Karst Modeling of a Miocene Carbonate Build-up in Central Luconia, SE Asia: Challenges in Seismic Characterization and Geological Model Building

Abstract

Abstract The development of the Central Luconia carbonate build-ups were strongly influenced by the interplay between eustatic sea-level and basinal tectonics. The Alpha field addressed here is one of the seismically best imaged isolated carbonate platforms in Central Luconia and dendritic features, interpreted as karst, were found to be very prominent throughout the field. Karst is a diagenetic facies, an overprint in sub-aerially exposed carbonate bodies produced and controlled by dissolution and migration of calcium carbonate in meteoric waters. The exploration well in the Alpha field encountered total losses while drilling which was believed to be as a result of drilling in to karst (common in Central Luconia carbonates). Some of the best features from Petrel, Landmark and Shell software packages have been integrated to generate an attribute volume for karst interpretation. Using Petrel Geobody, multiple 3D box probes with short time gate and optimum clipping value were used to extract the karst Geobodies. Using a short time gate box probe is crucial for CPU/memory performance and most importantly, to have better control on editing noise and non-karst signal later. Karst has a big impact on well planning (problem with losses and completion length), the hydrocarbon volume in-place as karst provide substantial secondary porosity; and well management and development strategy (wells nearer to karst are more likely to water-out quicker). This study incorporated an integration of well data, seismic characterization and geological understanding of the field and near-by analogues in building a subsurface model for field development. Geology The Central Luconia Province is located in offshore NW Borneo. The province is delineated by two major strike-slip faults, the West Baram in the northeast and the Rajang line in the southwest (Figure 1). Seafloor spreading in the South China Basin during the Oligocene to middle Miocene resulted in formation of a horst graben system that which controlled distribution of subsequent reefal carbonate growth. At mid-late Miocene reefs developed preferably on horst blocks (Mohammad Yamin Ali & Abolins[1]). Two major factors controlling carbonate sedimentation in the Central Luconia province are the regional tectonics and eustatic sea level changes. Tectonics played a role in creating horst and graben structures which served as basement for the onset of carbonate deposition and exerted an influence on the size and shape of the build-ups (Ting et al[2]). The latter also dictated the type of the depositional facies and their distribution which governed the reservoir properties of the carbonate platform.