At first approximation, computing volumes mean solving the following equation:

HCPV = BRV * NTG * PORO * So

• HCPV is the Hydrocarbon Pore Volume in reservoir conditions. This is the volume we are after.
• BRV is the Bulk Rock Volume. It represents the whole volume of the geological layer being the reservoir. The BRV is delimited by a top and a bottom horizon as well as potentially by fault laterally.
• NTG is the Net-To-Gross. It is the fraction of the BRV in which oil is found. The NTG factor allows removing the fraction of the BRV which is full of water (in case a water zone exists) as well as the volumes populated with non-reservoir rocks (like shales above the water zone in a conventional clastic reservoir). Depending on the work at hand, the NTG can sometimes also exclude the portion of the reservoir rocks with too low porosity and/or too low oil saturation.
• PORO is the average Porosity within the part of BRV full of oil.
• So is the average oil saturation within the part of the BRV full of oil.

None of the input parameters are known for certain. The limited data we have about our reservoir only gives us an approximation of them. As a result, it is unwise to compute only a single, deterministic HCPV value. It is recommended to compute a range of volumes which reflect the range of possible values taken by each of the input parameters of the equation.

Further, it is also important to note that the equation takes average porosity and So values as input. This point will be expanded upon later in the next chapter.