Net to Gross Calculation Petrel Calculator
Use this premium net to gross calculator to estimate net reservoir thickness, deductions, and net to gross ratio for Petrel style subsurface interpretation workflows. Enter gross interval thickness and non-reservoir deductions to calculate a clean NTG value that can support reservoir modeling, volumetrics, facies quality review, and communication between geology, geophysics, and reservoir engineering teams.
Interactive NTG Calculator
Expert Guide to Net to Gross Calculation in Petrel Workflows
Net to gross, usually abbreviated as NTG, is one of the most important descriptive ratios in subsurface reservoir characterization. In practical terms, it tells a geoscientist or reservoir engineer how much of a mapped gross interval is expected to behave like effective reservoir rock. When teams search for a reliable method for net to gross calculation in Petrel, they are usually trying to standardize a workflow that starts with geologic interpretation and ends with something operationally useful for static modeling, volumetric estimation, and uncertainty analysis.
At its simplest, the relationship is straightforward: net to gross = net thickness / gross thickness. The challenge is not the arithmetic. The challenge is deciding what qualifies as net, what should be excluded, and how to apply consistent cutoffs across wells, horizons, facies domains, and simulation zones. Petrel is commonly used because it helps teams integrate well logs, seismic interpretation, facies models, property modeling, and zonation in one shared environment. A robust NTG workflow inside or alongside Petrel allows interpreters to move from descriptive geology to quantitative reservoir inputs with less ambiguity.
Core principle: Gross interval thickness captures the full stratigraphic package of interest, while net thickness captures only the portion that meets your chosen reservoir quality or pay criteria. Every NTG result is therefore only as strong as the cutoff logic behind it.
Why NTG matters in reservoir modeling
NTG affects far more than a single percentage on a report. In many petroleum and subsurface studies, NTG influences hydrocarbon pore volume estimates, connectivity expectations, facies distribution logic, and development planning. If your NTG assumption is too high, the model may overstate reservoir quality, deliverability, and reserves. If it is too low, it may under-represent opportunities in thin bedded or heterogeneous intervals.
- Volumetrics: NTG directly impacts rock volume considered productive or storage effective.
- Static models: It helps distribute sand, reservoir facies, and effective rock proportions in grid cells.
- Dynamic simulation: Lower NTG often implies reduced flow pathways and more compartmentalization.
- Well planning: A strong NTG map supports landing zone selection and completion targeting.
- Risk assessment: NTG uncertainty often forms a major input to low, base, and high case scenarios.
Understanding gross, net, and non-net intervals
Gross thickness is normally measured between two interpreted stratigraphic surfaces, such as a top reservoir and base reservoir horizon. Within that gross interval, not all rock contributes equally. Some layers may be shale rich, tight, cemented, water bearing, or below cutoffs for porosity, permeability, or hydrocarbon saturation. Those layers are usually deducted when defining net.
In a Petrel style interpretation, geoscientists often begin with well log review and establish one or more cutoffs, such as:
- Maximum shale volume for reservoir classification.
- Minimum porosity for effective storage.
- Minimum permeability proxy or facies class.
- Maximum water saturation for net pay.
- Local depositional rules for channel, lobe, shoreface, or carbonate facies.
Those rules can define different products. For example, net reservoir may exclude obvious shale and tight streaks, while net pay may apply an additional water saturation or hydrocarbon mobility filter. The calculator above is intentionally practical: it lets you begin from gross interval thickness and deduct non-reservoir and non-pay components to estimate a clean NTG ratio quickly.
Core formula for net to gross calculation
The standard formula is:
NTG = Net Thickness / Gross Thickness
To convert to percentage:
NTG % = (Net Thickness / Gross Thickness) x 100
Suppose your gross interval is 120 m. If 25 m is shale or non-reservoir and another 15 m is low-quality or non-pay rock, your net thickness becomes 80 m. The net to gross ratio is therefore 80 / 120 = 0.667, or 66.7%. That one number can then be used in map-based comparisons, zonal statistics, stochastic property modeling, and volumetric sensitivity cases.
How this supports a Petrel workflow
Petrel users typically do not think of NTG as an isolated spreadsheet ratio. They think of it as a model-ready property that can be derived, scaled, distributed, and quality checked. In actual field studies, the workflow often follows this sequence:
- Interpret top and base reservoir surfaces from seismic and well ties.
- Define zone framework and layering strategy.
- Review well logs, core, and petrophysical interpretation.
- Choose cutoffs for facies, shale volume, porosity, and saturation.
- Calculate net and gross at well locations.
- Upscale the results to model cells or populate NTG trends.
- Compare maps against depositional understanding and analog fields.
- Run uncertainty scenarios for low, base, and high NTG distributions.
In many clastic reservoirs, NTG trends align strongly with depositional architecture. Channel axis sands tend to show higher NTG than levee or overbank settings. Shoreface systems may show moderate to high NTG with vertical heterogeneity. Deepwater lobe complexes can appear high quality in cores yet remain laterally variable. Carbonate systems add another layer of complexity because effective net may depend as much on diagenesis and fracture distribution as on lithology alone.
