Publications

Seismic reflection anomaly in the form of bright flat spots have been observed and drilled in the Mundu Formation. This formation is a Pliocene bioclastic limestone reservoir containing biogenic gas, which is widely spread in the Madura Strait. Many gas discoveries in Mundu Formation confirmed that the flat spot is associated with direct hydrocarbon indicators (DHI). In well logs, the DHI normally correlates with a single gas water contact (GWC). An interesting finding here is that many wells encountered two fluid contacts, the lowest mobile gas and the paleo-GWC. Fluid replacement method was used to model the physical variations of the seismic trace response at the bright spot level with varying gas saturations. When good seismic quality is available, this method could identify the two fluid contacts and quantify gas saturation of the interval. The model confirmed the zone between two contacts as residual gas zone. Observation on the occurrence of shallow gas and paleo-GWC suggests that shallow gas shows can be used as an indicator of paleo-GWC existence. The study concludes that the flat spot should be the main requirement for any drilling prospects targeting gas in the Mundu Formation. The characteristics of two fluid contacts can be indicated by the shallow gas occurrence and identified in the seismic trace. The ability to distinguish two fluid contacts would reduce the uncertainty in quantifying reserves. Keywords: Bright spot, fluid contact and fluid replacement modeling