Selecting a surficial geochemistry survey method

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

How does one select a method(s) for a surface geochemical exploration program? The choice of method(s) depends on the kinds of questions you hope the survey results will answer.

• What are the objectives of the survey—to demonstrate the presence of an active petroleum system in a frontier area, to high-grade previously defined exploration leads and prospects, or to determine the type of petroleum (i.e., oil vs. gas) likely to be encountered?
• What other data are presently available for the area of interest (satellite imagery, aeromagnetics, gravity, seismic, etc.)?
• What geochemical methods have previously been used successfully in the area of interest or in a geologic analog area?
• What limitations are imposed by the survey area (onshore or offshore, deep water or shallow, jungle or desert, mature basin or remote area, budget and personnel constraints)?

Direct vs. indirect methods

As a generalization, direct hydrocarbon methods are preferred over indirect methods (see Surficial geochemistry and hydrocarbon detection methods) because they can provide evidence of the very hydrocarbons we hope to find in our traps and reservoirs. Additionally, chemical and isotopic analysis of these hydrocarbons, especially the high-molecular-weight hydrocarbons, can provide insight into the nature and maturity of the source rock that generated these hydrocarbons.

Offshore methods

The table below lists the principal geochemical methods used for offshore exploration.

Medium to be Sampled	Target to be Analyzed	Methods
Atmosphere	Hydrocarbons	Radar or laser
Water surface	Oil slicks or sheens	Satellite, airborne sensors (radar, multispectral imaging, hyperspectral imaging, laser, fluorescence), or direct sampling
Water	Dissolved hydrocarbons (low molecular weight (LMW), high molecular weight (HMW), or aromatics)	Marine sniffer, water analysis
Sea bottom	Hydrocarbon macro- or microseepage	High-resolution seismic data, side-scan sonar, direct sampling (gravity core, vibracore, piston core, jet core, etc.)
	Hydrocarbon-induced alteration	Topographic, acoustic, and temperature contrasts; sediment sampling for microbial or geochemical indicators

Onshore methods

The table below lists the principal geochemical methods used for onshore exploration.

Medium to be Sampled	Target to be Analyzed	Methods
Land surface	Oil and gas macroseeps, stains, impregnations	Geologic mapping; historical records; satellite and airborne sensors (multispectral, hyperspectral); direct sampling of seeps, stains
	Hydrocarbon microseeps	Soil or sediment sampling for hydrocarbon analysis
	Hydrocarbon-induced alteration	Soil or sediment sampling for indirect microbial or geochemical indicators, aeromagnetic, electrical, or radiometric
Soil air	[ http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=light%20hydrocarbons Light hydrocarbons]	Probe or adsorptive collectors
	Nonhydrocarbons	Probe or adsorptive collectors
Soil or sediment	Light hydrocarbons, aromatics	Sample disaggregation and/or acid extraction for chromatography, UV-fluorescence
	Nonhydrocarbons or diagenetic anomalies	Hydrocarbon-oxidizing microbes, soil salts (i.e., carbonates, chloride, iodine, sulfate, etc.); clay minerals; trace metals; magnetic susceptibility, aeromagnetics, ground magnetics; electrical (IP, CSAMT, resistivity, MT); radiometrics

Recommendations

Whenever possible, use more than one geochemical survey method, for example, combine a direct method with an indirect method. The use of multiple methods can reduce interpretation uncertainty because seepage-related anomalies tend to be reinforced while random highs and lows tend to cancel each other. If surface conditions or budgetary constraints preclude the use of direct hydrocarbon detection methods, the next best choice is the indirect method most closely linked to hydrocarbons and hydrocarbon accumulations (microbial, helium, and perhaps certain magnetic and radiometric methods).

See also

• Surficial geochemistry and hydrocarbon detection methods
• Surficial geochemical survey objectives
• Designing a surficial geochemical survey
• Surficial geochemical interpretation guidelines

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