Not an o&g expert either but this gives a basic explanation and a good point from which to google.... http://en.wikipedia.org/wiki/Petrophysics Quite interesting reading!
Week 46 Wrap: Copper hits 2mth low as Trump threatens net zero pace; US rally fades; HotCopper’s picks
AN1 CuFe sells its Crossroads gold royalty to Northern Star for $4M cash; says better than waiting for mine launch