Fluid Substitution on Ogbon-1
Learning objectives
- Run the full fluid-substitution workflow over the brine sand of a real well, Ogbon-1
- Read the before-and-after logs: Vp down, density down, Vp/Vs down, impedance down, and less for oil than gas
- Trace the seismic consequence to a brightened or flipped reflection at the top of the sand
- Close the part: bounds, critical porosity, fluids, and Gassmann built a machine that still needs the dry frame as input
The Deliverable, on a Real Well
Everything in this part has been building toward one product. The Ogbon-1 well, shared with the Petrophysics course, carries a clean brine-saturated sand beneath a shale. An interpreter asked whether that sand would light up if it held gas does not answer in the abstract: they run Gassmann down the interval and produce a fluid-substituted log. That log, and the reflection it implies, is the deliverable, and the figure builds it track by track over the real well.
Reading the Substituted Logs
The workflow of Part 4.2 runs at every depth sample in the sand. Read the in-situ brine , , and density from the logs, recover the dry frame by inverting Gassmann, hold that frame fixed, and put gas in. The output is a second set of logs laid beside the first, and they do exactly what Part 4.3 promised. drops across the sand. The density drops. The ratio falls, the same divergence that makes gas a direct hydrocarbon indicator. And the impedance drops furthest of all, because the velocity and the density both fell and impedance is their product. Move the selector from gas to oil and every track moves the same way but less, oil being the softer contrast against brine (Part 4.4). Above and below the sand, in the shale, the in-situ and substituted logs lie on top of each other, because there is no reservoir fluid there to substitute.
The payoff sits at the top of the sand, where it meets the shale cap. Under brine the impedance step across that boundary is one size; under gas, with the sand's impedance dropped by about a seventh, the step deepens or even changes sign, and the modeled reflection brightens or flips polarity. That modeled brightening is the bright spot the seismic courses study from the other side, and producing it here, a fluid-substituted log and the reflection it implies, is precisely what fluid substitution is for. An interpreter can now say, before a single new trace is shot, whether gas in this sand would be loud or quiet, and by how much.
The Part Closes
Trace what Parts 2 through 4 built. Bounds bracket what any mixture of the ingredients can do (Part 2). Critical porosity locates the dry frame between those bounds (Part 2.6). The fluids were given their properties across the reservoir range (Part 3). And Gassmann couples the fluid to the frame to predict the saturated rock (this part). Together they are a complete machine for one question, how a fluid change moves the seismic, and on Ogbon-1 it just answered that question end to end.
But the machine ran on one input it took on faith. Gassmann needed the dry frame handed to it, and here we recovered that frame by inverting Gassmann on the in-situ brine log. That only works where a saturated log already exists to invert. Where a rock has never been logged, or where you want to predict a frame from grains, contacts, cement, and pore shape alone, the dry frame has to be built, not borrowed. Building it is the whole of what comes next: the granular contact models of Part 5, where sands are made from grains touching at points, and the inclusion models of Part 6, where stiff rocks are built around pores with shapes. The dry frame stops being an input and becomes the thing we model.
References
- Smith, T. M., Sondergeld, C. H., & Rai, C. S. (2003). Gassmann fluid substitutions: A tutorial. Geophysics, 68(2), 430-440.
- Mavko, G., Mukerji, T., & Dvorkin, J. (2009). The Rock Physics Handbook (2nd ed.). Cambridge University Press.