Hi OldisGold,I don't have a drilling background but have been...

Hi OldisGold,

I don't have a drilling background but have been involved, on the seismic interpretation side, in drilling a number wells over the years during my time in the industry.

From what I've seen and understand - as the well is drilled readings of background gas are measured and analysed for their Alkane fractions. These are usually the C1 to C4 but predominantly C1 & C2. As a generality - no seal is a perfect seal - so micro seepages of hydrocarbons can occur through the accumulations top seal. These micro seepages are extremely small but over time migrate up through the rock strata and effectively commingle with the background gas - that is virtually always present in a hydrocarbon system like this. The result is that background gas readings closer to the top seal of a trapped accumulation can show a percentage increase in the higher C fractions i.e. usually C5 to C10. This percentage increase, depending on the rock lithologies, will increase the closer the drilling gets to the accumulation's top seal.

This can obviously provide information to the geotechnical team before the objective is even drilled. On occasions the change in tone of company reports or the companies decision making can allude to a change in their thinking about an objectives risk.

Also, unfortunately, if this information leaks out inside trading can / will occur.

I've posted below an internet cut & paste.

Hope this helps.

Cheers
VOGC