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Carbonate rocks frequently exhibit velocity-porosity relationships less predictable than those found in siliciclastics because of complex and heterogeneous pore structures. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rock's physical properties. In this preliminary research, velocity measurements of 20 core plugs of a Miocene carbonate platform in Central Luconia, Sarawak were made under dry-saturated condition at variable confining pressures and at effective pressures of 20MPa. The cross plot of measured velocity and porosity shows scattering along the Wyllie's time average equation. The large scattering in most cases will result in large uncertainties in seismic inversion and calculations of porosity volumes. Digital image analysis (DIA) of thin section images were used to derived quantitative pore geometric parameters. Four DIA parameters proved to best describe several aspects of the pore system: dominant size (DOMsize), perimeter over area (PoA), roundness (?) and aspect ratio (AR). Combination of porosity, microporosity and these four geometric parameters improves coefficient of determination (R2) velocity estimation from 0.9219 to 0.9974. Introduction The Central Luconia Province is a broad and stable continental shelf, characterized by extensive development of Late Miocene carbonates. About 56 carbonate build-ups are proven to contain commercial quantities of non-associated gas in carbonate reservoirs. The Central Luconia Province contains about 40% of the total non-associated gas reserves of Malaysia (Yamin and Abolins, 1999). Carbonate rocks frequently exhibit velocity-porosity relationships less predictable than those found in siliciclastics because of complex and heterogeneous pore structures. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rock's physical properties. This research aimed at verifying the inadequacy of porosity prediction from sonic velocities in Miocene carbonate reservoirs of Central Luconia and at investigating the main causes for the deviations between measured and calculated porosity. A total of 20 core plugs of a Miocene carbonate platform from a well in one of the carbonate platform of Central Luconia, Sarawak, were used for this preliminary verification.

Abstract. Limestones are particularly susceptible to drastic early diagenesis modifications, mainly cementation and dissolution. During the early Miocene, a major tectonic deformation has caused a widespread of uplift in Sabah. This has resulted change in depositional environment from deep to shallow marine, which favours the deposition of Gomantong Limestone. This study aims to investigate the diagenetic history of Gomantong Limestone in East Sabah. Thorough understanding of the diagenetic processes may provide data to unravel the tectonic activities which affected the reservoir quality of the carbonates. Combining the data from comprehensive petrographic analysis, and Scanning Electron Microscopy (SEM) of 30 samples, two main cements type were identified. These are microcrystalline cement and Mg-calcite cement of granular and blocky mosaics which are dominantly seen in all samples. The sequence of diagenesis events are determined as (1) micritization; (2) grain scale compaction; (3) cementation (pore-filling); (4) mechanical compaction and cementation infilling fractures and (5) chemical compaction. These diagenetic events are interpreted as reflection of changes in diagenetic environment from shallow marine to deep burial. The massive cementation in the Gomantong Limestone has resulted into a poor reservoir quality.