# Hydrogen Index (HI)

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Critical elements of the petroleum system Evaluating source rocks Carol A. Law Web page AAPG Store
Figure 1 Decreasing HI trend for a source rock beginning to generate hydrocarbons at a depth of approximately 2200 m. Copyright: Ungerer et al.;[1] courtesy John Wiley & Sons.

Gross trends of hydrogen indices (HIs) can be used as a maturation indicator. The hydrogen index is calculated from Rock-Eval data using the following formula:

${\mbox{HI}}={\mbox{S2}}/{\mbox{TOC}}\times 100$

where S2 is the amount of hydrocarbons generated through thermal cracking of nonvolatile organic matter in mg/g of rock[2] and TOC is total organic carbon in %.

## How to apply HI

Hydrocarbon generation zones can be indicated in the HI data for a uniform source section when HI decreases with depth. Inconsistencies due to changes in organic facies or the chemistry of the source rock can produce shifts in the HI data which are not indicative of maturation trends. Therefore, be sure the source is of uniform character when applying this concept.

Figure 1 shows a decreasing HI trend for a source rock beginning to generate hydrocarbons at a depth of approximately 2200 m7,217.848 ft.