Surficial geochemical interpretation guidelines

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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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The presence of hydrocarbon macroseeps or microseeps in the area of a geochemical survey is direct evidence that petroleum has been generated. Hydrocarbon seepage at the surface represents the end of a petroleum migration pathway. These hydrocarbons may represent hydrocarbon leakage from an accumulation or leakage along a carrier bed or other migration pathway. Anomalies defined by multiple samples from one or more survey lines may indicate the location of discrete structural or stratigraphic targets within the survey area.

Anomalies and vertical migration

If the basin or play is characterized by predominantly vertical migration, then the correlation of a strong geochemical anomaly at the surface with a possible trap at depth suggests that the trap is charged with hydrocarbons. Conversely, if the trap is not associated with a positive geochemical anomaly, we assume the trap is not charged with hydrocarbons.

Anomalies and lateral migration

If the structural or geologic setting of the area suggests that microseepage may be predominantly lateral or pathway selective, such as along dipping stratigraphic surfaces and unconformities, the interpretation will be more difficult since geochemical anomalies may then not be located vertically above a trap. Which of these migration scenarios is more likely in your area of investigation? What is the relationship of the anomalies to outcrop geology, mapped structural closures, stratigraphic pinch-outs, faults, or basement highs? Because relationships between surface geochemical anomalies and subsurface accumulations can be complex, proper interpretation requires integration of surface geochemical data with geologic, geophysical, and hydrologic data. The importance of such integration cannot be overstated.[1]

Hydrocarbon composition from macroseeps

Hydrocarbon seep composition can play an important role in evaluating the exploration potential of a basin, play, or prospect. Petroleum in most visible oil and gas seeps (i.e., macroseeps) generally has been altered by processes such as biodegradation, water washing, and evaporative loss of volatile components. Despite these changes, chemical and iso-topic analysis of such seeps can enable inferences about the nature of the source rock facies and maturity as well as permit correlation with known source rocks and reservoired petroleum.

Hydrocarbon composition from microseeps

Obtaining compositional information from the analysis of hydrocarbon microseeps is more difficult because microseeps generally consist of only light hydrocarbons (methane through pentane). Sometimes, however, the heavier gasoline-range and aromatic hydrocarbons are also present. One can infer the composition of the migrating petroleum from these light hydrocarbons from soil gas/hydrocarbon ratios, carbon isotopic composition of soil gases, fluorescence characteristics of soil or sediment extracts, and chromatographic analysis of such extracts. A detailed discussion of these methodologies is beyond the scope of this article, but published examples of such analyses and their interpretations include Abrams[2] Barwise and Hay[3] Belt and Rice[4] Brooks et al.[5] Horvitz[6] Jones and Drozd[7] Kornacki[8] Piggot and Abrams[9] Schiemer et al.[10] Stahl et al.[11] and Thrasher et al.[12]

See also

References

  1. ↑ Thrasher, J., A., Fleet, A., J., Hay, S., J., Hovland, M., Duppenbecker, S., 1996, Understanding geology as the key to using seepage in exploration: the spectrum of seepage styles, in Schumacher, D., Abrams, M., A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 223–241
  2. ↑ Abrams, M., A., 1996, Interpretation of methane carbon isotopes extracted from surficial marine sediments for detection of subsurface hydrocarbons, in Schumacher, D., Abrams, M., A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 309–318.
  3. ↑ Barwise, T., Hay, S., 1996, Predicting oil properties from core fluorescence, in Schumacher, D., Abrams, M., A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 363–371.
  4. ↑ Belt, J., Q., Rice, G., K., 1996, Offshore 3D seismic, geochemical data integration, Main Pass project, Gulf of Mexico: Oil & Gas Journal, vol. 94, no. 14, p. 76–81, and vol. 94, no. 15, p. 100–102.
  5. ↑ Brooks, J., M., Kennicutt, M., C., Carey, B., D., 1986, Offshore surface geochemical exploration: Oil & Gas Journal, October 20, p. 66–72.
  6. ↑ Horvitz, L., 1985, Geochemical exploration for petroleum: Science, vol. 229, p. 821–827., 10., 1126/science., 229., 4716., 821
  7. ↑ Jones, V., T., Drozd, R., J., 1983, Predictions of oil or gas potential by near-surface geochemistry: AAPG Bulletin, vol. 67, p. 932–952.
  8. ↑ Kornacki, A., S., 1996, Petroleum geology and geochemistry of Miocene source rocks and heavy petroleum samples from Huasna Basin, California, in Schumacher, D., Abrams, M. A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 413–430.
  9. ↑ Piggott, N., Abrams, M., A., 1996, Near-surface coring in the Beaufort and Chukchi Seas, northern Alaska, in Schumacher, D., Abrams, M., A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 385–399.
  10. ↑ Schiemer, E., J., Stober, G., Faber, E., 1985, Surface geochemical exploration for hydrocarbons in offshore areas—principles, methods and results, in Petroleum Geochemistry in Exploration of the Norwegian Shelf: London, Graham and Trotman, p. 223–238.
  11. ↑ Stahl, W., Faber, E., Carey, B., D., Kirksey, D., L., 1981, Near-surface evidence of migration of natural gas from deep reservoirs and source rocks: AAPG Bulletin, vol. 65, p. 1543–1550.
  12. ↑ Thrasher, J., A., Strait, D., Lugo, R., A., 1996a, Surface geochemistry as an exploration tool in the South Caribbean, in Schumacher, D., Abrams, M., A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 373–384.

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