For high-porosity siliciclastic sediments, we find that the controls on velocity and formation factor are surprisingly different from what has been previously assumed based on low-porosity sediments. It is well known that velocity depends on both lithology and porosity at low porosities. However, well log data from Ocean Drilling Program Leg 156 to the Amazon Submarine Fan show that velocities of these high porosity (30-60%) sediments fit a single velocity/porosity relationship, regardless of lithology. The only lithology effect on velocity is indirect, associated with the different porosities of sandy and muddy sediments.

At low porosity, it is well known that resistivity and formation factor depend on both porosity and clay content. Archie's law is typically regarded as appropriate for clean sandstones because the conductivity of the formation is due to the connectivity of the pores, and shale conductivity is higher than sandstone conductivity because clay surface conduction dominates pore conductivity in low-porosity shales. However, porosity vs. formation-factor crossplots for Amazon Fan demonstrate that high-porosity shaley sediments are actually less conductive than sands: at a given porosity, sediments with more than 40% shale exhibit much higher formation factors than do sediments with less shale. We conclude that pore tortuosity is more important than clay conduction as a control of formation factor in high-porosity clay-rich sediments.

These revised formation-factor and velocity relationships are potentially useful for log-based determinations of porosity and water saturation in high-porosity sediments. In addition, they suggest that the combination of two surface geophysical techniques - multichannel seismic and electrical - can provide a unique determination of both porosity and sand/shale content for high-porosity siliciclastic sediments.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas