This paper discusses the source rock quality of the lacustrine shales within Groups K, L and M in the southern flank of the Malay Basin. This study is made possible through the use of state-of-the-art technique of gas chromatography / mass spectrometry / mass spectrometry or GCMSMS to provide highly selective measurements of biomarkers which are typically in low concentrations in source rock extracts, oils, and especially in condensates. Since only one well dataset is available, only the vertical variation in the source rock quality of the lacustrine shales is discussed. Stratigraphically, there is a noticeable change in the source rock quality within the three groups. In general, the TOC content of the lacustrine shale sequences in Groups K, L and M range from 0.35 to 2.00 wt%. Kerogen composition of these shales varies, showing mixtures of Type I to Type III indicating variable contributions from algal, bacterial and higher plant organic matter deposited in highly to less oxidising environments. This is indicated by hydrogen index (HI) values ranging from 137 to 403. Group L lacustrine shales provide the best oil-prone source rock with TOC values of 0.45 to 1.95 wt% and HI values in the range of 300 to 400 indicating predominantly Type II kerogens. The variation in the source rock quality within the Groups K, L and M may be due to a combination of organic source input and factors controlling the preservation of organic matter within the environments of deposition. This observation is supported by data from screening and microscopic analyses of whole rocks and alkane and biomarker analyses of source rock extracts. It appears that Groups L and M shales, deposited in a lacustrine environment, received more algal input compared to terrigenous organic matter in a less oxic condition resulting in relatively better organic matter preservation. This is shown by the lower Pr/Ph ratio in the range of 3.1 to 4.0, lower Tm/Ts ratio, moderate to high abundance of C30-diahopane and low abundance of tricyclics and gammacerane. On the other hand, the younger Group K had more fluvial influence and consequently received relatively more terrigenous organic matter input being deposited in a more oxidising environment. This is indicated by the higher Pr/Ph ratio (5.1 to 6.2), higher abundance of oleanane, predominance of C29-steranes compared to C27-and C28-steranes, and trace amounts of tricyclics and gammacerane. It is also observed microscopically that Group K has higher abundance of terrigenous-derived vitrinite particles available for measurements as opposed to Groups L and M. The marked change in organic facies within the lacustrine shales from Groups L and M to Group K is reflected in the evolution of the Malay Basin i.e. the transition from synrift to post-rift phase during the L and early part of K times.
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 C 29 steranes compared to the relatively low concentrations of C 28 and C 27 steranes, and the absence of C SD 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 pristanelnC 17 (>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).
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