Bulletin of the Geological Society of Malaysia, Volume 61, December 2015, pp. 59 – 73
1PETRONAS Malaysia Petroleum Exploration, Level 16, Tower 1, PETRONAS Twin Towers,
Kuala Lumpur City Centre, 50088 Kuala Lumpur, Malaysia
2JX Nippon Oil and Gas Exploration (Deepwater Sabah) Limited
Level 51, Menara 3 PETRONAS, Kuala Lumpur City Centre, 50088 Kuala Lumpur, Malaysia
3Independent Geoscience, Oil and Gas Consultant
*Corresponding author: firstname.lastname@example.org
Abstract: A review of clastic sandstone reservoirs in the Penyu Basin and Tenggol Arch area, adjacent to the south-western flank of the Malay Basin revealed that most deep reservoirs are affected by diagenetic alteration of reservoir mineral components. Furthermore, fluid inclusions in quartz are seen at distinct stratigraphic reservoir levels. These inclusions occur in Oligocene reservoirs in Groups L and M and in Miocene reservoirs of Groups K and H. However, to-date no oil inclusions have been found in Groups I and J. There appear to be two distinct populations of fluid inclusions in the so-called ‘oil quartzes’:
(i) oil inclusions in allochthonous, detrital quartz grains. These inclusions are thought to be of the primary type, formed when oil was incorporated in growing quartz crystals, and oil is seen encapsulated in ‘loose’ quartz grains. There is no evidence of destructive diagenesis or cementation in these relatively shallow (Miocene) host reservoirs, and oil migration is not confirmed by other indicators such as bitumen, which is often found in deeper carrier beds together with oil quartzes. The origin of these oil quartzes is somewhat controversial, and cannot be determined with certainty. They were probably shed into the basin from eroded granitic basement horsts and ridges.
(ii) in situ oil inclusions in quartz cement. The occurrence of oil encapsulated in quartz cement (secondary or tertiary inclusions) indicates oil migration had preceded quartz cementation. So-far, oil-bearing inclusions attributed to fractures could not be confirmed with certainty. Assuming a relatively constant temperature gradient in the basin during the Miocene, quartz cementation will have started at a palaeo-depth of ca. 2000 m or at 105 0C, and porosity was mostly destroyed by a depth of ca. 3000 m and 130 0C. Occasionally overpressures are observed. According to this model, oil had migrated into and percolated within reservoirs during the Miocene, but became locked in closed pores as quartz cement invaded pore spaces under increasing overburden and temperature. Consequently, fluid inclusions in quartz for this model suggest that depths of greater than 3000 m below mud line (BML) are likely to encounter sandstones with deteriorating reservoir properties in the study area.
Keywords: diagenesis, inclusions, Penyu Basin, quartz cementation, Tenggol Arch