Many (exploration) teams use the area-depth method to calculate volumes in subsurface traps. The roots of this method go back to when hydrocarbon exploration was mainly map based and 3D modeling on workstations was not very common.
Even though it may seem like an outdated method it is still a relatively easy and solid methodology to calculate volumes in the subsurface. It is also a useful tool to asses uncertainties in the Gross Rock volume (GRV) caused by different top structure maps (velocity effect mostly), different contacts and different reservoir thicknesses.
The concept is shown below:
Basically the contours of a depth map are used to calculate the area within the contour. This is done for each contour at a fine enough contour-interval to sample the structure. These areas are plotted against depth and the area under the graph up to the contact depth is the (approximated) volume in which hydrocarbons occur. This volume between the top structure and the contact is called the Gross Rock Volume (GRV).
To account for the reservoir thickness either a constant reservoir thickness can be applied by shifting the top structure Area-Depth relationship down with the reservoir thickness. If there is a varying thickness a different Area-Depth relationship for the top and base structures can be used.
The GRV is then multiplied by multiple factors to end up with the Hydrocarbons Initially in Place (HCIIP)