CARBONATE-SHELF EVOLUTION DURING THE OLIGOCENE TO EARLY MIOCENE: INSIGHTS FROM SHELF ARCHITECTURE, LITHOFACIES, AND DEPOSITIONAL MODELS OF THE KUJUNG FORMATION, OFFSHORE EAST JAVA, INDONESIA

Oligocene-Miocene carbonates are prolific hydrocarbon reservoirs in Southeast Asia. Extensive subsurface data for this stratigraphic section has become available through exploration and production activities. A carbonate shelf in the study area showed an evolution in shelf architecture and lithofacies during this period. Despite the economic importance, complexity, and data availability, there have been few published studies on a comprehensive description and interpretation of carbonate-shelf evolution in the region. We utilized data from nineteen wells and 1,300 km(2) of 3D seismic data from offshore East Java to study the Oligocene-Miocene Kujung Formation. An average of 700 m stratigraphic sections from this formation were analyzed. This interval spans 13.5 million years (My) from the Rupelian, Chattian, through Aquitanian constrained by numerical ages from Sr-87/Sr-86. The Kujung Formation is subdivided into the Rupelian-Chattian mixed-siliciclastic-carbonate shelf (MSCS) and the Aquitanian carbonate-buildups shelf (CBS) based on shelf architecture and lithofacics. The boundary between the MSCS and CBS is interpreted to be near the Oligocene-Miocene boundary at 23 Ma. Accumulation rates in the CBS are up to three times greater than that of the MSCS. We propose new depositional models for the Kujung Formation, which was used to discuss the dominant controls on shelf evolution. This study suggests that although climate played a role in dictating environmental conditions during the Oligocene-Miocene, the dominant factors controlling carbonate-shelf evolution in the study area appear to have been antecedent topography, routing of siliciclastic sediment, and patterns of sea-level fluctuation.