Seismic lithology and depositional facies architecture in the Texas Gulf Coast basin : a link between rock and seismic

Amplitude variation with offset (AVO) is a seismic attribute containing valuable lithology information. Although AVO analysis can provide three-dimensional lithology information, the technique has not been sufficiently utilized in many geologic interpretations owing to the difficulty in handling pre-stack seismic data. Moreover, most previous AVO analyses have solely focused on detecting gas sands. This study suggests a convenient method to extract a seismic lithology attribute using an AVO crossplot method. A crucial step for AVO analysis is to understand relationships between AVO attributes and rock properties. Missing petrophysical data can be estimated by proper empirical relationships. However, a uniform rock property assumption disregarding lithology variation has to be avoided in order to predict an accurate AVO response. Synthetic seismic modeling enables us to predict AVO crossplot responses depending on variations in each rock property. A change in the Vp/Vs or Poisson’s ratio affects the slope of the AVO crossplot trend. A change in the impedance contrast across the interface causes rotation and scattering of AVO crossplot points. An AVO crossplot of actual seismic data qualitatively coincides with the synthetic modeling result based on the mixed lithology assumption. A seismic lithology attribute cube is created by muting the unimportant lithology AVO crossplot trend and highlighting the preferred lithology crossplot trend. Based on the seismic lithology attribute, this study attempted to interpret depositional facies. Depositional features are easily recognized on seismic horizon slices. Combining seismic lithology and proportional horizon slicing methods enhances interpreting depositional facies architecture and stratigraphic sequence. Seismic lithology is especially useful for interpreting the depositional facies that do not have distinct channelized features. Various shorezone depositional facies are recognized including shoreface sheet sand, barrier bar, lagoonal delta, and muddy facies. Moreover, high-frequency stratigraphic sequences are identified by seismic lithology maps by interpreting successive horizon slices.