Geochemistry of selected crude oils from Sabah and Sarawak

Abstract

A total of 35 crude oils from 15 fields from offshore Sabah and Sarawak have been analysed and characterised using liquid and gas chromatography, and gas chromatography-mass spectrometry. The normal alkane distribution shows that the oils are of three different types, namely, i) normal, non-waxy crude oils; ii) waxy oils (high proportion of C20+ n-alkanes); iii) biodegraded oils (less abundance ofn-alkanes relative to isoprenoids). Biological marker distributions (i.e. steranes and triterpanes) show that the oils were derived from terrigenous source rocks containing mixtures of different types of landplant derived organic matter, including resins. Features of these distributions include the presence of relatively high concentrations of 18n (H)-oleanane, a number of resin-derived compounds, a predominance of C29 steranes compared to the relatively low concentrations of C28 and C27 steranes, and the absence of CSD steranes in all the samples. Source rock matter for these oils were probably deposited under peat swamp environments, as indicated by the high pristanelphytane (>3.0) and pristanelnC17 (>1.0) ratios. Biological marker distributions indicate that the oils were generated from source rocks having maturities of 0.5% to at least 0.8% VRe (Vitrinite Reflectance Equivalent).