Microbiological Indicators of Subsurface Hydrocarbon Accumulations

Abstract

The measurement of toxin tolerance in the microbial communities present in stream sediments has been demonstrated to be an effective exploration tool. The technique relies on the fact that metals of, or associated with, ore deposits often represent toxins to microbial communities. The microbes adapt to the presence of these toxins by genetic mutation of replicon structures called plasmids that then carry the coding that allows survival in their presence. The level of tolerance developed is proportional to the concentration of toxin present and shows measurable, statistically meaningful variance at levels normally below the detection thresholds of standard inorganic techniques. Hydrocarbons reaching the surface by macro- and microseepage, the metals adsorbed to them, and the geochemical changes they bring about in the near-surface environment also represent toxins or stresses to microbial communities that they must adapt to. Adaptation is again accomplished by genetic mutation and is measured by collecting a sediment sample, separating the microbial population from it, and exposing splits of the sample to several discrete concentrations of the hydrocarbon or metal being used as a seepage indicator. Tolerance to higher levels of toxin is interpreted to be indicative of the strength or volume of hydrocarbon seepage. The authorsmore » have undertaken this work using marine samples from the Gulf of Mexico because that setting has fewer surface geochemical variables than subaerial settings. Both gas and oil accumulations can be measured by this technique, and evidence for both macro- and microseepage has been found. Examples from the Mississippi Canyon area will be used to illustrate the efficacy of the technique.« less