Hi Blue 47,No pressure ah ......Apologies for all the subsurface...

Hi Blue 47,
No pressure ah ......
Apologies for all the subsurface lingo below.

Given the subsurface information released by IVZ to date, I have been scratching my head on what could turn Mukuyu-2 into a poor subsurface outcome. Remember, there is always risk in oil and gas exploration, and the best way to mitigate/minimise that risk is thorough Research.

So, could Mukuyu-2 be drilling through a large, residual hydrocarbon column in the Upper and Lower Angwa sands? i.e. the hydrocarbons originally trapped in the structure, have since leaked and the gas shows and fluorescence are from residual hydrocarbons.

• I have seen drilling breaks into clean sandstones with accompanying fluorescence and good oil/gas shows on mud log data, that after wireline logging, turned out to be from residual hydrocarbons.
• Residual hydrocarbons in clean, saline, water-wet sandstones are unlikely to show elevated resistivities on MWD data. Mukuyu-2 has elevated resistivities in clean sands with strong gas shows and fluorescence.
• That said, residual hydrocarbons in clean, fresh water-wet sandstones, will likely show elevated resistivities on the MWD data.
• RISK: If the connate/formation water in the Upper and Lower Angwa sands is fresh, resistivities on the MWD data will be elevated.
• I am unsure if IVZ has reported saline or fresh water in the Upper and Lower Angwa sands. We want saline!

Case Study - Gulf of Mexico

• Based on strong mud log oil and gas shows (samples had dull yellow fluorescence with a fast-streaming cut and a bright milky residue ring) in clean sandstones in the G-1 well, which already had confirmed overlying gas pay from wireline logs, the decision was made to refurbish a production platform to be placed over the field to tap the mud log interpreted oil in the Lower O Sands. See diagram below on seismic data.
• When the G2 well was drilled from the platform to tap the Lower O Sand which had strong mud log oil and gas shows, the resistivities from the open hole wireline logs showed that the sand was water-wet and only contained only residual oil. i.e no elevated resistivities.
• LESSON LEARNED - The company could have run the gamma ray, resistivity MWD tool while the drilling rig was on location, to drill and log the Lower O Sand. If the MWD tool had been run, this would have shown that the strong mud log oil and gas shows in the Lower O sand were in response to residual oil only and not moveable oil.

So, could Mukuyu-2 be drilling through tight sandstones (low porosity)

• Tight sandstones can have strong gas shows with accompanying fluorescence.
• Tight sandstones can have elevated resistivities on MWD data.
• Generally, you do not see drilling breaks into tight sandstones because in most cases, the original porosity has been infilled with secondary cement, which make the sandstone more coherent.
• Mukuyu-2 is seeing multiple drilling breaks into clean sandstones which is indicative of preserved porosity.
• I think Mukuyu-1 had porosities, measured from open hole wireline logs, of around 15%, which is certainly not tight! This helps at Mukuyu-2 which is also 400-500m shallower, which also reduces the risk of porosity degradation due to compaction / depth of burial.

Anyway, just sharing some thinking here.

Kind regards OldGeo