My friend Fred Gyllenhammar is an old badger that never stops digging. He has reprocessed all public domain logs from the Norwegian Continental Shelf and found several hundreds of missed pays.
My specialty is oversimplification, so to me the physics is very straightforward. The hydrocarbons pops out of the water saturation and is basically a function of the resistivity. Hydrocarbons set up a resistivity anomaly, so it is dependent on the background resistivity. High salinity pore fluids give a low background resistivity and low salinity gives a high background resistivity.
But if you have a poor knowledge of the salinity, do you go with some default value? From processing all these logs, Fred’s experience is that salinity of formation water is “all over the place”. You need to study salinity from area to area.
So if you use salinity values from the Southern North Sea with its Zechstein salt domes, and apply them to the Northern North Sea, with no Zechstein, you will have a too low background resistivity and therefore find too many false positives.
My favorite story is this, and I admit I invented it myself, but as the journalists will tell you: “never check a good story”. In earlier times, when oil companies were only interested in dinosaur-discoveries, they didn’t want small discoveries because the costs of reservoir testing would be too high. So by decreasing the salinity they could change a small discovery to a dry well. No need to do testing, no extra rig time cost!
I our times, when exploration is more tail-end and focused on additional reserves close to producing fields, Fred’s library would be a very valuable asset. Because as we know: a dry well is an exploration killer.
Gyllenhammar use IP (Interactive Petrophysics) in his log analysis, integrating wireline logs with mudlogging data. He has the best background for evaluating log results; starting his career as a mud-logger and wellsite geologist, 23 years later he was exploration manager and have been exploration advisor for several oil-companies. To day he interprets all released wells in Norway, UK and Holland. Making salinity maps and finding missed pay in 15% of the wells.
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