The Rock Physics Template
Learning objectives
- Build the rock-physics template as a crossplot of acoustic impedance against the velocity ratio, meshed with constant-porosity and constant-fluid curves
- See why one axis is ambiguous and two are diagnostic: impedance alone cannot separate a gas sand from a soft brine sand
- Anchor the mesh: a brine sand at porosity 0.20 sits near impedance 7.3 and ratio 1.86, while a gas sand at the same porosity sits near 5.6 and 1.49
- Read the template as local, built from a model calibrated to one field, not a universal chart
Two Axes Instead of One
A calibrated model can do more than predict one velocity. Run it across a range of porosities, and for each porosity across a range of fluids, and it fills a plane with points. The rock-physics template, in the form Odegaard and Avseth set out, plots acoustic impedance on one axis against the velocity ratio on the other, and connects the model points into a mesh: curves of constant fluid running one way, curves of constant porosity running the other. The reason for two axes is the whole point. Impedance alone is ambiguous. A tight brine sand and a porous gas sand can carry the very same impedance, because gas lowers velocity while brine keeps it up, and the porosity difference trades against the fluid difference until the product with density lands on the same number. On impedance alone they are indistinguishable. The velocity ratio breaks the tie, because it responds to fluid in a way impedance cannot hide.
Reading the Mesh
Take the template built for a quartz sand and read its corners. A brine sand at a porosity of 0.20 sits near and ; drop its fluid to gas at the same porosity and the point moves to and . That is the fluid effect drawn as a vector: gas pulls the point down, to lower impedance, and hard to the left, to a much lower velocity ratio. The leftward move is the diagnostic one. Impedance fell by a quarter, which fluid and porosity can both do, but the ratio collapsed from 1.86 to 1.49, and only fluid does that, because gas guts the P-wave velocity while leaving the shear velocity, and thus the shear part of the ratio, almost untouched. Now hold the fluid and change porosity: a brine sand moves from at porosity 0.20 down to 5.78 at 0.30, sliding along its fluid trend with the ratio drifting only from 1.86 to 1.97. Porosity moves points along each fluid curve; fluid jumps points between the curves. Oil sits in between, its trend near , closer to gas than to brine because even a little free hydrocarbon drops the ratio sharply.
Why the Template Is Local
The mesh is not a chart you copy from a textbook. It is the output of a rock-physics model with particular parameters, the mineral, the critical porosity, the coordination number, the pressure, all set by calibration to a particular field, the exercise the previous section ran on Ogbon-1. The mesh drawn here uses a friable soft-sand frame, the right instrument for an unconsolidated field; Ogbon-1 itself calibrated to the stiff-sand family, so its template would be built from that fit instead. Change the mineral to a dirtier sand, change the pressure to a shallower reservoir, change the frame from friable to cemented, and the whole mesh shifts and tilts. A template built for one field will mislocate the porosities and mispredict the fluids of another. That is why an RPT is always drawn local: you calibrate the model to your well, then let it draw the mesh for your rocks. The chart earns its diagnostic power by being specific. With the mesh built and its two axes understood, one axis for what fluid, the other tracking porosity, the template is ready to be used the way it is meant to be used, which is to take a point of unknown origin and read its porosity and fluid straight off the plot. That is the closing section.
References
- Odegaard, E., & Avseth, P. (2004). Well log and seismic data analysis using rock physics templates. First Break, 22(10), 37-43.
- Avseth, P., Mukerji, T., & Mavko, G. (2005). Quantitative Seismic Interpretation. Cambridge University Press.