Bulletin of the Geological Society of Malaysia, Volume 27, November 1990, pp. 137 – 159
Sarawak Shell Berhad
Abstract: The D18 Field is located 56 miles north-west of Bintulu in sub-block 4Q-15 of the Balingian geological province, offshore Sarawak. The productive reservoirs comprise Early Miocene (Cycle II) lower coastal plain and delta plain deposits. The field is a tectonically complex structure, bounded to the south by a series of en echelon reverse faults and is fault and dip closed to the north, west and east. The internal geometry is complicated by the presence of numerous cross cutting WSW/ENE and N/S trending faults.
The field was discovered in 1981 and following encouraging appraisal of the eastern part, an eight-slot production platform was installed. Five development wells were drilled and the field came on stream in 1986. The field produced at an initial rate of 5300 BOPD, although a rapid production decline was observed in some of the wells. The poorer than expected results combined with the presence of several different fluid contacts and variable sand distribution raised questions about the lateral extent and degree of interconnectedness of the reservoir sands.
A detailed reservoir geological study (including ten wells and approximately 1,400 ft of core) was therefore undertaken to develop a geological model for the Cycle II sediments which could be used to determine the characteristics (sand quality/heterogeneity) and architecture (geometry/lateral extent/connectedness) of the reservoir sands. This was integrated with the results of a 3D seismic study which was undertaken to determine the extent of reservoir level faulting.
The Cycle II deposits have been sub-divided into an Upper, Middle and Lower interval. The main productive reservoirs occur in the Middle Cycle II. Four genetic sandbody types are identified, namely fluvial and distributary channel, crevasse, mouth-bar and shallow marine sands. A geological model was proposed which envisages the north-west to north-easterly progradation and abandonment of small delta lobes in a river-dominated lower delta plain setting.
Detailed log correlation in the Middle Cycle II interval indicated the considerable lateral extent of both the shallow marine sands which occur field wide (> 22,000 ft), and the crevasse and mouthbar sands which can be correlated over distances of 3,000 to 10,000 ft. These sands are thin (10 to 30 ft) and exhibit a wide range in reservoir quality depending upon their location relative to either the proximal or distal parts of the abandoned delta lobe margin (shallow marine sands) or the active distributary channel (crevasse and mouthbar sands). The thicker (30 to 50 ft) and better reservoir quality fluvial and distributary channel sands are of more restricted lateral extent (typically 800 – 2,500 ft) and can only occasionally be correlated between wells on the current well spacing.
The geological model has provided an improved understanding of the distribution of the reservoir sands and recoverable reserves in the D18 Field. The study has indicated that different fluid contacts observed in laterally extensive sands are probably the result of offset by sealing faults. These faults compartmentalise the D18 Field into several fault bounded blocks each containing isolated reservoir sands capable of supporting their own fluid columns. Detailed mapping of the reservoir sands within the fault bounded blocks has enabled more accurate determinations of hydrocarbon volumes, predictions of ultimate well recoveries and the production potential of the D18 Field.