Heterogeneity of the Hydrocarbon Distribution in Tertiary Sediments of North-East Sarawak

Abstract

Abstract There are tremendous spatial and temporal variability within the sedimentary rock types of the Belait (Early-Mid Miocene) and Lambir (Mid-Miocene) Formations in North Eastern Sarawak. With the current renewed interest in oil and gas exploration in this part of Malaysia, evaluating the distributions of hydrocarbon resources in both Formations will be of valuable interests for the industry. The IR spectra of both Formations showed presence of aliphatic and aromatic components. Total organic carbon results for both Formations show that majority of the samples from the Belait Formation possess much higher percentage of organic carbon contents than the Lambir Formation. In comparing the E4/E6 values, it is observed that the high E4/E6 values give rise to more aromatic components and low E4/E6 values show strong presence of aliphatic peaks in the spectra. Positive relationship exists between the mean pore size and E4/E6 in the Lambir Formation, whereas, in the Belait Formation, the same trend is observed between the porosity and E4/E6 ratios. Variability in these geochemical patterns indicate that there are differences in the quality of hydrocarbon between the Formations. The study shows that the variation of aromatic and aliphatic bonds found in the samples suggests differences in terrestrial origin and marine origin and a complicated depositional pattern temporally and spatially. Variability in the kerogen type discussed above indicates differences in the quality of hydrocarbon between the Formations. Heterogeneity within the same samples here suggests that different rates of migration might occur within the reservoir quality rocks of the Lambir Formation. This indicates differences in show type between both Formations. This study reemphasizes the importance of FTIR in current emerging trends in hydrocarbon research. Introduction North-West Borneo has been producing oil and gas since the 1903 with first onshore well drilled around Miri area in 1910 (Liechti et. al., 1960). Common rock Formations in this area includes Belait Formation (Mid - Upper Miocene), Lambir Formation (Lower - Upper Miocene), Tukau Formation (Lower Oligocene) and the Mid-Miocene Miri Formation (Figure 1). The Belait Formation which is found in the North Eastern Sarawak has been interpreted as coastal and coastal plain deposits from a range of sedimentary environments associated with a relatively large delta (Wilford, 1961) or as fluviatile (Hutchinson, 2005). Towards the coast the environment changes into fully marine, and accordingly the Belait Formation merges gradually into the more marine Lambir or Miri Formations or the uppermost part of the Setap Shale Formation (Liechti et. al., 1960). The Formation consists predominantly of sandstones and alternating shales with minor limestone and marl in some places (Haile, 1957; Liechti et. al., 1960; Hutchinson, 2005).