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How can Microbial Geochemical Exploration (MGCE) differentiate shallow gas pockets?

By 1) compositional analysis (usually only dry methane for shallow gas) and by 2) by integrating the geometry, areal extent and lateral variation of the measured microbial  anomalies with the subsurface seismic and geology. The apical distribution of microbial anomalies and background will approximate the shape and extent of the HC accumulation(s) near-vertical below surface. Since shallow gas pockets or hydrates usually have much different and larger geometries than deeper dip/fault dependent closures or stratigraphic traps with specific shapes, they can quite easily be recognized, even when stacked above deeper targets. Most shallow gas pockets can also be identified on seismic by their bright amplitude responses.   If from biogenic origin, than we can distinguish from thermogenic using isotope analysis. The latter can also identify non-HC gases such as CO2, which even the best 3D seismic or CSEM cannot.

Is there a concept behind the microbes preference for e.g. butane (C4) instead of methane (C1), so it can be used to differentiate migrated thermogenic HC accumulations at reservoir level instead of shallow gas pockets (biogenic) or coal bed methane in the overburden strata?

Specific obligate microbes and their enzymens 'eat' and breakdown specific HC molecules e.g. C2 or C4 only. There is a direct, positive relationship between microbial anomalous activity and dynamic microseepage (ref. AAPG Memoir 66. 1994).

Only when there is a scarcity in 'food' due to ineffective or declining  microseepage (due to e.g. reservoir depletion), then certain microbes may starts to 'eat' 2 different types of molecules.  If abundant dry CH4 biogas, then there will only be certain microbes living on C1.  And biogas can be distinguished from thermogenic by isotope analysis. CBM gas from shallow (antricite) coal seams will only be released at small background levels unless underground mined (escaping mine gas), or after hydraulic well fracing. The latter logical response however has not been field tested by a MGCE survey.

Can your lab analysis distinguish biogenic from thermogenic gas?

Yes definitely, by isotopic analysis, which is optional and on client’s request.

How can MGCE differentiate biodegraded Hydrocarbons?

Biodegradation into tar at shallow levels is typical  for large amounts of heavier HC's leaking to or near surface due to accumulative macroseepage, like in the Orinoco Belt of Venezuela or in Canadian Tar Sands. Trace HC's leaking by dynamic microseepage are however broken down by the microbes into alcohols, aldehydes, organic acids and eventually greenhouse CO2 - see graph below.


Can the phenomena above create anomalous microbial accumulations (and thus be misleading)?

Not to our knowledge, experience or from literature/publications.