A refinery’s Nelson Complexity Index (NCI) is a weighted measure of its secondary processing capacity relative to its crude distillation capacity. Crude distillation is the baseline; other units add weight according to their complexity factors and capacity. A higher NCI indicates a more complex equipment configuration—not necessarily a more profitable or efficient refinery.
How the Nelson Complexity Index is calculated
The calculation begins with atmospheric crude distillation, assigned a factor of 1. Each other processing unit receives a factor relative to that baseline. Its factor is multiplied by its capacity as a share of the refinery’s atmospheric distillation capacity; the contributions are then added, including the crude distillation baseline. This means both the type of equipment and its capacity affect the result.
Oil & Gas Journal’s 2005 explanation gives these generalized examples of factors:
| Processing unit | Example complexity factor |
|---|---|
| Atmospheric distillation | 1 |
| Vacuum distillation | 2 |
| Delayed coking | 6 |
| Fluid catalytic cracking | 6 |
| Reforming | 5 |
| Hydrocracking | 6 |
| Hydrotreating | 2 |
| Alkylation or polymerization | 10 |
These are examples from that article, not a universal factor table for every publisher or refinery. Methodologies can differ: Eni says its refinery tables use standard factors modified as necessary, and describes factors in relation to costs relative to primary distillation. Compare published NCI values with the source and method in view.
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What a higher NCI tells you—and what it does not
A higher NCI means a refinery has more secondary processing capacity, or capacity weighted more heavily under the method used. Such equipment can separate crude fractions further, upgrade lower-value products, increase octane, or remove sulfur and other pollutants. The U.S. Energy Information Administration (EIA) summarizes the relationship: “The larger the Nelson index of a refinery, the more complex it is.”
NCI describes equipment and capacity, not how well a refinery is operating. On its own, it does not establish profitability, efficiency, emissions, or specific product yields. Those outcomes also depend on feedstock, market conditions, operating choices, costs, and regulation.
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How to compare NCI figures responsibly
Before comparing two figures, check that they refer to the same kind of subject and were calculated on a comparable basis:
- Year or reporting period: refinery configurations and published figures can change.
- Scope: distinguish an individual refinery from a regional or global average.
- Source and method: publishers may modify standard complexity factors, and published data series can be revised.
- Equipment context: note major conversion and treating units when the source identifies them.
- What the figure measures: treat it as a structural complexity indicator, not as a stand-in for performance or returns.
Examples in published EIA data
Individual refineries: dated 2012 examples
In its October 11, 2012 explainer, the EIA cited Phillips 66 values of 7.0 for the Ferndale refinery in Washington and 14.1 for the Los Angeles refinery in California. The Ferndale example was associated with fluid catalytic cracking, alkylation, and hydrotreating units. The Los Angeles example included fluid catalytic cracking, alkylation, hydrocracking, reforming, and coking units. These are historical figures from that article, not current refinery values.
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Global aggregate: figures reported in a 2019 analysis
An EIA analysis published in 2019, using Eni World Oil Review and IEA data, reported these global aggregate NCI figures:
| Year | Global aggregate NCI | Source and reporting context |
|---|---|---|
| 2000 | 7.9 | EIA’s 2019 analysis using Eni World Oil Review and IEA data |
| 2005 | 8.2 | EIA’s 2019 analysis using Eni World Oil Review and IEA data |
| 2010 | 8.7 | EIA’s 2019 analysis using Eni World Oil Review and IEA data |
| 2015 | 9.1 | EIA’s 2019 analysis using Eni World Oil Review and IEA data |
| 2016 | 9.3 | EIA’s 2019 analysis using Eni World Oil Review and IEA data |
| 2017 | 9.3 | EIA’s 2019 analysis using Eni World Oil Review and IEA data |
The EIA cautions that older time series assembled from earlier Eni publications are not strictly comparable because the data were revised. Treat the figures above as the series presented in that 2019 analysis, with its stated sources and years, rather than as current global values.
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Origin of the index
W. L. Nelson developed the index in the 1960s through a series of articles for Oil & Gas Journal. The crude distillation unit was assigned a factor of 1, while other equipment was weighted according to its complexity and cost relative to that unit. The index provides a compact way to summarize refinery configuration, but its meaning depends on the factors and capacity data used to calculate it.
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