you can't turn gas in the mud to gas saturation... there are way too many variables.
Porosity is the space between the grains and the practical maximum porosity of a sandstsone reservoir is about 40% of the volume. Ie 60% is rock, 40% space (porosity).
The hydarcarbon saturation is a percentage of the porosity.
ie 70% gas saturation implies that 30% of the porosity is water and 70% of the porosity is gas.
I hope that explains it
