Bulletin of the Geological Society of Malaysia, Volume 32, Nov. 1992, pp. 185 – 211
PETRONAS Petroleum Research Institute, Lot 1026, PKNS Industrial Area, 54200 Hulu Kelang, Selangor
Abstract: The ‘Tigapapan Unit‘ is one of several Miocene hydrocarbon-bearing reservoirs in the northwestern Sabah Basin. This unit consists of bioclast-rich, clastic-carbonate mixture, and has been interpreted as progradation storm shoal deposits.
Cathodoluminescence, geochemical and isotopic studies indicate that the sequence had undergone at least 9 stages of cementation which were responsible for complete occlusion of interstitial pores. Each stage represents one distinctive cement texture which was precipitated at specific temperature and burial condition. The diagenetic evolution was first introduced by (Ca1) a syndepositional, accicular and bladed iron-free marine calcite, followed by (Do1) early methane-derived dolomite resulting from degradation of biogenic methane during uplifting and induced faulting, (Ca2) bladed calcite, (Ca3/Ca4) blocky, vein-filling Fe-calcites, (Do2) clay-associated dolomite, (Do3/D04) Fe-dolomite and ankerite, and (Ca5) late iron-rich calcite. Each cement stage is composed of distinctive Sr4+, Mn2+, Mg2+, Fe2+, Ca2+, Si4+, and Na+ concentrations, reflecting changes in palaeopore fluid systems with time. The cement stages are correlatable over the whole unit.
The cementation occured from very early to late phases of diagenesis at near surface to 2.0 km depth. With progressive burial and temperature increased, oxygen isotope values become strongly negative (for example, stage Ca4: δ13C = +2.70 ‰ PDB, δ18O = -5.70 ‰ PDB). Oxygen isotope data indicate that the cements were precipitated at temperatures in the range of 20-75°C. Carbon isotope data suggest that the source of carbon varies from marine dissolved carbonates to fermentation of organic matter, biogenic methane, and possibly hydrothermal fluids.
Strontium isotope dating of Do1 (with δ13C = -34.7 ‰ PDB, δ18O = +2.60 ‰ PDB) and Do2 dolomites indicates that these two dolomitization events took place at 10.5 Ma and 8.9 Ma, respectively. The Do1 dolomite was precipitated from the methane-derived brine exactly at the boundary between the Serravallian and .Tortonian stages after a major uplifting, induced faulting, and beginning of sealevel fall. The Do2 dolomite was precipitated at an intermediate to deep burial diagenesis during smectite-illite transformation.