Bacteria are ubiquitous in the environment but will concentrate where a food source is available. Hydrocarbon microseepage from oil and gas reservoirs provide this food source. Microbial activity provides a catalyst for the various "redox" chemical reactions that occur within a live seepage area. Oxidation of the hydrocarbons provides an electron rich environment for these reactions.
The relationship between soil bacteria and fungus is very specific and related to fungal exudates that either promote microbial growth or are toxic to them. Carbohydrates, acetates, and formates are some of the exudates produced. Hydrocarbon seepage and alteration byproducts will affect the diversityof the microbial taxa by influencing the exudates, or substrate, provided by the fungi to the bacteria. Whether the geochemical signature is apical or halo for a Soil Biota Assay with be dependent on this relationship.
Bacterial are available that use specific substrates such as ethane, propane, or butane. US Geochemical uses a method that is non-specifice and primarily looks at aerobic bacteria that live in very near surface soils which can oxidize any number of organic substrates. The culturing is fairly rapid (72 to 96 hours) and results in a color change that is equivalent to the microbial concentration.
Fungal methods react in a similar way. Cultures appear with 96 hours and are digitally rendered for image analysis. Shades of gray are analogous to concentration and plotted for spatial comparison.
Bacteria, despite being extremely small and inconspicuous, are the most numerous organisms on this planet, with typically more than 100 billion bacteria per teaspoon of agricultural soil. That's fairly well known, but a strong case can also be made that bacteria represent the dominant life form on Earth.
Multivariate traverse showing background through production. There is a noticeable increase in concentration relative to the productive well. <click the image for more detail>