Fractured Carbonate
Learning objectives
- See why the fractures are the reservoir in a tight carbonate
- Run the Part 8 rock-physics chain to an azimuthal AVO response
- Read fracture strike and intensity from the ellipse
- See why acoustic and isotropic-elastic both fail here
The Brief
The third capstone needs the most elaborate engine of all. A tight carbonate has almost no matrix permeability; it produces only where it is naturally fractured, so the fractures ARE the reservoir. The question is not merely whether fractures exist, but which way they run and how intense they are, because a well has to be steered to cross them. Characterise the fracture set from surface seismic.
The Build
This is the whole rock-physics chain of Part 8 in one shot. A carbonate background plus an aligned fracture set, a strike, a crack density, and a fluid, becomes an HTI medium, and its azimuthal AVO response is an ellipse. The long axis of that ellipse points along the fractures; its ellipticity scales with their intensity. Set the fracture parameters and read the ellipse straight back.
The Debrief
Which engine? The azimuthal, anisotropic workflow, and nothing less. An acoustic model returns a single stacked amplitude with no fracture information. Even an isotropic-elastic model, with full AVO, is azimuth-blind: it gives an offset trend but no orientation. Only a model that carries the anisotropy and varies with azimuth can recover a strike and a density. Here the target IS the anisotropy, so a model that averages over azimuth throws away the entire answer.
This is the one capstone where the most complete engine is the only fit-for-purpose choice, the exact opposite of the fault dataset. It also exposes the method's limit: switch the fluid to brine and the ellipse shrinks, because wet fractures weaken the P azimuthal signal, which is why shear-wave splitting is kept as the fluid-blind backstop. The next capstone moves the problem into time, detecting a CO2 plume between two surveys.