Print this pageUsing Soil Gas Composition to Distinguish 
Hydrocarbon
Type
in Areas of Multiple Petroleum Producing Horizons
RICE, GARY K., JOHN Q. BELT, JR., HARVEY H. HOWELL and LANCE HARWERTH, and CHAD A. BOWERMAN
Soil gas data is frequently used to detect near-surface hydrocarbon seepage.
It can also be used to calculate measures of soil gas composition. Composition
indicators include Ethane/Propane ratios (EPR) and a new Ethane Composition
Index (ECI). ECI is one of several related composition measurements derived from
"common mode" reduction of hydrocarbon data.
Hydrocarbon composition data exhibits unique properties: 1. Composition is
independent of concentration. 2. Composition data is normally distributed. The
first property attenuates large spatial variations in hydrocarbon concentrations
caused by vertical migration pathways. Therefore, composition data is more
evenly distributed over a reservoir. The second property allows direct
application of statistical data analysis. Histograms, duster analysis, and
discriminate analysis detect and separate multiple compositions into distinct
groups.
This project tested EPR, ECI, and other composition measures in a Study Area
at the southern end of the Eastern Shelf of the Midland Basin. The Study Area
included southern Concho, northern Menard and eastern McCulloch Counties and is
a complex multiple petroleum horizon area. Composition data spanning 12 years
were evaluated for effectiveness in discriminating among known hydrocarbon
reservoirs to target specific petroleum reservoirs with commercial viability.
Resulting composition information proved successful in refining geologic models
that yielded several new field discoveries.