geology · geochemistry
Rock-Eval Source Rock Indices (PI, HI, OI)
PI = S1/(S1+S2); HI = (S2/TOC)×100; OI = (S3/TOC)×100
click formula to derive ↑
Inputs
mg HC/g rock
mg HC/g rock
mg CO2/g rock
wt%
Description
Computes the three standard source-rock quality indices derived from Rock-Eval programmed pyrolysis (Peters 1986): the Production Index (PI), which tracks thermal maturity, and the Hydrogen and Oxygen Indices (HI, OI), which are TOC-normalized measures of the kerogen's hydrogen- and oxygen-richness used for kerogen typing on a pseudo-Van Krevelen (HI-OI) diagram.
Variables
| Symbol | Unit | Description |
|---|---|---|
| PI | — | Production Index |
| HI | mg HC/g TOC | Hydrogen Index |
| OI | mg CO2/g TOC | Oxygen Index |
| S1 | mg HC/g rock | Quantity of free (already-generated) hydrocarbons thermally volatilized from the rock sample during the initial, lower-temperature stage of Rock-Eval pyrolysis. |
| S2 | mg HC/g rock | Quantity of hydrocarbons generated by thermal cracking of kerogen during the higher-temperature pyrolysis stage — a measure of the remaining hydrocarbon-generating potential of the sample. |
| S3 | mg CO2/g rock | Quantity of CO2 generated during pyrolysis, used as a proxy for the oxygen content of the kerogen. |
| TOC | wt% | Total organic carbon content of the rock sample, typically measured alongside the Rock-Eval pyrolysis run. |
Assumptions
- S1, S2, S3, and TOC were all measured on the same sample under standard Rock-Eval pyrolysis conditions.
- The sample is not significantly affected by drilling-mud contamination or weathering, either of which can distort S1 and TOC independently of the source rock's true character.
- HI/OI-based kerogen typing (not computed directly by this calculator, but the standard use of its outputs) assumes the sample is at low-to-moderate thermal maturity — HI declines with increasing maturity regardless of original kerogen type, which can obscure kerogen typing in more mature samples.
Limitations
- PI increases with thermal maturity but can also be elevated by hydrocarbon staining, drilling-mud contamination, or migrated (non-indigenous) hydrocarbons — a high PI alone does not prove in-situ thermal generation.
- HI and OI become less reliable kerogen-type indicators at higher thermal maturity, since HI declines toward zero regardless of original kerogen type as the source rock is depleted of its generative potential.
- These index definitions are standard, widely reproduced formulas confirmed consistently across multiple independent secondary sources (AAPG Wiki's Production Index and Hydrogen Index pages, IFPEN's Rock-Eval documentation), but Peters (1986), the AAPG Bulletin paper they are attributed to, is paywalled and was not directly page-accessed this session. Treat the formulas as secondary-source-corroborated, not a page-checked citation from the primary paper directly.
Use Cases
- → Kerogen typing from cuttings or core: Plot HI against OI (a pseudo-Van Krevelen diagram) across a well or field to characterize kerogen type (oil-prone Type I/II vs. gas-prone/inert Type III/IV) in an unconventional or conventional source-rock evaluation.
- → Maturity screening alongside vitrinite reflectance: Use PI as a quick, low-cost cross-check on thermal maturity trends established from vitrinite reflectance or the ΔlogR/TTI calculators elsewhere in this discipline.
Related Calculations
Region Notes
Global
Always screen for drilling-mud (especially oil-based mud) contamination before trusting S1/PI values from cuttings samples — contamination artificially inflates S1 and PI independent of the source rock's true maturity.
References
Primary source
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