How To Calculate Net To Gross In Petrel

Use this interactive calculator to estimate net-to-gross ratio, gross thickness, or net thickness for reservoir intervals commonly interpreted in Petrel. Enter your values, choose the calculation mode, and review the charted interval breakdown instantly.

Net to Gross Calculator

Expert Guide: How to Calculate Net to Gross in Petrel

Net to gross, often written as NTG, is one of the most important ratios in reservoir characterization. In practical terms, it tells you what portion of a gross geological interval is considered reservoir-quality rock after applying your interpretation criteria. In Petrel, that usually means you are comparing a net interval, such as net reservoir, net sand, or net pay, against a gross interval bounded by interpreted tops and bases. The ratio is simple, but getting a reliable answer depends on consistent cutoffs, accurate layering, and a disciplined workflow.

The most basic formula is straightforward: NTG = Net Thickness / Gross Thickness. If the result is multiplied by 100, you get a percentage. For example, if a reservoir zone is 40 meters gross and 24 meters net, then NTG is 24 divided by 40, which equals 0.60, or 60%. In Petrel, the challenge is rarely the arithmetic. The challenge is defining what counts as net, setting your interval limits correctly, and making sure the inputs represent the same zone and scale.

What Net to Gross Means in Reservoir Modeling

Geoscientists and reservoir engineers use NTG because it bridges geology and volumetrics. Gross thickness captures the full stratigraphic interval under study. Net thickness captures only the fraction that meets your reservoir or pay criteria. Depending on your project, net may be defined using one or more of the following:

• Lithology cutoffs, such as sand fraction above a threshold.
• Petrophysical cutoffs, such as porosity greater than a selected value.
• Fluid cutoffs, such as water saturation below a limit.
• Permeability cutoffs for dynamic or producibility workflows.
• Combined pay criteria, where multiple filters must be satisfied at once.

Because each project defines net differently, there is no universal NTG value that is “correct” in every field. A clean fluvial channel body may show very high NTG, while a heterolithic tidal environment may show lower NTG even when the gross interval is thick. This is why Petrel users need to document cutoffs, facies logic, and interval boundaries carefully. The quality of your NTG map or property model depends directly on those decisions.

The Core Formula Used in Petrel Workflows

When calculating net to gross manually or validating Petrel outputs, these are the three most common formulas:

1. Net to Gross = Net Thickness / Gross Thickness
2. Gross Thickness = Net Thickness / (NTG / 100)
3. Net Thickness = Gross Thickness x (NTG / 100)

These formulas are exactly what the calculator above uses. If top and base depths are known, gross thickness can also be estimated from the interval itself:

Gross Thickness = Base Depth – Top Depth, using the absolute difference where needed.

Quick example: Suppose a modeled zone is 38 ft thick from top to base, and only 22.8 ft satisfies your net reservoir criteria. NTG = 22.8 / 38 = 0.60, or 60%.

How to Calculate Net to Gross in Petrel Step by Step

1. Define the Gross Interval

Start by making sure your horizon interpretation is stable. In Petrel, the gross interval usually comes from the distance between a top surface and a base surface for a zone. If those surfaces are inconsistent, your gross thickness map will also be inconsistent. This is especially important in structurally complex settings where fault relationships and erosional truncations change the apparent thickness rapidly.

2. Decide What “Net” Means for the Study

Before any calculation, identify whether you are measuring net reservoir, net sand, or net pay. Those are not the same thing. Net reservoir may use facies plus porosity. Net pay may add saturation and permeability cutoffs. If one team member uses porosity greater than 10% and another uses porosity greater than 12%, your NTG values will diverge immediately even if the same well and the same grid are used.

3. Apply Cutoffs Consistently

In Petrel, cutoffs can be applied in logs, upscaled well data, facies models, or 3D grid properties. The key is consistency. If your gross interval is measured on a zonation framework but your net thickness is filtered from a different layering or property resolution, the ratio can become misleading. For best practice, define and document:

• The source data used for gross thickness.
• The exact cutoffs used for net identification.
• The scale of calculation, such as log sample, layered grid, or simulation cell.
• Whether values are measured vertically, stratigraphically, or along cell geometry.

4. Compute the Ratio

Once net and gross are aligned, Petrel can calculate NTG as a property or you can verify it externally. If net thickness equals 12 m and gross thickness equals 30 m, then NTG is 12/30 = 0.4 = 40%. If you are populating a grid property, you may represent NTG as a fraction between 0 and 1. If you are reporting to management or in reserves documentation, you may present it as a percentage.

5. Validate Against Wells and Maps

After calculation, check whether the NTG distribution makes geological sense. Compare it with well markers, facies belts, depositional trends, and thickness maps. A high NTG ribbon aligned with a channel fairway may be reasonable. A patchy pattern that cuts across structural and depositional boundaries may indicate a problem with cutoffs, gridding, or interpolation.

Common Petrel Use Cases for NTG

In real projects, users calculate NTG in Petrel for several reasons:

• Static model building: to represent reservoir proportion in each cell or layer.
• Volumetric estimation: to constrain hydrocarbon pore volume realistically.
• Property modeling: to guide porosity, facies, and permeability distributions.
• Well correlation and zonation: to compare net quality across wells.
• Uncertainty analysis: to test how cutoffs affect STOIIP or GIIP.

