Net-to-Gross
Learning objectives
- Define net-to-gross as net thickness over gross thickness
- Read NTG from a layered interval and its cutoff
- Explain how NTG multiplies through the volumetric equation
- Contrast a blocky sand with an interbedded sequence
Pay Becomes Thickness
Net pay is a thickness, and the single number that carries it into the volumetric equation is the net-to-gross ratio,
The gross interval is the whole reservoir from top to base; the net is the sum of the beds that pass the cutoffs.
From a Half to the Volume
A clean blocky sand has a net-to-gross near one; an interbedded sand-shale sequence can drop to a half or less. That fraction is not a detail: it multiplies straight through to the hydrocarbon volume, so two reservoirs of the same gross thickness and the same sand quality hold oil in proportion to their net-to-gross. Half the net is half the oil.
The Cutoff in Feet
Raising the porosity floor drops the thin and tight beds out of the net, the gross holding fixed while the net and the ratio fall. It is the same judgment call as the cutoffs of the last section, now measured in feet. The hard case is the thin-bedded reservoir, where individual sands sit below the resolution of the logs: the tool averages a sand and its shale together, reading a single muddy value that hides net pay, and a naive cutoff then throws away rock that would in fact produce.
References
- Worthington, P. F. (2010). Net pay: what is it? What does it do? How do we quantify it? How do we use it? SPE Reservoir Evaluation & Engineering, 13(5).
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.