An alternative stratigraphic scheme for the Sarawak Basin

The present stratigraphic scheme for the Tertiary of Sarawak (Ho, K.F., 197 8. Stratigraphic framework for oil exploration in Sarawak. Bulletin of the Geological Society of Malaysia, 10, 1-14), which subdivides the entire sedi mentary succession into eight sedimentary cycles, is based on the generic s edimentary cycle concept. Each cycle starts with a transgressive basal part , followed by a regressive unit, which is in turn overlain by the basal tra nsgressive unit of the next cycle. The limitation of the present scheme is, however, in its general applicability; for instance, when one tries to ide ntify the equivalents of marine surfaces within non-marine sediments. This is critical for some areas in Sarawak where the sediments were deposited pr edominantly within lower coastal plain to upper coastal plain environments, normally barren of foraminifera. Among other inadequacies of the scheme is the lack of basin-wide association between the cycle boundaries and seismi c reflectors. In practice cycle boundaries, picked on the composite well lo gs, often do not agree with the seismic pick. The scheme is, however, well accepted and continues to be used. The alternative stratigraphic scheme for the Sarawak Basin, which will be discussed in this paper is based on seque nce stratigraphic concepts, with the utilisation of unconformity or its cor relative conformity as the stratigraphic boundary. This scheme has been gen erated as the result of a programme of basin-wide seismic mapping of the Sa rawak Basin. Seven regional unconformities within the Tertiary sedimentary sequences have been mapped. The sedimentary units between the unconformitie s can be recognised as 'Sequences'. The oldest unconformity is that between the basement (Belaga Formation) and the overlying Tertiary sediments, main ly of Late Oligocene age. Since the sediments in the basin are mainly of Te rtiary age, the oldest unit of the succession is referred to as the Tertiar y One Sequence (T1 S). The next younger sequence is called Tertiary Two Seq uence (T2 S) and so on. The proposed scheme, which is based on a more conve ntional and multi-disciplinary approach, can be understood and appreciated, not only by palaeontologists, but by all members of exploration and produc tion teams. The scheme should be able to facilitate prediction of stratigra phic levels. not only after the well has been drilled, but also prior to dr illing. (C) 1999 Elsevier Science Ltd. All rights reserved.