Depth conversion: the map you hand over is not the map you made
Learning objectives
- Fit a V0 plus kt velocity function to a checkshot and apply it to a time map
- Explain how a high-velocity overburden body creates apparent structure in time
- Predict the direction in which a single-well velocity model biases a prospect
- Say what a second well improves and what it leaves untouched
Everything so far has been in time. Time is not a place. Nobody drills to 550 milliseconds, and the volume you are about to compute is in cubic metres of rock, so at some point the map has to be converted, and the conversion needs a velocity.
You have one well. You fit a function to its checkshot, you apply it everywhere, and you get a depth map. That is not a shortcut and it is not a mistake to be warned about. With one well it is the only thing anybody can do.
The error has a direction
A high-velocity body sits in the overburden above the crest of this structure. Waves travelling through it arrive sooner, so the reflector beneath appears shallower in time than it really is. Part of the relief on the time map is not geology at all; it is pull-up.
Converting with the true, laterally varying velocity removes it, and the prospect loses roughly a third of its area and half its closure. Converting with a single function fitted at a well that sits on that body does something worse than fail to remove it: the elevated velocity is applied to the whole survey, and the prospect comes out larger than the time map suggested. The reader who does every step of the arithmetic correctly ends up further from the truth than when they started.
What a second well buys
Compare the one-well and two-well models and watch the two numbers move independently.
The depth error roughly halves, because the fit now averages two different overburdens instead of one. The closure barely changes at all. That is the point worth carrying: the error was never in the average velocity, it was in the fact that velocity varies laterally and a single @@V_0 + kt@@ function cannot represent variation of any kind. A second well fixes your depths and leaves your structure.
Fixing structure needs a velocity model that varies across the map, which means seismic velocities, or many wells, or an interpretation of what the anomalous body actually is. That is the work depth conversion really asks for, and it is why the step is rarely as quick as it looks on a schedule.