Net To Gross Calculation Petrel

Use this interactive calculator to estimate net-to-gross ratio, net reservoir thickness, non-net interval, gross rock volume, and net rock volume for Petrel style reservoir workflows. It is designed for geoscientists, petrophysicists, and reservoir modelers who need a fast, transparent way to validate net-to-gross inputs before building property models and volumetric cases.

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Expert Guide to Net to Gross Calculation in Petrel

Net-to-gross, often shortened to NTG, is one of the most important ratios in reservoir characterization and static modeling. In the simplest possible terms, it answers a practical question: what proportion of the gross interval is actually considered net reservoir? If your gross thickness is 100 meters and your net thickness is 60 meters, the net-to-gross ratio is 0.60 or 60%. In Petrel, this ratio is not just a descriptive statistic. It can become a controlling property in facies modeling, petrophysical filtering, volumetrics, upscaling, and uncertainty analysis.

When people search for a net to gross calculation Petrel workflow, they are usually trying to solve one of three problems. First, they want to verify that their net interval picks are mathematically correct before loading values into a model. Second, they want to understand how Petrel treats net properties in gridded cells, zonal averages, or discrete facies frameworks. Third, they need a practical way to communicate net-to-gross assumptions to reservoir engineers, management teams, or partners. A small change in NTG can materially affect hydrocarbon pore volume, reserve estimates, and development economics.

This page gives you both a working calculator and a deeper interpretation guide. The calculator provides a fast calculation for thickness-based and volume-based checks. The written guide explains what NTG means, how the formula works, how it is commonly applied inside Petrel, and why cutoff selection is often more important than the arithmetic itself.

What Net to Gross Means in Reservoir Modeling

Gross thickness refers to the full interval being studied. It may be a formation, member, zone, flow unit, or stratigraphic package. Net thickness is the subset of that interval that passes predefined cutoffs such as shale volume, porosity, permeability, and fluid saturation. In a deterministic workflow, the equation is straightforward:

Net-to-gross ratio = Net thickness / Gross thickness

Net thickness = Gross thickness × Net-to-gross ratio

Non-net thickness = Gross thickness – Net thickness

Inside Petrel, the ratio may be represented in several ways: as a cell property, a zonal average, a facies proportion trend, a volumetric modifier, or a quality-control parameter. Depending on the project setup, net rock may be defined from logs, interpreted electrofacies, core-calibrated cutoffs, or seismic-guided property transforms. That means two teams can use the same gross interval and still obtain different NTG values because their net definition is different.

Why Net to Gross Calculation Matters in Petrel

• Volumetrics: NTG directly influences net rock volume and, when combined with porosity and saturation, hydrocarbon pore volume.
• Facies modeling: In object and pixel based workflows, NTG often constrains the amount of reservoir facies placed inside a zone.
• Upscaling: Cell scale net properties can change when moving from fine geocellular grids to simulation grids.
• Uncertainty analysis: P10, P50, and P90 reserve cases are highly sensitive to shifts in NTG assumptions.
• Development decisions: Completion strategy, well placement, and perforation interval selection frequently depend on where net rock is concentrated.

Step by Step: How to Do a Net to Gross Calculation for Petrel

1. Define the gross interval. Confirm the top and base of the target zone or layer package.
2. Set your net criteria. Typical criteria may include minimum porosity, maximum shale volume, minimum permeability, and maximum water saturation.
3. Measure net thickness. This can come from well picks, cell filtering, property cutoffs, or facies mapping.
4. Apply the formula. Divide net thickness by gross thickness to get the NTG ratio.
5. Convert to volume if needed. Multiply area by gross thickness to get gross rock volume, then multiply by NTG to estimate net rock volume.
6. QC inside Petrel. Check maps, histograms, cross sections, and zone statistics to confirm values are geologically consistent.

Suppose you have a gross thickness of 120 m and a net thickness of 78 m. The NTG ratio is 78 ÷ 120 = 0.65, or 65%. If the closure area is 2.5 km², the gross rock volume is 300,000,000 m³, and the net rock volume is 195,000,000 m³. That is exactly the type of first-pass check the calculator above is designed to provide.

Typical Net to Gross Ranges by Depositional Environment

Net-to-gross can vary dramatically depending on depositional setting, accommodation, sand distribution, heterogeneity, and diagenetic overprint. The table below summarizes common industry analog ranges used for screening and conceptual modeling. These are not universal constants, but they are useful as a practical benchmark when quality controlling Petrel models.

