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Hydrocarbon Composition: The Other Dimension of Geochemical Exploration

Gary K. Rice¹

¹GeoFrontiers Corporation, Dallas, TX


While concentration remains the workhorse in using geochemical exploration for mapping oil reservoirs, composition is concentration's reliable sidekick. Concentration measures how much hydrocarbon is in a geochemical sample. Composition measures its hydrocarbon makeup or signature.

Composition is measured by a ratio of one hydrocarbon concentration to another hydrocarbon concentration. A simple and common example Ethane to Propane Ratio (EPR) is simply the ethane concentration divided by the propane concentration that is expressed as:

Brackets denote concentration. When concentration units are the same in both the numerator and in the denominator, units cancel and the ratio, EPR, has no units. A slightly more complex composition measure is Ethane Composition Index (ECI):

ECI has a limited range usually from 50 and 80. ECI is approximately normally distributed which is useful for statistical analysis.

Composition lends itself to pattern recognition techniques. Differences in surface signatures can be seen by comparing histograms.


Examples of composition differentiating among multiple oil pays in the Permian Basin illustrate the effectiveness and benefit of this important other dimension to geochemical exploration.

In geologically favorable settings, composition mapping adds significant value to geochemical exploration by not only discriminating oil plays from gas plays, but by also providing information about hydrocarbon reservoirs represented by geochemical features.



AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014