Abstract

This paper demonstrates synergy of geophysics and geology in a preliminary study delineating a gas reservoir at Pantai Pakam Timur Field. The objective is a sandstone unit in the lower Keutapang Formation corresponding to a seismic amplitude anomaly "brightspot" at 1300 msec.

The study utilized preserved amplitude processing of 2D seismic profiles plus subsurface data from four wells. Seismic parameters of amplitude distribution, amplitude variation with offset (AVO), and tuning effects were used to define the thickness and distribution of the gas reservoir. Analysis of well logs provided pay thickness information, and production test data proved the presence of gas in the reservoir.

Analysis of seismic amplitude distribution shows that amplitude values vary with reservoir pay thickness. Amplitudes also change in relation to variations in reservoir porosity or gas content The AVO response shows strongly negative AVO gradients in the area of high amplitudes, and the area with a strong AVO anomaly corresponds to the gas reservoir.

Analysis of tuning effects is necessary because the objective reservoir is thinner than the 32 msec dominant period of the seismic data. Pay thickness in all wells is below tuning thickness, and stronger amplitudes may simply indicate thicker pay. These higher amplitudes, however, could also reflect changes in lithology.