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Using Time-Lapse Seismics to Reveal Heterogeneities That Matter for Understanding the Dynamic Behavior of a Giant Carbonate Gas Field, Offshore Sarawak, Malaysia

Abstract

A giant carbonate gas field in the Central Luconia Province, offshore Sarawak, Malaysia, has been producing for more than 8 years. Currently the main challenge is predicting and preventing water breakthrough at existing producers. In order to assist near-term reservoir management and to evaluate the remaining potential of the field an improved and integrated understanding of the field was required. On a field scale, the platform interior developed in a stratigraphically uniform fashion, with the most critical flow elements being low permeability layers composed of deeper-water, argillaceous, carbonate wackestones deposited during flooding events. In contrast, the superimposed geobodies are manifold and complex. For example, there are multiple phases of karstification either being stratabound or associated with faulting. Faulting occurred at numerous times and at different locations and levels. Along the eastern and southern margins of the platform, fault reactivation of deeper-seated extensional faults bounding the underlying structural fault-block high has been observed. In addition, various styles of reef-margin development have been recognized either as long-lived reef margins or as more localized reefs that developed atop previous platform interiors due to backstepping and platform break up. All the producing wells are situated in the northern, crestal area of the field, whereas the field extends a further 5 km to the south from the southernmost producer. A reservoir surveillance program has been in place since first gas with three wells being dedicated to monitor fluid contact movements on a yearly basis. In addition to a 1992 base survey, two time-lapse (4D) seismic surveys have been successfully acquired in 2008 and 2012. Fluid contact monitoring could be confidently tied to the 4D response and the high quality 4D seismic data was used to delineate sweep patterns. The information obtained indicates that gas has likely been produced from a much wider area including the southern area. Most importantly, the 4D seismic data revealed which of the many heterogeneities or geobodies matter for understanding the dynamic behavior. In what follows, the geosciences community can extract valuable information from the dynamic responses of this mature carbonate field in order to predict the impact of reservoir heterogeneities on hydrocarbon flow behavior and use them as analogs for new field developments and carbonate reservoirs with less data.