What's the difference between net sand vs gross sand thickness?

Answer 1

Gross sand is the thickness of the sand top to bottom.
Net sand is the number of meters in this sand which has porosity greater than a set cutoff (~7%).
Net pay is the number of meters of net sand that has movable oil in it.

Answer 2

Net sand thickness refers to the total thickness of sandstone reservoirs without the inclusion of non-reservoir material, such as shale. Gross sand thickness, on the other hand, includes all layers of sandstone, regardless of their reservoir potential. Net sand thickness is a more accurate measure of the potential reservoir volume available for fluid storage and movement.