ABSTRACT: Growth Architecture, Faulting and Karstification of a Middle Miocene Carbonate Platform, Luconia Province, Offshore Sarawak, Malaysia

Abstract

The Mega Platform is a 30- 50-km-large and 1.2-km-thick middle Miocene carbonate platform located in the Luconia Province, offshore Sarawak, Borneo. The platform originated in the late early to early middle Miocene on a regional fault-bounded structural high, first aggraded and then backstepped during a series of third-order sea level fluctuations during the middle Miocene (TB2.32.6). The Jintan Platform termination with an area of 8 12 km is one of the prominent backsteps toward the top of the Mega Platform. Three-dimensional (3-D) seismic indicates that growth on Jintan ceased relatively early with continued carbonate aggradation in adjacent smaller terminations (M1, M1-East). Spectacular reservoir architecture and diagenesis are revealed by the seismic. Several transgressive, aggradational, and progradational cycles are overprinted by repeated karst events. Dissolution features and bank-margin collapse are aligned to a deep-seated regional fault system, which periodically became reactivated during carbonate growth. A large triangular-shaped graben formed during one of the faulting periods but subsequently healed by a prograding reef-margin sequences. Two alternative scenarios are presented to explain the ultimate demise of the platform. The first proposes drowning resulting from a combination of subsidence and eustatic sea level rise. The second evokes a much-later drowning, which was preceded by a long period of exposure resulting from a second-order sea level fall and an initial decrease in subsidence caused by the onset of tectonism in Borneo during the late Miocene. In any case, following a hiatus of about 5 m.y., the platform was finally buried by deep-marine siliciclastics that prograded into the basin from the large delta systems of northwest Borneo. Recognition of growth architecture, faulting, and karstification is a key to exploiting the hydrocarbon reservoirs of the Mega Platform. A 30-m-thick low-porosity and -permeability layer shields the gas trapped in Jintan from the underlying aquifer. Penetrated by only one well, the extent of the layer and areas of breaching caused by faulting and karstification are identified on seismic. Interpretation of the seismic is critical to assessing whether and how the underlying aquifer is felt during reservoir depletion and whether there is pressure communication between adjacent reservoirs connected via the aquifer. Cores and logs from three wells provide ground truthing of reservoir architecture, karst features, and faulting derived from the interpretation of reflection and inversion seismic. The interpretation is then imported into static and dynamic 3-D models to constrain reservoir properties, predict dynamic behavior, and guide optimum field development.