geology · stratigraphic
Net Rock Volume
NRV = Area × Gross Thickness × NTG
click formula to derive ↑
Inputs
acres
ft
fraction
Description
Computes net rock volume — the portion of gross rock volume that meets reservoir-quality cutoffs — by applying a net-to-gross ratio to the same Area × Gross Thickness box-model volume used by gross_rock_volume. Net-to-gross is supplied directly as a fraction (from net-pay log analysis at well control points) rather than requiring a dedicated net-to-gross calculator to already exist, the same self-contained-inputs pattern this platform's own ooip/giip calculators use for porosity and water saturation.
Variables
| Symbol | Unit | Description |
|---|---|---|
| NRV | acre-ft | Net Rock Volume |
| A | acres | Areal extent of the structure or mapped closure — from a structure contour map, isopach map, or 3-D interpretation. |
| hg | ft | Gross (total) vertical thickness of the interval, before any net-to-gross, porosity, or saturation cutoffs are applied. |
| NTG | fraction | Fraction of the gross interval that meets reservoir-quality cutoffs (porosity/Sw/clay-volume) — typically established from net-pay log analysis at well control points. |
Assumptions
- NTG is a single representative value for the whole mapped area — in reality net-to-gross commonly varies across a field and would be more rigorously handled as its own map, not a single scalar.
- Area and gross thickness carry the same single-value box-model simplification and limitations as gross_rock_volume.
- The NTG value supplied already reflects appropriate reservoir-quality cutoffs (porosity/Sw/clay-volume) established from net-pay log analysis — this calculator does not itself derive NTG from logs.
Limitations
- Same single-value Area × Thickness box-model limitation as gross_rock_volume, compounded by treating net-to-gross as a single scalar rather than a spatially varying property — for a field with significant net-to-gross variability, a map-based or multi-well-averaged NTG would be materially more accurate than one representative value.
- The GRV × NTG relationship is standard, widely-used reservoir-engineering content — corroborated across multiple secondary sources, same as gross_rock_volume — but no primary/textbook source was directly page-accessed this session for this specific relationship (SEG Wiki and ScienceDirect both returned 403). Attributed to Dake's *Fundamentals of Reservoir Engineering* on the same attribution-not-page-checked basis as gross_rock_volume — same honest-disclosure standard, not softened for being built alongside a now-established batch.
- Does not itself apply porosity or water saturation — see this platform's ooip/giip calculators for the remaining steps to hydrocarbon volumes in place.
Use Cases
- → Reservoir-quality volume estimation: Narrow a gross rock volume estimate down to the portion that is actually reservoir-quality, once a representative net-to-gross ratio is available from well control.
- → Comparing candidate development areas: Rank prospects or development areas by net rock volume rather than gross rock volume alone, when net-to-gross varies significantly between them.
Related Calculations
Region Notes
Global
Net-to-gross varies widely by depositional setting — clean, blocky channel/bar sandstones can approach NTG of 0.8-1.0, while heavily interbedded or laminated shaly sequences common in many unconventional plays can fall well below 0.3; always confirm the NTG value used matches the specific reservoir facies being volumed, not a field-wide average that may not apply locally.
References
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