RE: Good Sign or Bad Sign?9 Apr 2021 16:43
@dai2belts
happy to have a quick look. but please bear in mind I am not a geoscience.
First simple read, I could gather the following:
1. Good porosity range, but no mention of permeability (how connected are the porous media).
2. several good overburden (top layer, shallower regions) containments - shales, muds
3. source migration is defined via charged active faults, filling into trap (they see evidence of trap present from the seismic amplitude anomaly - usually this gets a geologist over excited that they think they discovered a gas field! - see point 8), with some released through closed (then charged open) fissures (along the fault lines).
5. Karoo accumulation (prospect) defined in the deeper section. Lake Bed including the upper section of the large Red sandstone as the shallower prospect.
6. trap mechanism proposed for Karoo is a simple 3 way dip closure with the fault sealing in the 4th direction, therefore I suspect a drill target near the crest close / parallel to the fault might be a good drill target to confirm yay, or nay. Looking at the simple section, i can see why they proposed the deeper section could provide a good trap. If this is missing, then the upper structure might hold the escaped He migrating upwards via the faults (forming the Lake Beds target) or be in both.
7. Re lake bed trap, it mentions possibility of a shallow 4 dip closure
8. they alluded to seismic anomalies on structural high to be trapped gas bearing confirming trap mechanism. I buy this. . This is mostly the case for hydrocarbons fields but needs confirmation from drill bit.
9. from the very little seismic cross section they shared (figure 6.3), I cant see any gas plumes - so migration is controlled via leak paths (faults). therefore there is a high likelyhood it have also migrated into traps in the overburden (3 way dip + fault system) provided the charged faults returns to closure / healed.
10. (not in the report) Presence of water usually helps as connate / formation water usually contains CO2 and Ca2+ ions and the release of this through faults would help "healing" through calcite drop out and cementation over time.
Risk:
1. how leaky are those faults? and are they still leaky now?
2. risk of drilling near or through faults (its now deemed conventional but still some risks that needs managing, like loss of muds etc).
anyhow, thats a mathematician gleaning over geology... need a cup of tea now .
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