Concepts and Methods for the Prediction of Reservoir Hydrocarbon Type Using Ratios of Mudlog Gas Chromatography C1-C5 Gases

Raymond M. Pierson
Aera Energy LLC, 10000 Ming Avenue, Bakersfield, CA 93311

The application of Mudlog Gas Ratio Analyses for the purposes of predicting the hydrocarbon type from which the gases were liberated during drilling is essential in understanding the reserves. Using various gas ratios, it becomes possible to predict and interpret the hydrocarbon source type. This is possible based on the premise that rock cuttings from any particular formation “produce” the gases, or the hydrocarbon vapors they contain, into the drilling mud system. These same gases are detectable at the surface with the use of Gas Chromatography. It is reasonable to assume that the same formation, if completed, would produce gases of a similar composition whether the source hydrocarbon is a dry gas, wet gas, condensate, light oil or heavier hydrocarbon liquid. The composition is then reconstructed through the use of Gas Ratios and becomes a help in “fingerprinting” the source hydrocarbons (not the source rock).

This presentation begins with an overview of basic concepts, then presents various analytical tools and techniques, discusses data applications and concludes with examples of how the ratios are integrated into and enhance reservoir description. Applications and uses include determination of “best show in well”, identification of hydrocarbon flow units, volume of liberated gases, gas volume comparisons, and many other analyses including the identification of unconventional gas from coals and shales.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California