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[[file:evaluating-source-rocks_fig6-14.png|thumb|{{figure number|1}}. Copyright: Ungerer et al.;<ref name=Ungerer1991>Ungerer, P., P. Forbes, and J. L. Rudkiewicz, 1991, Computer modeling of the interplay between generation and expulsion of petroleum: a new advance in synthesizing geochemical data: Proceedings of the 13th World Petroleum Congress, Buenos Aires, p. 179–189. ''Discusses the relationships between a number of maturation indicators and petroleum generation and migration.''</ref> courtesy John Wiley & Sons.]]

[[file:evaluating-source-rocks_fig6-14.png|thumb|{{figure number|1}}Decreasing HI trend for a source rock beginning to generate hydrocarbons at a depth of approximately 2200 m. Copyright: Ungerer et al.;<ref name=Ungerer1991>Ungerer, P., P. Forbes, and J. L. Rudkiewicz, 1991, Computer modeling of the interplay between generation and expulsion of petroleum: a new advance in synthesizing geochemical data: Proceedings of the 13th World Petroleum Congress, Buenos Aires, p. 179–189. ''Discusses the relationships between a number of maturation indicators and petroleum generation and migration.''</ref> 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:

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: