FLUID CONTACT ANALYSIS BASED ON FLUID REPLACEMENT MODELING ON DIRECT HYDROCARBON INDICATOR FEATURE IN THE MUNDU FORMATION, EAST JAVA BASIN

摘要

Seismic reflection anomaly in the form of bright flatrnspots have been observed and drilled in the MundurnFormation. This formation is a Pliocene bioclasticrnlimestone reservoir containing biogenic gas, whichrnis widely spread in the Madura Strait.rnMany gas discoveries in Mundu Formationrnconfirmed that the flat spot is associated with directrnhydrocarbon indicators (DHI). In well logs, thernDHI normally correlates with a single gas waterrncontact (GWC). An interesting finding here is thatrnmany wells encountered two fluid contacts; thernlowest mobile gas and the paleo-GWC.rnFluid replacement method was used to model thernphysical variations of the seismic trace response atrnthe bright spot level with varying gas saturations.rnWhen good seismic quality is available, this methodrncould identify the two fluid contacts and quantifyrngas saturation of the interval.rnThe model confirmed the zone between tworncontacts as residual gas zone. Observation on thernoccurrence of shallow gas and paleo-GWC suggestsrnthat shallow gas shows can be used as an indicatorrnof paleo-GWC existence.rnThe study concludes that the flat spot should be thernmain requirement for any drilling prospectsrntargeting gas in the Mundu Formation.rnThe characteristics of two fluid contacts can bernindicated by the shallow gas occurrence andrnidentified in the seismic trace. The ability torndistinguish two fluid contacts would reduce thernuncertainty in quantifying reserves.