Publications

In the West Natuna Sea, Cumi-Cumi PSC, Middle Miocene age Intra-Muda Fm. sandstones are draped over the Boundary High, an Oligocene inversion feature. The Intra-Muda interval is characterised by a strong seismic amplitude anomaly over the crest along with 'flatspots' around the flanks of the structure. Tenggiri-1, a well penetration on the feature, found the sandstones to be gas charged, and as a consequence the interval has for many years been regarded as a potential hazard to drilling. Recent commercial opportunities have made gas more economically attractive, and various geophysical techniques were used to build a reservoir model for the Intra-Muda Fm. over Boundary High using 2D seismic data.Intra-Muda Fm. sandstones are buried to only 1100 feet subsea over the Boundary High and appear on seismic at around 400ms TWT. This shallow depth of burial results in a high-resolution seismic image with frequencies as high as 150 Hz. The low pore pressure means that gas within the pore spaces significantly reduces the interval velocities in the reservoir sandstones relative to surrounding mudstones, which results in clear Direct Hydrocarbon Indicators (DHI's).The excellent quality of the seismic data allowed amplitude mapping on stacked data, which was used to define the extent and relative magnitude of the seismic anomaly. Seismic stacking velocities show significant slowing in the Muda section over the feature, and they have been used to predict areas of thickened gas pay. The high quality and frequency of the data allowed detailed seismic facies mapping. This was particularly useful in combination with 2D zero-offset seismic modelling to estimate net pay thickness in the various seismic facies types observed. The high frequencies present within the seismic data allow sandstone units as thin as 5ft to be resolved. Amplitude versus offset (AVO) processing was also used to define the extent of gas saturated reservoir. The method of crossplotting P-wave (P) amplitude against Gradient (G) proved extremely successful in picking the top and base of sandstone units and verified the results of the zero-offset seismic forward modelling.The modelling work was later confirmed by the drilling of the Mako-1 well, which encountered a thickness of gas bearing sandstone within the range predicted and very close to the most likely case. Well results also verified the GWC predicted by modelling.