Carbonate platforms in the Central Luconia is the most important province in the Sarawak Basin for gas production in the region. These carbonates are economically significant containing 65 trillion cubic feet of gas in place with minor contribution of oil rims. Over 200 carbonate buildups have been seismically mapped, out of which 120 still remain undrilled. These provide potentially attractive exploration targets and incentives to discover the remaining hydrocarbons in the region. The Central Luconia Province is a key geological unit for understanding the distribution of hydrocarbon resources in Malaysia. Although the first hydrocarbon was discovered more than 60 years ago,nevertheless, little effort has been made until now to address the proper facies scheme, cyclicity and reservoir quality of these Tertiary carbonates of Central Luconia.
This paper documents the various facies based on the qualitative and quantitative description of cores of five wells cores in three different carbonate platforms with different cyclicity, carbonate buildups and reservoir quality. Detailed petrographic and petrophysical data have been validated to establish the standardized facies scheme. The combination of facies with cyclicity is very important to understand the depositional setting of the sediments and reservoir behavior. Because the sediments which are deposited during the transgressive phase can act as a barrier for hydrocarbon to migrate vertically.
Eight facies types have been recognized qualitatively and quantitatively (Table-1), namely F-1 coated grain packstone, F-2 coral (massive) lime pack-grainstone, F-3 oncolite lime grain dominated packstone, F-4 skeletal lime/dolo packstone, F-5 coral (platy) lime mud dominated packstone, F-6 coral (branching) lime dominated pack-grainstone, F-7 cross bedded skeletal lime packstone, and F-8 bioturbated carbonate mud stone (chalk). These eight facies reflect different depositional environments ranging from lagoon to off reef settings. Overall, cores from the five wells consist of 80% of limestone, 15% of dolomitic limestone and 5% of dolomite and argillaceous limestone. The facies consist of eight dominant components comprising, 35%of coral, 30% of red and green algae, 30% of skeletal debris with 5% of other bioclasts. Moldic porosity is dominant (up to 45%), with the remaining are interparticle, intraparticle, vuggy and fracture porosity.
Six regressive cycles are recognized within the carbonates based on the core and well logs. The 0.1 to 50m thick facies is stacked in vertical sequence within the 50-100m thick transgressive and regressive cycles. The reservoir intervals are divided into seven zones, based on porosity, permeability and major diagenetic events, representing high and tight porosity. The best reservoir intervals are consistently found in most of the regressive cycles, which composed of skeletal lime packstone, coarse coral fragments with touching vugs. The systematic semi-quantitative characterization, portrayed the buildup as coarse limestone with cyclical grain size variations creating high-permeability layers.