Seismic rock physics of the South Sumatra basin coal, Indonesia

Abstract

We have collected some coal samples in different ranks from Muara Enim, South Sumatra Basin, Indonesia. Laboratory ultrasonic velocity measurements of gas and brine saturated coals samples in different rank present that dynamic elastic properties of coal depend on coal rank, overburden pressure and pore pressure. In this paper, we investigate the relationships between coal rank and pressure with dynamic elastic properties of coals. We also present the effect of various coal rank and pore pressure to the AVO response. Coal elastic properties are strongly affected by pressure. Coal velocities increases as overburden pressure increases, while velocities decreases as pore pressure increases. The increasing in pore pressure causes the increasing in AVO intercepts and gradients, while the increasing of coal rank tends to decrease AVO intercept and gradients. We determine the sensitivity of elastic properties to the variations of pore fluid for all coal samples. Pimpedance, S-impedance, and MuRho are good fluid discriminator for lignite, bituminuous and anthracite coal samples whereas they can be used as lithology discriminator for all coal samples. Another Lame parameter, LambdaRho, is only sensitive to discriminate fluid in lignite and bituminuous coal samples. Poisson’s Ratio and VP/VS Ratio are only capable to separate the pore fluid in lignite coal sample. The AVO modeling example offered in this paper shows that AVO may be used as an aid for pressure delineation in the CBM reservoirs, to delineate the area with high fracture density or high pore pressure.