Rock Eval analysis using hydrogen index (HI) and oxygen index (OI)

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Critical elements of the petroleum system
Chapter	Evaluating source rocks
Author	Carol A. Law
Link	Web page
Store	AAPG Store

Obviously, the two main elements of hydrocarbons are hydrogen and carbon. Therefore, once we know the amount of total organic carbon (TOC) present in a rock, we then need to know the amount of hydrogen present in order to assess overall source rock quality. The amount of oxygen and hydrogen present in the kerogen defines the kerogen as type I, II, or III and if the rock will be oil or gas prone.

Hydrogen index

The hydrogen index (HI) represents the amount of hydrogen relative to the amount of organic carbon present in a sample. The S2 curve of a Rock-Eval analysis can help us determine the total amount of hydrogen in milligrams of hydrogen to grams of sample according to the following formula:

${\mbox{HI}}={\mbox{S2}}{\mbox{ (Mg/g)}}/\%{\mbox{TOC}}\times 100$

Oxygen index

The oxygen index (OI) represents the amount of oxygen relative to the amount of organic carbon present in a sample. The S3 curve of a Rock-Eval analysis can help us determine the total amount of oxygen present in a sample according to the following formula:

Figure 1 Modified Van Krevlen diagram. Copyright: Tissot and Welte; courtesy Springer-Verlag.

${\mbox{OI}}={\mbox{S3}}{\mbox{ (mg/g)}}/\%{\mbox{TOC}}\times 100$

Determining kerogen quality

The type of kerogen present in a rock determines its quality. Type I kerogen is the highest quality; type III is the lowest. Type I has the highest hydrogen content; type III, the lowest. To determine the kerogen type present in a source rock, plot the hydrogen and oxygen indices on a modified Van Krevlen diagram (Figure 1).

Caveat

Use the HI/OI technique only to determine source rock quality (kerogen type) of immature rocks. HI and OI change as a source rock matures (the amount of hydrogen and oxygen relative to carbon decreases and the HI/OI ratios converge toward the origin of the plot, leading one to a more gas-prone type III interpretation). Therefore, in mature rocks HI and OI are not indicative of the original kerogen quality.

See also

• Kerogen types
• Kerogen type and quality: visual assessment
• Pyrolysis gas chromatography

External links

find literature about Rock Eval analysis using hydrogen index (HI) and oxygen index (OI)

• Rock Eval 6 — Product page at Vinci Technologies
• Original content in Datapages
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