I knew from early in my time at BP that the previous generation of explorers had used surface seeps to locate oil reservoirs in the Middle East – find a surface seep in the middle of a map-able anticline…….. Then when I was involved in BP's Gulf of Mexico exploration, I understood that you could see seeps from space on satellite imagery, you could get out there and sample the sea surface, you could send down an ROV to sample the chemo-synthetic creatures that lived on oil seeps coming out of the sea bed, you could see the pathways of these seeps on seismic data.
All these are macro-seeps
, detectable in visual amounts, flowing up discontinuities, typically faults, to the surface, argued by some to be offset from their origins in a reservoir. There's always a debate about whether seeing them is more positive than not seeing them, and doubt as to whether any of the petroleum has ever been in a reservoir. So interesting but not as conclusive as you might think.
And then there is micro-seepage
, detectable in analytic amounts, flowing vertically from a source rock or reservoir (but see paragraph above regarding doubts), favoured mechanism being micro-buoyancy (transport in buoyant micro-bubbles).
What happens next can be summarised as:
Hydrocarbons, chiefly methane through pentane, migrate upwards from source rocks and reservoirs to the surface.
When upward-migrating light hydrocarbons reach near-surface oxidizing conditions, aerobic hydrocarbon-oxidizing bacteria consume methane (and other light hydrocarbons) and decrease oxygen in pore waters.
With this development of anaerobic conditions, the activity of sulphate-reducing bacteria results in sulphate ion reduction and oxidation of organic carbon to produce reduced sulphur species and bicarbonate ion.
Highly reactive reduced sulphur species can then combine with iron to form iron sulphides and oxides. Iron sulphide can be in the form of pyrite, marcasite, magnetite, pyrrhotite, greigite, or maghemite.
As a result of bacterial sulphate reduction, sulphate ion concentration is decreased. In addition, bicarbonate is added to pore waters, raising pH and thus promoting precipitation of isotopically light, pore-filling carbonate cements.
If collecting soil samples for analysis, one should be able to demonstrate the presence of hydrocarbons and also find impacted bacteria.
Also, either from the ground or the air or space, there are several possible observations:
Changes in rock colours, for example, whitening of red beds.
Geophysical measurements, detecting ‘unusual' minerals: Magnetic, Electrical, Gamma Ray
Impact on vegetation: Reflectivity, ‘Health/Stress'.....[Links to agricultural expertise and experience]
Many such observations have been well documented; there is a fascinating example based on the DNA of microbes here
and the sort of ‘heat map' shown in this article is a very desirable output of micro-seepage observations.
In my mind though, it is an open question as to whether such a ‘heat map' shows a “sweet spot” in a shale or the presence of a conventional reservoir.