The character of the Cenozoic sedimentary sequence of NW Borneo exhibits the interplay of eustatic sea level change and the commonly dramatic effects of tectonic events. In this paper, we show how the adoption of a unified stratigraphic nomenclature, tied to the Global Sequences of Haq et al. (1988), can help unravel the superimposition of eustatic and tectonic processes. We also assess the importance of these processes in terms of the genesis, character and the distribution of the basin fills. The results give us a greater understanding of the key exploration plays in the region and a framework to link the play elements.

The Half-Graben sub-province in West Luconia, offshore Sarawak is characterised by a series of NNW trending, SW dipping extensional faults, creating significant sub-basins within the half-grabens. Structural extension is interpreted to have taken place mainly during the Middle Miocene. The timing is supported by age dating of tilted syn-rift carbonate wedges at the base of the half-graben. These carbonates probably drowned as a result of the half-graben formation and the related rapid subsidence and influx of clastics. Middle Miocene fluviomarine to shelfal sediments later filled the half-grabens. The beginning of the Upper Miocene coincides with the end of rifting. Post-rift sediments in this area are separated by two main hiatuses: the Upper Miocene (SB 3.1, 10.6 Ma) and the Lower Pliocene (SB 3.4, ~5.6 Ma) unconformities, which are consistently observed throughout the study area. They can be identified by the presence of lowstand features such as erosional truncations and channel incisions. Thick, seismically transparent transgressive shales lie above the unconformities. Pliocene to Recent fluviomarine to shelfal sedimentation is dominated by sea-level fluctuations; evidence of the latest lowstand is still present in the form of a sea-bottom trough/channel, which is interpreted as the Proto Rajang/Lupar River. The Miocene-Pliocene boundary was also the time of the last major structural deformation during, which large, highly faulted, anticlinal structures formed locally within the NW part of the area. The origin of these anticlines may be attributed to wrench related inversion on some of the extensional faults. The structural and sedimentation history of the area produced a variety of trapping configurations and a diverse portfolio of leads.

The D35 oilfield was discovered in 1983 and over the next seven years, six appraisal wells were drilled to test the reservQir and hydrocarbon distribution. The productive reservoirs comprise exclusively of channel deposits of Early to Middle Miocene age, which exhibit highly variable lateral reservoir distribution. Development drilling in the field began in 1993 with fifteen wells drilled from two platforms. ' Results from this iriitial development campaign indicated that a secondary objective, the Cycle III Channel Sand was absent over much of the field. ~ resulted in a downward revision in the inplace oil volumes for this reservoir unit and downgraded its prospectivity. As part of efforts to enhance the structural and stratigraphic understanding of D35 prior to further development, a 3D survey was acquired in 1995 aimed at giving superior lateral and vertical resolution. The initial result of the interpretation appears promising. Seismic amplitude extractions at and near the Cycle III Channel Sand level have revealed features which are diagnostic of reservoir geometry and has helped explain much of the disappointing results from the first round development. More significantly the new 3D has highlighted additional scope for hydrocarbon recovery from this reservoir. DEVELOPMENT HISTORY The D35 field is located in the Balingian geological province some 130 km offshore and to the northwest of Bintulu town Sarawak (Fig. 1). The field was discovered in 1983 following which 2 appraisal wells (D35-3, D35-4) were drilled encountering the hydrocarbon accumulation in the main field area. Between 1986 and 1990 a further 3 appraisal wells (D35-5, D35-6 plus D35-6 sidetrack and D35-7) were drilled to test the northern and western limits of the field. Development drilling in D35 was undertaken from late 1993 to early 1994 and comprised of 14 wells drilled from two drilling platforms D35DP-A and D35DP-B. First oil production came on stream in October 1994.

The 035 oil field in the Balingian province offshore Sarawak was discovered in 1983. It came on-stream in late 1994 and currently produces around 14,000 sth/d. An infill development drilling campaign has been planned for late 1997. A multidisciplinary study for the 035 field, including quantitative interpretation of the seismic for porefill prediction, was undertaken to support the development drilling. A high resolution 30 seismic survey was acquired over the 035 field in 1995. As the seismic data was of reasonably good quality, an AvO study was initiated. True amplitude near and far offset migrated sub-stacks were created, and amplitudes extracted at objective levels. AvO attributes were calculated and the responses at the infill target locations were evaluated by comparing the results with those observed at well locations. The results of this study showed that the brine-bearing and oil-bearing reservoirs have different AvO responses. Thus, the results gave an indication of the porefill at the reservoir level, and this in turn assisted in optimising the revisit target locations. In conclusion, AvO is aJ). additional tool that can be used in screening drilling targets and for risk management in a multidisciplinary team environment. The technique offers added information to reduce uncertainties as presented here in the 035 study.