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Surficial geochemical interpretation guidelines (view source)
Revision as of 18:28, 18 January 2014
, 18:28, 18 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Surface geochemical exploration for petroleum
| frompg = 18-1
| topg = 18-27
| author = Dietmar Schumacher
| link = http://archives.datapages.com/data/specpubs/beaumont/ch18/ch18.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
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.<ref name=ch18r46>Thrasher, J., A., Fleet, A., J., Hay, S., J., Hovland, M., Duppenbecker, S., 1996b, 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</ref>
==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<ref name=ch18r3>Abrams, M., A., 1996b, 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.</ref> Barwise and Hay<ref name=ch18r6>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.</ref> Belt and Rice<ref name=ch18r8>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.</ref> Brooks et al.<ref name=ch18r9>Brooks, J., M., Kennicutt, M., C., Carey, B., D., 1986, Offshore surface geochemical exploration: Oil & Gas Journal, October 20, p. 66–72.</ref> Horvitz<ref name=ch18r16>Horvitz, L., 1985, Geochemical exploration for petroleum: Science, vol. 229, p. 821–827., 10., 1126/science., 229., 4716., 821</ref> Jones and Drozd<ref name=ch18r17>Jones, V., T., Drozd, R., J., 1983, Predictions of oil or gas potential by near-surface geochemistry: AAPG Bulletin, vol. 67, p. 932–952.</ref> Kornacki<ref name=ch18r20>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.</ref> Piggot and Abrams<ref name=ch18r28>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.</ref> Schiemer et al.<ref name=ch18r38>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.</ref> Stahl et al.<ref name=ch18r42>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.</ref> and Thrasher et al..<ref name=ch18r45>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.</ref>
==See also==
* [[Designing surface geochemical surveys]]
* [[Hydrocarbon detection methods]]
* [[Geochemical survey objectives]]
* [[Selecting a survey method]]
* [[Designing a geochemical survey]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch18/ch18.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Surface geochemical exploration for petroleum]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Surface geochemical exploration for petroleum
| frompg = 18-1
| topg = 18-27
| author = Dietmar Schumacher
| link = http://archives.datapages.com/data/specpubs/beaumont/ch18/ch18.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
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.<ref name=ch18r46>Thrasher, J., A., Fleet, A., J., Hay, S., J., Hovland, M., Duppenbecker, S., 1996b, 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</ref>
==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<ref name=ch18r3>Abrams, M., A., 1996b, 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.</ref> Barwise and Hay<ref name=ch18r6>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.</ref> Belt and Rice<ref name=ch18r8>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.</ref> Brooks et al.<ref name=ch18r9>Brooks, J., M., Kennicutt, M., C., Carey, B., D., 1986, Offshore surface geochemical exploration: Oil & Gas Journal, October 20, p. 66–72.</ref> Horvitz<ref name=ch18r16>Horvitz, L., 1985, Geochemical exploration for petroleum: Science, vol. 229, p. 821–827., 10., 1126/science., 229., 4716., 821</ref> Jones and Drozd<ref name=ch18r17>Jones, V., T., Drozd, R., J., 1983, Predictions of oil or gas potential by near-surface geochemistry: AAPG Bulletin, vol. 67, p. 932–952.</ref> Kornacki<ref name=ch18r20>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.</ref> Piggot and Abrams<ref name=ch18r28>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.</ref> Schiemer et al.<ref name=ch18r38>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.</ref> Stahl et al.<ref name=ch18r42>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.</ref> and Thrasher et al..<ref name=ch18r45>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.</ref>
==See also==
* [[Designing surface geochemical surveys]]
* [[Hydrocarbon detection methods]]
* [[Geochemical survey objectives]]
* [[Selecting a survey method]]
* [[Designing a geochemical survey]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch18/ch18.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Surface geochemical exploration for petroleum]]