Organic Facies Variation in Lacustrine Source Rocks in the Southern Malay Basin

Abstract

This paper attempts to look at, in more detail, the source rock quality of the lacustrine shales within the 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 linked-scan gas chromatography / mass spectrometry / mass spectrometry or GCMSMS to provide highly sensitive measurements of biomarkers which are typically in low concentrations in source rock extracts and oils, and especially so 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% (Fig. 1). Kerogen composition of these shales varies, showing mixtures of Type II and Type III indicating variable contributions from algal, bacterial and higher plant organic matter deposited in a highly to less oxidising environment (Fig. 2). This is indicated by hydrogen index (HI) values ranging from 137 to 403. Group L lacustrine shales seem to provide the best oilprone 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 (Fig. 2).