Volumetrics

HIP = Rock Volume x Porosity x Hydrocarbon Saturation(x Expansion or / Shrinkage Factor)

• Gross Interval - interval from top to base of resevoir flow unit (Gross int. in Figure 1),

• Net Interval or Gross Sand - interval satisfying shale cut-off criteria (Gross sd. in Figure 1),

• Net Sand - interval satisfying shale and porosity cut-off criteria,

• Net Pay - interval satisfying shale, porosity and saturation cut-off criteria.

Cut-off criteria are levels determined by petrophysical analysis to eliminate non-productive rock (e.g., 10% porosity is often used as a cut-off in an oil reservoir - rock with less than 10% porosity is non-productive). Unless otherwise specified,

Net/ Gross ratio (N/G) usually refers to Net Sand /Gross Interval. As you can see N/G may be subject to confusion and should therefore always be defined in usage.

_{STOIP = 7758 x}~~^{GRV x N/G x}~~ ^φ ~~^{x S}o~~
		B_o
where:
STOIP	: stock tank oil-in-place (STB)
GRV	: gross rock volume (acre-ft),
N/G	: net/gross ratio (fraction),
φ	: porosity (fraction),
S_o	: oil saturation (fraction) (1- Sw),
B_o	: oil formation factor (RB/STB)	(Expansion factor = 1/Bo
			which is a "shrinkage factor" as Bo >1)
and Gas-in-place (GIP):
GIP = 43560 x GRV x N/G x φ x S_g
		B_g
where:
GIP	: gas-in-place (ft³)
GRV	: gross rock volume (acre-ft),
N/G	: net/gross ratio (fraction),
φ	: porosity (fraction),
S_g	: gas saturation (fraction),
^Bg	: gas formation factor (RCF/SCF) (Expansion factor = 1/Bg as Bg <1)