Gas Cloud and Pushdown
Learning objectives
- Explain velocity pushdown from a slow gas cloud
- Compare the flat depth model with the sagged time section
- See how a time artefact mimics a structural low
- Justify velocity modelling and depth conversion
The Brief
The last capstone is the most cautionary. A time section shows a promising structural low, a possible trap, directly beneath a shallow gas cloud. Is it real closure or an artefact? A well proposal hangs on the answer, and only a model can settle it.
The Build
Gas in the overburden is slow. A wave crossing the gas cloud takes extra time, so any reflector beneath it arrives later, and on a time section a later arrival is drawn deeper. A perfectly flat reflector therefore appears to sag into a low, precisely under the cloud. Compare the two views: in depth the reflector is flat, the truth; in time it dips into an apparent low whose size scales with how slow the gas is. The low is not structure, it is the delay.
The Debrief
Which engine? You need the velocity model and a depth-domain view, ideally depth migration, not a time section read literally. A convolutional time section, or any time-domain image, reproduces this pushdown faithfully, and reading it at face value would map a trap that does not exist. Only by modelling the velocity and depth-converting do you recover the flat reflector and expose the low as an artefact.
This is the darkest fit-for-purpose lesson in the course: the right data, read the wrong way, invents geology. And it closes the capstones on their central point. Across all five, the same tools delivered five different verdicts, convolution here, the wave equation there, azimuthal anisotropy for the fractures, Gassmann-plus-noise for the plume, and a velocity model for the gas cloud. The only thing that changed was the question. That is fit-for-purpose modelling, and it is the whole discipline. The next part reviews the course and puts it to a final exam.