DOLOMITIZATION IN MIOCENE CARBONATE PLATFORMS OF CENTRAL LUCONIA, SARAWAK: CHARACTER, ORIGIN, AND IMPACT ON RESERVOIR PROPERTIES

The occurrence of dolomite has been reported in several Miocene carbonate platforms of the Central Luconia Province, Sarawak. However their character, origin, and impact on reservoirs properties have received little attention. This study aims at conducting a thorough and comprehensive investigation of the dolomite texture(s) present in two Miocene carbonate platforms of Central Luconia, their most probable origin, impact on reservoir properties, and an assessment of how the lateral distribution will likely be. A total of sixty five (65) core plugs, thirty (30) from the North Platform and thirty five (35) from the South Platform, were obtained and analyzed with microscopic and geochemical techniques. Results of the analyses show that the dolomites of the two carbonate platforms have distinctly different textures and considerably different diagenetic features and history. A mimetic replacement dolomitization is predominantly observed in the North Platform succession, where the dolomite retains the original precursor limestone texture. In the South Platform, dolomite is present in mostly non-mimetic replacement style, obliterating the original texture of precursor limestones. Dolomite crystals in both platforms are commonly planar euhedral, with a minor proportion of planar subhedral developed only in the deeper section of the South Platform. The size of the crystals ranges from < 10 μm to 180 μm. Stable isotope values and trace elements content show that pervasive dolomitization was most likely caused by diluted seawater that circulated on, or near the mixing zone area. Pore-filling and pore-lining dolomite cement may have precipitated from mixed-water in the mixing zone. An assessment of the geometry of the dolomite bodies, based on the proposed dolomitization model suggests that dolomites could have formed elongated dolomite bodies throughout the platforms, forming massive bodies that mimic the lens shape of a mixing zone. However, their thickness and the depth at which they will be encountered will most likely vary