Practical interpretation thresholds
There is no universal NTG threshold that applies to every basin. However, many interpretation teams use broad screening ranges to communicate rock quality and continuity expectations. The table below shows common descriptive categories used in reservoir discussions. These are screening ranges rather than regulatory standards.
| NTG range | Interpretation category | Typical geologic implication | Modeling consequence |
|---|---|---|---|
| Below 0.30 | Low NTG | High shale content, discontinuous sand, or limited effective reservoir | Lower connected volume and stronger uncertainty in flow pathways |
| 0.30 to 0.50 | Moderate low NTG | Mixed lithologies, stratified pay, heterogeneity likely significant | Requires careful facies modeling and sensitivity analysis |
| 0.50 to 0.70 | Moderate to good NTG | Reservoir intervals are more continuous but still internally variable | Often suitable for base case volumetrics with clear zonal controls |
| 0.70 to 0.90 | High NTG | Thick reservoir-prone section with relatively limited deductions | Higher confidence in effective sand distribution and continuity |
| Above 0.90 | Very high NTG | Clean, thick reservoir or highly selective gross interval picking | Check cutoff bias and ensure gross interval has not been overly narrowed |
Comparison with porosity and permeability statistics
NTG should never be interpreted in isolation. A high NTG interval with poor porosity or poor permeability may still disappoint. Likewise, a moderate NTG interval can be highly valuable if the net beds have excellent reservoir quality and strong connectivity. The next table uses widely cited educational ranges to show how NTG is often interpreted alongside typical porosity and permeability expectations in clastic reservoirs.
| Reservoir quality screen | Porosity range | Permeability range | Common NTG implication |
|---|---|---|---|
| Tight to poor | Below 10% | Below 1 mD | Even with moderate gross thickness, effective net may be very limited |
| Fair | 10% to 15% | 1 to 10 mD | Net intervals may be present but often require strict cutoff management |
| Good | 15% to 20% | 10 to 100 mD | Moderate to high NTG zones often become development targets |
| Very good | 20% to 30% | 100 to 1000 mD | High NTG sections can strongly improve volumetrics and deliverability |
Where to get authoritative geoscience context
For broader subsurface and petroleum geology context, the following sources are useful because they provide technical references, educational material, and data products from highly credible institutions:
- U.S. Geological Survey for basin, stratigraphic, and energy resource studies.
- U.S. Department of Energy Office of Fossil Energy and Carbon Management for subsurface energy and reservoir related technical resources.
- Stanford University Department of Geology for academic geoscience and reservoir geology learning resources.
Common mistakes when calculating NTG
One of the biggest errors in net to gross work is inconsistency. Teams may compare one well interpreted with a porosity cutoff to another well interpreted with a facies cutoff and then wonder why the NTG map appears geologically noisy. Another frequent issue is using a gross interval that is too broad in one area and too narrow in another. Since NTG is a ratio, even small changes in either the numerator or denominator can produce misleading spatial trends.
- Mixing cutoffs across datasets: Keep logic consistent across wells and zones.
- Ignoring vertical resolution: Thin beds may be under-resolved in logs or seismic-derived properties.
- Confusing net reservoir with net pay: These are related but not identical products.
- Failing to validate against core: Core and image logs may reveal heterogeneity hidden in standard logs.
- Overlooking scale effects: Well-based NTG and grid-cell NTG may differ after upscaling.
Using the calculator effectively
The calculator on this page is designed for fast screening and communication. To use it correctly, start with a defensible gross interval thickness. Then estimate your shale or non-reservoir deductions, followed by any non-pay or low-quality thickness that should also be excluded. The output reports:
- Net thickness
- Total deductions
- Net to gross ratio
- Net to gross percentage
This is especially useful during early interpretation workshops, Petrel project reviews, field analog comparisons, and well-by-well screening before a full property model is built. Because the chart separates gross, deductions, and net components visually, it also helps non-specialists understand the basis of the ratio quickly.
How NTG links to uncertainty and scenario planning
No subsurface interpretation is complete without uncertainty management. NTG often varies significantly across a field because of depositional changes, fault compartmentalization, diagenesis, and data density. A disciplined team therefore builds low, base, and high cases. The low case may use stricter cutoffs or more conservative continuity assumptions. The high case may use favorable facies interpretation or analog support. The base case generally reflects calibrated well control and the preferred static model narrative.
In a mature Petrel workflow, those scenarios are not just numbers. They become alternative NTG maps, facies proportions, and cell property distributions that can be tested in volumetric and simulation studies. This is why even a simple net to gross calculation remains fundamental. It is one of the few ratios that directly links geology, petrophysics, geostatistics, and reservoir engineering in a way that every discipline can understand.
Final takeaway
If you are searching for a practical method for net to gross calculation in Petrel, focus first on consistency, not complexity. Define a clear gross interval, apply transparent deduction rules, document your cutoffs, and review every result in its geologic context. NTG is a powerful metric, but only when the assumptions behind it are traceable and repeatable. Use the calculator above as a fast front-end tool, then carry the same logic into your Petrel interpretation, well analysis, and reservoir modeling workflow.