The Rock Physics of Group H in the North Malay Basin

Seismic amplitudes are determined by the vertical distribution of the elastic properties of the subsurface modulated by the effective seismic wavelet. Seismic AVO is determined by the changing relationships between acoustic impedance, Vp/Vs, and density. For the Lower Coastal Plain rocks of the North Malay Basin this results in quite complex seismic signals. The rocks comprise flood-plain shales, levees, crevasse splays, crevasse channels, and coal. Although there appears to be systematic variations in the individual elastic properties with clay content for different facies, the inter-relationships change. As clay content increases Vp/Vs tends to increase monotonically. Acoustic impedance, on the other hand, has the highest values for a mid-range of clay content. This means that contrasts in acoustic impedance do not necessarily correlate with contrasts in Vp/Vs. The vertical variations in facies in this environment of deposition can result, for example, in seismic events that brighten even when hydrocarbons are not present. In addition, seismic events are not expected to align horizontally within an angle gather, which has implications for seismic processing and conditioning. Coals and anisotropy introduce further complexity. Interpreting seismic amplitudes for facies and fluids becomes challenging, especially when these variations are below seismic resolution.