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Abstract: Surface Exploration Geochemistry: Numerical Unmixing of Soil Gas Data and Application for Hydrocarbon Exploration

Hans Von Der Dick, Richard E. Wyman, Dane Bosman

Soil hydrocarbon gas surveying is one of several surface exploration techniques proposed for oil and gas exploration. These surveys usually measure hydrocarbon gas concentrations and the data are used to define exploration fairways or to evaluate prospective areas. However, conventional, simple soil gas data may be misleading and prone to misinterpretation.

A systematic evaluation of several thousand soil gas data points from 30 field surveys indicates the high degree of uncertainty in the interpretation of conventional soil gas data. Many concentration driven anomalies follow a random pattern and are not necessarily related to subsurface oil or gas deposits. The scatter and high degree of randomness are largely attributed to wide background fluctuation, co-produced in-situ biological hydrocarbon gases and secondary alteration effects.

Even though conventional gas surveys may be of limited diagnostic value for exploration purposes, these data contain valuable information when treated as gas mixtures of various original By using numerical unmixing techniques, the small but critical fraction of gases due to hydrocarbon see page can be discriminated from other extraneous gases. Model calculations are used to systematically search for a defined seepage signal in noisy and complex soil gas data sets. Near surface gas samples that obey "model see page criteria" show a high degree of correlation with subsurface hydrocarbon occurrences.

Several exploration examples using this technique are provided and the statistical record from more than 50 prospects is discussed. Results show that the exploration risk is lowered when these discrimination techniques are incorporated into exploration programs.

Advantages and limitations of this new approach are discussed on the basis of the large-scale test program. Limitations are recognized in exploration areas with very deep targets and tight seals. The advantages of this technique are the reduction of false anomalies along with an improved level of interpretation.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada