Deep-marine rocks of the Belaga Formation (Cretaceous-Eocene) in the Sibu-Tatau area, Sarawak, show a variety of facies types, which are characterized by grain fabric, bed thickness, and sedimentary structures. The main facies types are (A) thick-bedded sandstone, (B) thinly-bedded heterolithic sandstone-mudstone interbeds, and (C) mudstone facies. These facies may be interpreted in relation to a submarine fan model, in which facies A represents a proximal position (near to source area) while facies B and C represent deposition in the middle to distal parts of the system, respectively. Within this general fan model, a detailed characterization of the facies can be made to understand the depositional processes operating in the deep-marine environment. Facies A, for instance, comprises massive and graded sand beds that appear to be linked genetically; the massive bed, often with floating mudclasts at the top, probably represent a debris flow deposit laid down over a pre-cursor turbidity flow deposit, which is commonly preserved as a thin graded bed at the base of the sandbody. Such linked debrite-turbidite facies association seems to be a common feature in the Belaga Formation, similar to many other deep-marine depositional systems, including the West Crocker in Sabah.

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.

The Tembungo field located offshore Sabah is a highly faulted anticlinal structure where oil and gas accumulations occur in different fault blocks. This paper discusses source rock potential, characteristics of Tembungo oils, and oil-oil and oil-source rock correlations between oils and source rocks from Tembungo and adjacent fields. The shales of the Tembungo wells have poor to fair source rock potentials mainly of Type III gas-prone organic matter and minor contributions from Type II oil-prone organic matter. Maturity data show that the organic matter in the Tembungo well sections are immature. The Tembungo crude oils from the different fault blocks are genetically similar, paraffinic, contain low sulphur and wax, and have moderate API gravity. The presence of C 24-tetracyclic terpanes, oleananes, C 80-resin triterpanes and predominance ofC 29-steranes in all the samples suggest that the oils are derived from source rocks of terrigenous origin containing different mixtures of land-plant organic matter including resins. GC and GeMS analyses indicate that the crude oils produced from the same fault block have similar biomarker distributions but some variations occur in oils from different fault blocks. These variations are interpreted as due to the effect of migration and biodegradation whereby each fault block has a separate fluid system and there is no or very little intermixing between them. Oil-oil correlation indicates that the oils in the study area have similar biomarker fingerprints and could have been generated from the same source rock type containing abundant terrigenous organic matter.

Extensive field-geology observations of the Kudat Peninsula, Sabah, resulted in substantial revision of its geological ages, stratigraphy and structure. Four geological terranes make up the peninsula. The (1) Northern Sabah Terrane is a large anticline representing an exotic crustal unit separated by the (2) Kudat Fault Zone from the (3) Slump Terrane that includes the wide area from Sikuati to Kota Marudu, consisting of mainly slope sediments with distinct slump intervals. The southernmost terrane is the (4) Mengaris Duplex formed by the West Crocker Formation. Easterly trending fold axes and reverse faults are drag phenomena associated with the Balabac Transcurrent Fault. The up to 6 km wide Kudat Fault Zone is a horst consisting of Early Cretaceous ophiolite and oceanic crust. Markings on steeply inclined fault surfaces indicate left-lateral wrenching among its latest activity. The thick amalgamated sandstone of the Northern Kudat Terrane has early to middle Eocene calcareous nannofossils. The Suang Pai limestone enclave among the sandstone possesses Middle Eocene benthic foraminifera and fragments of shallow-marine organisms. The limestone rests on deep-water mudstone of the same age. The Slump Terrane is characterised by scores of meters wide slump intervals composed of lower slope fan turbidites and also steep to overturned tectonic folds verging northeast. The Mengaris Duplex consists of latest Eocene to Oligocene turbidites belonging to the West Crocker Formation of Sabah.

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