Surficial geochemical exploration principles
| 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 |
| Store | AAPG Store |
Surface indications of oil and gas seepage have been noted for thousands of years, and such seeps have led to the discovery of many important petroleum producing areas. Over the past sixty years, numerous geochemical and nonseismic geophysical surface exploration methods have been developed. The application of these geochemical prospecting methods to oil and gas exploration has resulted in varied success and occasional controversy. Few question the fact that hydrocarbons migrate to the surface in detectable amounts, but many remain uncertain of how such information can best be integrated into conventional exploration and development programs.
Definition
Geochemical exploration for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons and their alteration products, which serve as clues to the location of undiscovered oil and gas accumulations.
Surface geochemical principles
The past decade has seen a renewed interest in surface geochemical exploration. Coupled with developments in analytical and interpretive methods, this interest has produced a new body of data and insights, many of which are summarized in AAPG Memoir 66.[1] There is now consensus with the following points:
- All petroleum basins exhibit some type of near-surface hydrocarbon leakage.
- Petroleum accumulations are dynamic and their seals are imperfect.
- Hydrocarbon seepage can be active or passive and is visible (macroseepage) or only detectable analytically (microseepage).
- Hydrocarbons move vertically through thousands of meters of strata without observable faults or fractures in a relatively short time (weeks to years).
- Migration is mainly vertical but can also occur over great distances laterally.
- Relationships between surface anomalies and subsurface accumulations range from simple to very complex.
Significance of anomalies
Surface indications of oil and gas seepage have been noted for thousands of years; such seeps have led to the discovery of many important petroleum producing areas. Although the discovery of a surface geochemical anomaly does not guarantee the discovery of commercially significant petroleum, it does establish the presence of hydrocarbons in the area of interest. Hydrocarbon seeps at the surface represent the end of the migration pathway. Traps and structures along such pathways should be considered significantly more prospective than those not associated with such anomalies.
Benefits
The potential benefits of a successful geochemical exploration program are many and include the following:
- Directly detect hydrocarbons and/or hydrocarbon-induced changes in soils, near-surface sediments, and/or on the sea floor.
- Document the presence of a working petroleum system in the area of interest.
- Permit high-grading of basins, plays, or prospects prior to acquiring leases or before conducting detailed seismic surveys.
- Permit postseismic high-grading of leads and prospects; generate geochemical leads for further geological or geophysical evaluation.
- Use geochemical data to infill between seismic lines and constrain mapping of AVO/amplitude anomalies between lines.
- Evaluate areas where seismic surveys are impractical or are ineffective due to geological or environmental factors.
- Provide methods applicable to both stratigraphic traps and structural traps, with the ability to locate traps invisible or poorly imaged with seismic data.
- Have little or no negative environmental impact (most surface geochemical methods).
See also
- Assumptions of surface geochemical exploration
- Limitations and uncertainties of surficial gechemistry
- Seepage activity and surficial geochemistry
- Macroseepage vs. microseepage in surficial geochemistry
- Geochemical surface expression
- Surficial geochemical case histories
References
- ↑ Schumacher, D., 1996, Hydrocarbon-induced alteration of soils and sediments, in Schumacher, D., Abrams, M., A., eds., Hydrocarbon Migration and Its Near-Surface Expression: AAPG Memoir 66, p. 71–89.
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