Integrated evaluation of a paleo gas water contact and residual gas zone in the Sirasun field, East Java Sea, Indonesia

Abstract

Abstract A strong seismic direct hydrocarbon indicator (SDHI) is present beneath the Sirasun gas field in the East Java Sea, north of Bali, Indonesia. The SDHI cuts across lithologic boundaries and coincides with the base of a residual gas zone which lies 33 ft (10 meters) below the current free water level (FWL). Recognition of the current gas water contact (GWC) and a paleo-GWC (the base of a zone of residual gas saturation) was achieved through the integration of mud log data, electric logs, wireline formation pressure data, drill stem test data, capillary pressure data, 3D seismic, and fluid substitution modeling. Formation pressure profile analysis integrating wireline (RFT) and drill stem test data (DST) was used to determine the present day free water level in the reservoir at -3316 ft (-1011 m) (subsea). These results were compared to estimates of the depth of the present and paleo-GWC's from electric log evaluation, mudlog gas shows while drilling, synthetic seismic correlations to 3D seismic, and 3D seismic interpretation. Fluid substitution modeling at reservoir conditions was used to quantify the effect of varying gas saturations on compressional wave velocity, bulk density, and seismic impedance. The fluid substitution modeling confirmed the observed log and seismic response at the GWC and the paleo-GWC and indicated that the mapped SDHI is a response to the paleo-GWC (base residual gas). Also, the fluid substitution modeling indicated that the gas saturation below the paleo-GWC at-3348 ft (-1020 m) is essentially zero and that no trapped gas exists below the seismic direct hydrocarbon indicator.