Issues in rock physics and implications for DHI interpretation

Seismic techniques for direct hydrocarbon indication rely on the changes in seismic character which accompany changes in pore fluid content. Interpretation and modeling of direct hydrocarbon indicators (DHIs) require an understanding of the factors affecting seismic velocities and reflection coefficients which are, in turn, dependent on pore fluid properties, rock frame properties, and fluid substitution within a given rock frame. Recent developments in rock physics have greatly increased our general understanding in these areas but have also revealed complications that require further research. In the past decade, notable progress has been made in several areas which are particularly important for DHI analysis. These include: Shear-wave velocity prediction. In 1985, in conjunction with Ray Eastwood, two of us (Castagna and Batzle) published the “mudrock trend” in GEOPHYSICS. This equation allows rough estimation of shear-wave velocities in clastic basins and has been extensively used in AVO analysis and other applications, particularly when little other information about rock properties is available. The following year, one of us (Han), along with Nur and Morgan, published detailed relationships between compressionaland shear-wave velocities, porosity, clay content, and effective stress for well consolidated shaly sandstones. A very robust algorithm for shear-wave velocity estimation in mixed lithologies, developed by Matt Greenberg and Castagna, was published in Geophysical Prospecting in 1992. Pore fluid properties. Another development in 1992 was a GEOPHYSICS