NTG is especially important because it multiplies into volumetric calculations. Even a modest shift in NTG can materially change in-place volumes. That is why teams often create low, base, and high NTG scenarios during uncertainty assessment.

Typical Net to Gross Interpretation Ranges

There is no universal classification, but the table below reflects common interpretation bands used in practical reservoir description. These are planning guidelines rather than fixed regulatory categories.

NTG Range	General Interpretation	Typical Geological Meaning	Modeling Implication
Below 0.30	Low NTG	Thin reservoir streaks, high shale content, or strongly heterolithic package	Expect lower connected reservoir proportion and greater sensitivity to layering assumptions
0.30 to 0.60	Moderate NTG	Mixed quality interval with meaningful non-reservoir fraction	Pay attention to cutoff choice and vertical communication
0.60 to 0.80	Good NTG	Reservoir-dominated interval with limited non-net beds	Often favorable for volumetrics, but still validate facies continuity
Above 0.80	Very high NTG	Clean reservoir package or amalgamated sands	Check that cutoffs are not overly permissive and confirm with well control

Real Industry Context: Why Precision Matters

Net-to-gross values feed directly into reservoir description, and reservoir description matters because the petroleum sector deals with very large resource and production volumes. Public agency statistics show just how significant modeling accuracy can be when scaled to basin and field level decisions.

Source	Published Statistic	Value	Why It Matters for NTG Workflows
U.S. Energy Information Administration	U.S. proved reserves estimates are published annually by hydrocarbon type and region	National totals are reported in billions of barrels and trillions of cubic feet	Small percentage changes in effective reservoir volume assumptions can scale into large reserve impacts
U.S. Geological Survey	USGS continuously assesses undiscovered technically recoverable resources across petroleum systems	Assessments are commonly reported in millions to billions of barrels of oil equivalent	Reservoir quality metrics such as NTG help link geologic interpretation to resource estimates
Bureau of Ocean Energy Management	BOEM publishes offshore reserve and resource evaluations for federal waters	Reported values cover large offshore provinces with substantial remaining resources	Accurate gross and net interpretation is fundamental in offshore volumetric and development screening

Those agency datasets are not NTG tables by themselves, but they show why geoscience inputs must be defensible. If your net definition is inconsistent across wells or your gross interval shifts between interpretations, the impact can ripple through mapping, volumetrics, reserves, and development plans.

Best Practices for Calculating NTG in Petrel

1. Use a single zonation framework. Your top and base boundaries should come from one approved interpretation version.
2. Document all cutoffs. Record porosity, shale volume, saturation, facies, and permeability thresholds.
3. Keep scale consistent. Do not compare log-derived net at fine resolution with gross thickness from a mismatched grid without proper upscaling.
4. QC the wells first. Many NTG anomalies start with poor well picks, depth shifts, or missing logs.
5. Map and cross-check. Compare NTG against facies maps, structure maps, and gross thickness maps.
6. Run scenarios. Low, base, and high cutoff cases are often more useful than one deterministic NTG value.

Common Mistakes to Avoid

Mixing Different Net Definitions

If some wells use net sand and others use net pay, the resulting NTG map will not be internally consistent. Always standardize the net definition before comparison.

Using Gross Thickness from the Wrong Interval

Gross must represent the same stratigraphic interval as the net. If gross is measured over a larger zone than the net picks, the NTG ratio will be artificially low.

Ignoring Resolution Effects

Thin beds may disappear during upscaling. That means grid-based NTG can differ from well-log NTG. This is not always an error, but it must be understood and documented.

Failing to Validate with Geology

A mathematically correct ratio can still be geologically wrong. If the map pattern contradicts known depositional trends, revisit the workflow.

Worked Examples

Example 1: Calculating NTG

A zone is 50 m gross and 32 m net reservoir. NTG = 32 / 50 = 0.64 = 64%. In Petrel, you might store this as 0.64 in the grid property and display it as 64% in reports.

Example 2: Solving for Gross Thickness

You have 18 m of net pay and want to know the gross interval if expected NTG is 45%. Gross = 18 / 0.45 = 40 m. This is useful during analog-based planning and early screening.

Example 3: Solving for Net Thickness

A mapped gross interval is 72 ft and analog NTG is 55%. Net = 72 x 0.55 = 39.6 ft. That estimate can then be compared with well control and facies interpretation.

Recommended Authoritative References

For broader technical context, review public resources from the U.S. Energy Information Administration, the U.S. Geological Survey Energy Resources Program, and the Bureau of Ocean Energy Management.

Final Takeaway

If you want to calculate net to gross in Petrel correctly, focus on more than the formula. Yes, the ratio itself is simple: net divided by gross. But robust NTG depends on disciplined interval definition, defensible cutoffs, proper scaling, and geological QC. In most projects, the biggest errors are not arithmetic errors. They are workflow errors, especially inconsistent net criteria and mismatched gross intervals.

Use the calculator above to validate your numbers quickly, but always tie the result back to your Petrel interpretation framework. When net, gross, and cutoffs are aligned, NTG becomes a powerful property for mapping reservoir quality, constraining volumetrics, and improving model realism.

Practical tip: If your Petrel model stores NTG as a fraction from 0 to 1, convert carefully when reporting. A model value of 0.68 should be communicated as 68%, not 0.68%.