Re: Log response in Shale Gas
to the first part of your question:
Shale gas is gas that has been generated in organic rich shales due to burial (high temperatures and pressures), and has not all been expelled out of the shale into more permeable rocks, such as reservoir sandstones or limestones. Such shales often give good gas shows on mud logs while drilling through them. In the past, when tested, they would give a short blow of gas because their permeability is very very low.
To be able to produce at economic rates, they need to be extensively fractured by high pressure fracking methods. However, once the gas in the artificial fractures has been depleted, the initial production declines rapidly, due to lack of permeability in the matrix.
There is an ongoing debate among geologists about whether the full life-cycle economics are profitable. Government regulators (e.g. SEC) are reviewing gas shale projects to find out if investors are being misled or not.
There´s a lot been written about gas shales. Let me know if you would like me to send you some references.