- First indications date from 1933. The idea of microseepage has a close association of surface ‘geochemical’ anomalies with subsurface petroleum generation and entrapment is well known (Laubmeyer, 1933; Horvitz, 1939 and 1985; Rosaire, 1940; Jones and Drozd, 1983; Faber, and Stahl, 1984; Price, 1986; Klusman, 1993; Schumacher and Abrams, 1996; and Abrams and Schumacher, in press).
- In 1994 an AAPG Hedberg conference was held on the topic of “Hydrocarbon Migration and Its Near-Surface Expression”. At this conference industry and academic geoscientists investigated the topic. Results of this are documented in AAPG Memoir No. 66, Schumacher and Abrams, 1996.
- Macroseepage: large concentrations of migrated hydrocarbons, visible oil staining, and odour. These migrate laterally via porous, permeable damage zones and/or faults, injectates or vents and can be easily detected at the surface. Onshore these can be seen as mud volcanoes, tar pits, and seeps. Offshore these can be mud volcanoes, pockmarks, and visible slicks.
- Microseepage: chemically detectable but far lower the concentrations than macroseepage. Nonvisible hydrocarbons or odour and vertical migration micro-fractures or grain boundaries in the rock.
- Microseepage occurs, with or without the presence of faults, as small gas bubbles, colloids of molecules of nano-meters size, travel vertically upwards to the surface. This is caused by buoyancy, like bubbles in champagne. They make use of tiny micro-cracks in the subsurface, present in every type of geology.
- Faults do not influence microseepage. This is due to the fact that the scale of macro to microseepage is very different. Macoseepage is visible and travels at a far lower speed, whereas microseepage is barely detectable but is much faster (1 km vertical in 6 months).
- Macroseepage is usually visible at the surface near fault or fracture boundaries.
- In order to get the best results, macroseep areas should be avoided when sampling.
- Larger hydrocarbon accumulations macroseep-areas could skew the results but as with climate effects, these will be filtered out. This is because macroseep/climate is not consistently a differentiating factor in the labels of the training set which is used.
- Microseep gas bubbles can take a couple of weeks up to some years to reach the surface, depending on the depth of the reservoir. Gas particles move to the surface rather fast with speeds of a few meters per day. Due to buoyancy, these bubbles travel multiple orders of magnitude faster vertically than horizontally.
Methane-oxidizing bacteria were the first type of bacteria studied to pinpoint petroleum accumulations. Mogilewskii (1938) described the possibility of using methane-oxidizing bacteria for gas exploration. Methane-oxidizing bacteria are also found in petroleum prospecting operations of the Soviet Union (Kartsev et al., 1959). Later Whittenbury et al., (1970) reported the isolation of more than 100 strains of methane-oxidizing bacteria. In the ’90s Hanson and Wattenberg (1991) and Hanson and Hanson (1996) gave overviews of the ecology of methylotrophic bacteria and their role in the methane cycle. Finally, microbial anomalies are proven to be reliable indicators of oil and gas in the subsurface (Beghtel et al, 1987; Lopez et al., 1993; Tucker and Hitzman, 1994).
- The concentrations of gas from microseepage are so small that surface accumulations are not immediately measurable. These small concentrations are, however, even so lightly, influencing the microbial ecosystem. Some of these microbes die and some flourish causing a (relatively) more easy parameter to detect than the infinitesimal concentration of actual hydrocarbons. The sensitive signal caused by the help of/presence of microbes is discriminated and identified by machine learning algorithms that look for those few distinctive microbes (50-200) in over 300,000+.
- The evidence of the hydrocarbon biomarkers can change within 1 year as a result of changes in the reservoir conditions. This occurs when capillary pressure is lower than displacement pressure, e.g. after producing a large part of the original gas in place. The length of this period is also dependent on overburden thickness.
- Micro-organism reservoir identification is based on the presence of hydrocarbon microseeps.
- Seepage is conditional on the capillary pressure being higher than displacement pressure (Pc > Pd) and therefore microseepage can stop if the pressure of a field is too low or has reached/is below hydrostatic.
- Over time the method can also identify bypassed pay or if a field is not fully produced yet.
- In the Biodentify studies today the database sample set does not distinguish between different oil or gases, just that there are hydrocarbons present.
- There may be differentiating species that are sensitive to various hydrocarbon types. As the database is populated with more samples of different soils and organisms, we could, in theory, make a determination of different types.
- Surface geochemical characteristics can be potentially used to evaluate charge type (gas versus oil), fluid quality (API gravity), physical properties (elemental sulphur content), and presence of non-hydrocarbon reservoir gases (CO2).
- Microseep is driven by buoyancy, which is going up vertically, 100-1000 times faster compared to the convection-diffusion equation, hence the resulting vector is predominately vertical.
- A Darcy type equation results in the fact that the flow through a relatively low permeability layer (a seal) is nearly vertical and the flux in a high permeable layer is mainly horizontal (this can be simplified by the “Dupuit-Forchheimer flow” where all horizontal flux is projected in the high permeable layers and between those a vertical flux is exchanged depending on the pressure difference between the layers and the resistance of the low permeable layer).
- If this very small amount of flux through the seal reaches higher permeable layers in the upper burden, the very small amount of gas is going upwards by buoyancy as microbubbles.
- The velocity belonging to buoyancy is orders of magnitude higher than the velocity belonging to ‘normal’ flow (m/d vs. m/yr) and therefore this small amount of gas will be nearly vertical.
- There have been many studies and publications dating back to the 1930s.
- More recently in 1994, an AAPG Hedberg conference was held on “Hydrocarbon Migration and Its Near-Surface Expression“, gathered to discuss the concept.
- From that AAPG Memoir No. 66, Schumacher and Abrams, 1996, 33 publications and Consensus:
- “Hydrocarbon accumulations are dynamic; seals are imperfect.
- All petroleum basins have some type of near-surface hydrocarbon leakage.
- Surface expression of leakage is not always detectable by conventional means.
- Hydrocarbon seepage can be active or passive, and it can be visible (macroseepage) or only chemically detectable (microseepage).
- Seepage expression, whether active or passive, is a function of many factors other than the mere presence or absence of active hydrocarbon generation and migration.”
- ” Migration occurs mainly vertically, but it can also occur over long distances laterally.
- Hydrocarbons can move vertically through thousands of meters of strata without observable faults or fractures in a relatively short time (weeks to years).
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