Soil Gas Studies

The identification, characterization and configuration of a thermogenic hydrocarbon surface anomaly in a soil gas survey is only possible by means of the analyses of a large number of samples in a specific area. Samples are collected and returned to the laboratory, where they are analyzed to characterize the composition and origin of gaseous hydrocarbons.

These measurements are performed to establish the presence/absence of seeping hydrocarbons. One of the most important factor in the application of surface geochemistry as an important auxiliary exploration tool is the definition of background and anomalous values using statistical procedures. Regardless of the geological setting, the results of a statistical treatment are interpreted as a whole anomaly, and values greater than 1, 2, or 3 standard deviations (SD) from the mean (M) characterized the 1st, 2nd and 3rd order anomalies. These values are converted to parts per million (ppm) and mapped. Interpretation of one of the populations is straightforward. Case histories all around the world have shown that generally only anomalies greater than 2nd order are important enough to be considered in the selection of potential hydrocarbon prospective plays.

SURFACE GEOCHEMICAL PROSPECTING METHODS

Surface geochemical prospecting methods may be subdivided into direct, indirect and non-conventional methods.

a) Direct methods:
Such methods allow the detection and quantification of hydrocarbons that are present in surface or very near surface soils, sediments, and rocks. Based on the hydrocarbon type and its occurrence, they can be classified in four main types:

- Free hydrocarbon gases absorbed within the soil, sediment or rock. The method includes direct collection and concentration.

- Absorbed hydrocarbon gases or chemically/physically connected to the soil, sediment or rock. The method includes direct sampling and mechanically/chemically induced release prior to concentration.

- Adsorbed and occluded hydrocarbon gases in crystals and mineral particles in soils sediments and rocks. The method includes direct sampling and mechanical, chemical, and acid-induced release prior to concentration.

- Gas, liquid and solid hydrocarbons held in soils, sediments and rocks or dissolved in water bodies. The method includes direct sampling, extraction with solvents and hydrochemical methods to induce release prior to concentration.

b) Indirect methods:
Such methods detect chemical, physical and biological disturbances caused by hydrocarbon concentrations in the surface. They include detection and quantification of inorganic elements and/ or compounds. The most important are:

- Redox potential:
- Surface gamma radiation, based on radiometrics survey;
- Noble gases in soils, based on direct concentration;
- Trace soil elements and mineralogy, based on mineral assemblages and concentrations;
- Inorganic hydrochemicals, based on water dissolved salts.

c) Non-conventional methods:
Such methods consist of the detection of hydrocarbons through indirect association, using methods such as:

- Microbiological analysis based on bacterial population distributions;
- Remote sensor method based on spectral behavior of soil or vegetation;
- Geobotanical method based on diversity and concentration of plant specimens.

The ideal geochemical prospecting method is one that combines at least two techniques. The use of such an approach greatly increases the diagnostic value of surface geochemistry prospecting.