Hydrocarbon Generating Potential of Terrigenous Shales and Coals of the West Balingian Province, Offshore Sarawak

Abstract

The Balingian Province is a proven hydrocarbon province in Sarawak and has been actively explored since the early 1920s. The Balingian Province is subdivided into West and East Balingian sub-provinces which are separated by N-S alignment of sub-basins or depocentres, namely the Acis, South Acis and Balingian sub-basins (Mazlan and Abolins, 1999). The sediments had been subdivided into 8 sedimentary units, called cycles, I - VIII, with Cycle I as the oldest (Ho, 1978). Major oil and gas accumulations are found in the western part of the province, whereas only minor accumulations occur in the eastern part. Geochemical characteristics of most oils indicate generation from mature source rocks containing high proportion of terrigenous organic materials (e.g. Awang-Jamil et al., 1991). Despite active exploration, uncertainty still surrounds the identity of the source rocks responsible for generating the oil and gas. Therefore, in this study, detailed geochemical investigation of the source-rocks of the West Balingian Province was carried out. The objectives of this study were to determine their hydrocarbon source potential and to delineate the oil generation threshold through assessment of maturity. For this purpose, 90 rock samples (ditch cuttings and cores) from three wells (DA-2, DB-2 and DE-1) drilled in the northwest area of the Balingian Province was subjected to a series of organic geochemical analyses. The two main lithologies analysed were shale and coal, selected from thermally immature to mature sections of Cycles I, II and III of the Tertiary sediments. Rock samples from Cycles IV - VIII were not included because they are generally immature (below 0.4% Ro) and therefore are not thought to have contributed to hydrocarbons accumulation in this area.