How deep is deep?28 Jan 2023 23:54
Earlier this morning (my time - evening for most of you), I had a brief message exchange with Graham concerning drillhole depth. To preclude conspiracy theories, I have never met the gentleman, and our conclusions arose independently. I had originally said to him that I would not make my thoughts public, since they could be readily misinterpreted, especially by those here of ill-will. I then looked at this BB, and saw that the topic had already been brought up by Jimmy, and also by requote of a GRH tweet. So, I will try to explain my thoughts. Let's try and work through some fairly complicated stuff. If you want a full technical explanation, try this:
https://csegrecorder.com/articles/view/true-depth-conversion-more-than-a-pretty-picture
If not for you, read on!
When you look at a seismic chart such as the ones in PRD's presentations, you will see that the ordinate (y axis) is labelled 'ms' and might assume this represents metres - but it is actually milliseconds, being the time taken for a seismic wave to travel down from the surface and be reflected from an underlying rock stratum back up to the surface - this is the two way time, or TWT. Seismic waves travel faster the denser the rock. As an example, gabbro is a very dense rock type, so waves pass through it very quickly, so a layer will appear 'thinner' on a TWT chart than a layer of less-dense sandstone, even though both rock layers are of equal real thickness.
To convert TWT to actual depth, it is necessary to know what the likely rock densities will be - for which of course you have to have a best guess at what thicknesses of which rocks you are likely to drill, a Catch-22 situation. No one can possibly know this for sure, but best indication is from nearby drillholes, where you already know rock types, densities & depths. PRD have done all this, and derived the most likely depth of the reservoir.
What the MOU-2 drill data have shown is an unexpected actual thickness of shale/mudstone, which is quite dense, and consequently appeared thinner on the TWT chart. This could mean that the expected reservoir is deeper than anticipated, and also possibly thicker. Is that good or bad? - Well you might have to drill an extra day, but if the base of the reservoir is lower than previously thought, that means the gas spill point (from where the observed gas cloud originates) is also lower, meaning that the gas fill extends across a greater area than previously calculated. That means more gas!
There was some speculation that the gas cloud could be due to leakage through the top seal. This is now seen not to be the case - there is no gas directly above the seal, which is made of impervious claystone. The source of the gas cloud is therefore most likely to be from the base of the reservoir structure.
Just theory atm, let's see what Paul has to say.