(From Wikipedia)
Hydrocarbon in place - amount of hydrocarbon likely to be contained in the prospect. This is calculated using the volumetric equation - GRV x N/G x Porosity x Sh x FVF
GRV - Gross rock volume - amount of rock in the trap above the hydrocarbon water contact
N/G - net/gross ratio - proportion of the GRV formed by the reservoir rock ( range is 0 to 1)
Porosity - percentage of the net reservoir rock occupied by pores (typically 5-35%)
Sh - hydrocarbon saturation - some of the pore space is filled with water - this must be discounted
FVF - formation volume factor - oil shrinks and gas expands when brought to the surface. The FVF converts volumes at reservoir conditions (high pressure and high temperature) to storage and sale conditions