Macroseepage vs. microseepage in surficial geochemistry

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Surface geochemical exploration for petroleum
Author Dietmar Schumacher
Link Web page
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Macroseeps

There is a seepage continuum from the smallest detectable levels to visible oil and gas seeps. The term macroseepage refers to visible oil and gas seeps. Macroseeps are very localized areas containing large concentrations of light hydrocarbons as well as, if available, high-molecular-weight hydrocarbons. They are localized at the termination of faults, fractures, and outcropping unconformities or carrier beds. These visible seeps have led to the discovery of many of the world's important oil and gas producing areas.[1][2]

Microseeps

Microseepage is defined as high concentrations of analytically detectable volatile or semivolatile hydrocarbons in soils, sediments, or waters. These invisible seeps are recognized only by the presence of anomalous concentrations of the following:

  • Light hydrocarbons (principally C1–C5)
  • Volatile or semivolatile high-molecular-weight hydrocarbons (such as 2–4 ring aromatics)
  • Hydrocarbon-oxidizing microbes
  • Hydrocarbon-induced alteration products

High-molecular-weight hydrocarbons may be present in ever-wet or intermittently wet environments; however, only volatile or semivolatile hydrocarbons are expected above the water table. Most surface geochemical methods, including both direct and indirect methods, were developed to detect microseepage.

Microseepage evidence

The existence of microseepage is supported by a large body of empirical evidence.[3][4][5][6] This includes the following:

  • Increased concentration of light hydrocarbons and hydrocarbon-oxidizing microbes in soils and sediments above hydrocarbon reservoirs.
  • Increased key light hydrocarbon ratios in soil gas over oil and gas reservoirs.
  • Sharp lateral changes in these concentrations and ratios at the edges of the surface projections of these reservoirs.
  • Similarity of stable carbon isotopic ratios for methane and other light hydrocarbons in soil gases to those found in underlying reservoirs.
  • The disappearance and reappearance of soil gas and microbial anomalies in response to reservoir depletion and repressuring.

Microseep migration

Research and field studies suggest that the dominant migration medium is as a continuous-phase, buoyancy-driven gas flow within carrier and reservoir rocks and capillary imbibition in the transition from sources and seals into carrier rocks. Hydrocarbon microseepage is predominantly vertical and is dynamic; migration rates range from less than 1 meter per day to tens of meters per day.[7][8]

See also

References

  1. ↑ Link, W. K., 1952, Significance of oil and gas seeps in world oil exploration: AAPG Bulletin, vol. 36, p. 1505–1541.
  2. ↑ Macgregor, D. S., 1993, Relationships between seepage, tectonics, and subsurface petroleum reserves: Marine and Petroleum Geology, vol. 10, p. 606–619., 10., 1016/0264-8172(93)90063-X
  3. ↑ Price, L. C., 1986, A critical overview and proposed working model of surface geochemical exploration, in Davidson, M. J., ed., Unconventional Methods in Exploration for Petroleum and Natural Gas IV: Dallas, Texas, Southern Methodist Univ. Press, p. 81–129.
  4. ↑ Klusman, R. W., 1993, Soil gas and related methods for natural resource exploration: New York, John Wiley & Sons, 483 pp.
  5. ↑ Klusman, R. W., and M. A. Saeed, 1996, Comparison of light hydrocarbon microseepage mechanisms, in D. Schumacher, and M. A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 157–168.
  6. ↑ Matthews, M. D., 1996a, Migration a view from the top, in D. Schumacher, and M. A. Abrams, eds., Hydrocarbon Migration and Its Near-Surface Expression:AAPG Memoir 66, p. 139–155.
  7. ↑ Arp, G. K., 1992, Effusive microseepage: a first approximation model for light hydrocarbon movement in the subsurface: Assoc. Petroleum Geochemical Explorationists Bulletin, vol. 8, p. 1–17.
  8. ↑ Abrams, M. A., 1992, Geophysical and geochemical evidence for subsurface hydrocarbon leakage in the Bering Sea, Alaska: Marine and Petroleum Geology Bulletin, vol. 9, p. 208–221., 10., 1016/0264-8172(92)90092-S

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