From Rock Volume to STOIIP

Cutting Down to Oil

Gross rock is not oil. The volumetric equation strips it down: $\text{STOIIP} = \text{GRV}\cdot \tfrac{N}{G}\cdot \phi \cdot (1-S_w) / B_o$. Net-to-gross keeps the reservoir rock, porosity keeps the pore space, and one minus water saturation keeps the oil-filled pores, leaving the hydrocarbon pore volume.

Reservoir to Surface

That hydrocarbon pore volume is measured at reservoir conditions, hot and under pressure, where the oil is expanded and holds dissolved gas. Dividing by the formation volume factor $B_o$ shrinks reservoir barrels to stock-tank barrels at the surface, giving STOIIP, the stock-tank oil initially in place. $B_o$ is typically 1.1 to 1.5, so reservoir volume overstates surface oil by ten to fifty percent.

Oil and Gas

The same chain gives gas initially in place (GIIP) using the gas formation volume factor $B_g$ in place of $B_o$. STOIIP and GIIP are volumes in place, not reserves: multiplying by a recovery factor, the fraction the field will actually produce, turns them into recoverable reserves. The widget shows each cut shrinking the bar.