.2 m) thick shale breaks, and the distribution of which appears to account for the variation in fluid contact observed in the reservoir. These shales, in conjunction with smaller scale clay drapes associated with internal cross-bedding, are expected to significantly reduce effective vertical permeability in the reservoir. A lower vertical permeability may result in increased ultimate recovery from the reservoir as the effects of gas cusping and water coning are retarded.
The model has since been tested by the drilling of four more development wells, and the results obtained support the sand package correlation in the reservoir. The study illustrates the importance of conducting a detailed reservoir geological study early in field development, in order to provide critical input for reservoir management and ultimately to optimize field recovery.
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