Depositional environment	Typical NTG range	Reservoir implication	Modeling note in Petrel
Deepwater channel sands	0.50 to 0.85	High quality sand concentration in channel axes, lower NTG in levees and margins	Use facies constrained trends and pay attention to lateral pinchout behavior
Shoreface and shallow marine sands	0.45 to 0.75	Can show strong vertical stacking with interbedded shales	Zonal averages may hide bed scale variability, so inspect vertical proportion curves
Fluvial channel belt systems	0.35 to 0.80	Good reservoir quality in channels, lower net in floodplain influenced intervals	Object based facies and channel proportion trends can improve realism
Tidal flat to estuarine settings	0.20 to 0.55	Frequent heterolithic bedding can sharply reduce effective net	Cutoff sensitivity is usually high, especially for shale volume and permeability
Delta plain and distributary complexes	0.30 to 0.70	Reservoir compartmentalization can be significant	Map continuity carefully across fault blocks and stacked lobes

How NTG Affects Volumetric Outcomes

One reason net to gross calculation receives so much attention is that it scales directly into volumetrics. If your gross rock volume remains constant, every increase in NTG increases the amount of rock that can host effective pore volume. This is why static modelers often build several NTG cases during field evaluation. A change from 0.45 to 0.65 may seem modest, but it can transform the interpreted commercial potential of a reservoir.

Case	Gross thickness	Area	Gross rock volume	NTG	Net rock volume
Low case	100 m	3.0 km²	300,000,000 m³	0.40	120,000,000 m³
Mid case	100 m	3.0 km²	300,000,000 m³	0.55	165,000,000 m³
High case	100 m	3.0 km²	300,000,000 m³	0.70	210,000,000 m³

The difference between the low and high case in this example is 90,000,000 m³ of net rock volume. If average porosity were 18%, that change would represent 16,200,000 m³ of additional pore volume before saturation effects are even considered. That is why a disciplined NTG workflow is essential.

Petrel Workflow Tips for Better Net to Gross Modeling

• Calibrate to core and test data: Log based net flags are helpful, but they are stronger when anchored to measured rock and fluid performance.
• Use consistent cutoffs across wells: If cutoff logic changes from well to well without clear justification, your NTG map may become biased.
• Separate geological net from engineering pay: Some teams use one net definition for static geology and another for producible pay. Be explicit about which one is being modeled.
• Honor stratigraphic architecture: Layering style matters. A uniform zone average may be mathematically correct but geologically misleading.
• Run sensitivity cases: Test low, base, and high cutoffs to understand how robust the volumetric outcome really is.
• Check scale effects: Fine scale NTG may not transfer directly to coarser simulation grids without loss of vertical detail.

Common Mistakes in Net to Gross Calculation

The formula itself is easy, but the interpretation can go wrong in subtle ways. A common error is mixing units, such as entering area in acres while mentally expecting square kilometers. Another is allowing net thickness to exceed gross thickness, which is physically inconsistent in a simple thickness based workflow. A third is forgetting that apparent net from logs may differ from effective flow net after permeability screening. In Petrel, these issues often appear as unrealistic facies proportions, abrupt map artifacts, or volumetric outputs that do not reconcile with well data.

Another frequent mistake is using a single field-wide NTG value where geological variation is clearly present. Deltaic and channelized systems are rarely uniform. Better practice is to calculate NTG by zone, facies belt, or structural compartment and then compare the spatial pattern to depositional expectations. If your model predicts high NTG in areas with known shale drapes or poor core quality, that is a prompt to revisit the input assumptions.

Recommended Quality Control Checks

1. Compare gross maps, net maps, and NTG maps side by side.
2. Validate the calculated net interval against cored wells and test intervals.
3. Review histograms to ensure NTG values sit within geologically reasonable ranges.
4. Inspect cross sections to confirm vertical continuity and layering logic.
5. Reconcile zonal NTG statistics with volumetric outputs and reserve cases.
6. Document every cutoff used so the workflow can be reproduced later.

Authoritative External Sources

For broader context on reservoir evaluation, energy data, and geological assessment methods, the following public sources are useful:

• U.S. Geological Survey for basin analysis, resource assessments, and subsurface geology references.
• U.S. Energy Information Administration for official energy statistics and reserve related market context.
• Stanford Earth at Stanford University for academic resources related to geoscience, sedimentology, and reservoir studies.

Final Takeaway

A strong net to gross calculation Petrel workflow combines simple arithmetic with rigorous geological judgment. The calculation itself is easy: divide net by gross. The difficult and valuable part is how net is defined, mapped, validated, and scaled into the model. If you use clear cutoffs, maintain unit consistency, compare results against analog expectations, and run sensitivity cases, NTG becomes a highly effective bridge between petrophysics, geology, and engineering. Use the calculator above whenever you need a quick validation of thickness and volume relationships before or during Petrel modeling.