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A refinery’s Nelson Complexity Index (NCI) is a weighted measure of its secondary processing capacity compared with its crude distillation capacity. It summarizes the refinery’s equipment configuration: a higher NCI indicates greater measured complexity, not necessarily higher profits, better efficiency, or a particular product yield.
What the Nelson Complexity Index measures
The NCI describes the types and relative capacities of processing equipment installed at a refinery. Crude distillation—the primary step that separates crude oil into fractions—is the reference point. It is assigned a complexity factor of 1; other units receive factors that reflect their complexity relative to that baseline.
The index is therefore a compact way to describe refinery configuration and capability. It is not a direct measure of how well a refinery is running or how much money it makes. The U.S. Energy Information Administration (EIA) explains that refineries add secondary units to improve separation, upgrade lower-value products, increase octane, or remove sulfur and other pollutants. EIA’s explainer states: “The larger the Nelson index of a refinery, the more complex it is.”
How the NCI is calculated
The calculation is a weighted sum. For each process unit, its complexity factor is multiplied by its throughput capacity as a share of the refinery’s atmospheric crude distillation capacity. Those contributions are added, including the crude distillation unit’s baseline contribution. As a result, both the kinds of units present and their relative capacities affect the total.
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Oil & Gas Journal’s 2005 explanation gives these generalized factors:
| Process unit | Generalized factor in Oil & Gas Journal (2005) |
|---|---|
| Atmospheric distillation | 1 |
| Vacuum distillation | 2 |
| Delayed coking | 6 |
| Fluid catalytic cracking | 6 |
| Reforming | 5 |
| Hydrocracking | 6 |
| Hydrotreating | 2 |
| Alkylation/polymerization | 10 |
These are generalized examples from that article, not a universal or current factor table for every refinery or publisher. Eni’s World Energy Review 2023 says its refinery tables use standard factors modified as necessary; its review relates the factors to costs relative to primary distillation. Reported NCI values should therefore be read with their source and methodology in mind.
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What a higher NCI does—and does not—tell you
A higher value signals a more complex equipment configuration under the method used. More secondary conversion capacity can let a refinery process fractions further and upgrade lower-value material into higher-value products. The index alone, however, does not establish that the refinery is more profitable, more efficient, or less polluting, nor does it specify its product yields. Those outcomes also depend on feedstock, market conditions, operating decisions, costs, and regulation.
How to compare published NCI values
Before interpreting two figures as an apples-to-apples comparison, check what each one describes and how it was calculated:
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- Period: Identify the year or reporting period. Refinery configurations and published values can change.
- Scope: Distinguish an individual refinery from a regional or global aggregate.
- Source and method: Name the publisher and note whether it modifies standard complexity factors.
- Equipment: When available, consider which conversion and treating units contribute to the value.
- Meaning: Treat the comparison as one of structural complexity, not operating performance or profitability.
Examples: refinery and global figures
The following examples have different scopes and dates, so they should not be read as a current ranking of refineries.
| NCI | Scope and period | What the cited source reports |
|---|---|---|
| 7.0 | Ferndale refinery, Washington; Phillips 66 reporting cited in an EIA article from 2012 | EIA associates the example with fluid catalytic cracking, alkylation, and hydrotreating units. |
| 14.1 | Los Angeles refinery, California; Phillips 66 reporting cited in an EIA article from 2012 | EIA associates the example with fluid catalytic cracking, alkylation, hydrocracking, reforming, and coking units. |
These are historical refinery examples from EIA’s October 11, 2012 article, not current values.
A separate EIA analysis published in 2019 reported these global aggregate NCI figures, using Eni World Oil Review and IEA data:
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|---|---|
| 2000 | 7.9 |
| 2005 | 8.2 |
| 2010 | 8.7 |
| 2015 | 9.1 |
| 2016 | 9.3 |
| 2017 | 9.3 |
These are global aggregates for the years shown, as presented in EIA’s 2019 analysis, not figures for a single refinery. EIA also cautions that older time series assembled from earlier Eni publications are not strictly comparable because the data were revised.
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Where the index came from
The EIA says W. L. Nelson developed the index in the 1960s through a series of Oil & Gas Journal articles. The crude distillation unit provides the factor-1 baseline, while other equipment is weighted according to its complexity and cost relative to that unit.
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