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The Penyu Basin is moderately explored but may still have undiscovered hydrocarbon potential for small to moderate size accumulations. Despite the much publicised Rhu-1 oil discovery made in 1991, the Penyu Basin, with only a couple of sub-economic oil discoveries made, has not had much success ever since. This was generally attributed to the poorly developed generative or immature source rocks most likely present in isolated half-grabens within the basin. The Penyu Basin was formed on continental crust, although the exact formation is not properly understood and most authors generally consider it as a pull-apart or "rift-wrench" basin. This is supported by the presence of major strike-slip and associated normal faults being the main basin-bounding faults. The initial half-graben basins developed into isolated lacustrine systems which provide source-rock facies that may have potentially charged traps in the synrift and post-rift sequences. Trap styles identified include compressional anticlines, basement drape structures and synrift stratigraphic/structural traps. Further data acquisition through the last two decades of exploration activities, such as new 3D seismic, geochemical fingerprinting and fluid inclusion investigations, and full tensor gradiometry (FTG) gravity data adding to the past understanding, has enabled a more refined review of the geology and also of the petroleum potential. Undoubtedly, more detailed mapping of new previously undetected structures, coupled with seismic amplitude analyses and advanced quantitative interpretation (QI) techniques may lead to a better understanding of the structural evolution and, hence, to an increase of hydrocarbon prospectivity by identification of additional plays, new leads and to a potential reduction of exploration risk.

Temperate and cold stages comparable to those of the last interglacial-glacial have alternated for ca . 2.4 Ma, a time-level regarded as the base of the Quaternary. A curve showing climatic fluctuations according to a number of glacials, interglacials, and temperate oscillations of small amplitude or short duration (interstadials) is given, and the value of pollen records in this context is discussed. Because the position of the individual intervals with known vegetational development on the total timescale is controlled by superposition, the lithostratigraphic position of some of them is reviewed. Basic differences in vegetational evolution between the Tiglian, Waalian, and later interglacials, as well as the extinction of certain trees at around the time of the transition of the Early-Middle Pleistocene, probably indicate lower temperatures during the glacials which have occurred in the past million years. Although the established record shows some resemblance to the oxygen-isotope curve of the deep sea, precise correlation is not yet possible. A tentative correlation is discussed.

Following the intense Palaeocene-Early Eocene Sarawak Orogeny (around 40-36 Ma), the South China Sea engulfed the northern shore of Borneo in present-day NW Sarawak, enveloping both the Luconia/Tinjar terrains and also rimmed the recently emerged and eroding Rajang Group hinterlands on the northern Borneo shore. With prevailing inner neritic depositional environment at that time, benthic foraminiferal limestone banks and ramps developed on sheltered shoals, separated from each other by clastic fairways with turbiditic channel deposits. By Early-Middle Oligocene times, carbonate deposition slowed as a consequence of increased subsidence and or, less likely, of a strongly global rising sealevel. After a pause in which clastics dominated the area, a second carbonate system formed during the Early-Middle Miocene times. These carbonates contain the first hard evidence of small bioherms, mainly corals and coralline algae. However, in the study area, there is not a single outcrop or well which shows an uninterrupted carbonate sequence from the Palaeogene to the Neogene. In addition, it is believed that the palaeo-edge of the platforms today lies somewhat masked by tectonic events, in particular, by a Late Miocene to Early Pliocene fold and thrust belt. Consequently, we believe, that both the Eo-Oligocene and Early-Middle Miocene carbonate systems are independent, not linked or vertically interconnected. Arguably, the presence of carbonates in two distinct systems points to a deepening, and later shallowing in a mega cycle. Within the mentioned hypothesis, the Eo-Oligocene carbonate system was formed during the deepening of the NW Borneo foredeep, whereas the Lower-Middle Miocene carbonates originated as the foredeep shallowed. The latter eventually disappeared with the establishment of a shallow, clastic shelf.