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This is a real petrophysical log analysis chart with resistivity logs (laterolog log on the last row with rainbow plots and induction log on the second row with red, orange, purple lines) and little relevant first row with PEF to this question.

enter image description here

I thought that the gas-water-contact (GWC) would be at 1277 due to the change in resistivity but my assistant said

"No GWC at 1277 m, the shift of neutron-density separation is rather triggered by the variation of shale content than a fluid change. If there is gas-water contact here, the resistivity should change abruptly since gas is very resistive while water is very conductive."

Please help me to understand his writing about large enough resistivity change.

P.s. I am studying Petrophysics TPG4175 in NTNU.

hhh
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    Most of us here are not in the petroleum industry. Can you cite your textbook about the resistivity of shale vs. water layers? – CuriousOne Dec 15 '14 at 15:30
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    @CuriousOne unfornately we don't have any course book (yet ~1k pages of lecture slides, petrophysics is a wild subject -- mix of physics, chemistry and practise. I provided answer below if you got interested in log analysis. You need to be very careful in analysing the chart, it is just basic thinking :) – hhh Dec 15 '14 at 16:49
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    Thanks. That's the kind of data I was looking for. Thanks for posting... and it looks like you answered your own question! – CuriousOne Dec 15 '14 at 16:58
  • @CuriousOne Thanks for encouraging: sometimes asking the right question is the trick to fool you do the right thing. When I tried to provide you the information, I realised the solution, best social teamwork -- I feel painful to say that I solved the problem. I just connected the dots :D – hhh Dec 15 '14 at 17:09

1 Answers1

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The region 1277 has the resistivity at about 200 ohms-m while 1291-1292m zone has the resistivity rising abruptly to the range of ~2k ohm-m (and later to 30k ohm-m at 1293m).

enter image description here

By the chart below we can classify the 200 ohm-m to probably be shale while the high pike to be very resistive zone. Because this is a hydrocarbon reservoir, it is some sort of mixture with short hydrocarbons like methane/ethane

enter image description here

so the GWC is at about 1291-1292m and very large amount of gas at 1293m.

P.s. Lecture slide 13/57 (unfortunately not online).

hhh
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