Hydrogen Index (HI)
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:
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 depth::2200 m.