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. Copyright 2011, International Petroleum Technology Conference.