GUIDELINES FOR SURFACE GEOCHEMICAL SURVEYING

David Boleneus
Registered Professional Geologist
Spokane, Wash.

This article classifies the various methods employed and provides guidelines useful in design of orientation, reconnaissance, and detail surface geochemical surveys for oil and gas exploration.

It is based on a user manual for surface geochemistry in petroleum exploration the author assembled in 1989.

Surface geochemistry is perhaps better termed surface prospecting because it employs both geochemical and geophysical techniques to detect petroleum, its constituents, or products subsequently caused by reactions near the earth's surface. It should not be confused with geochemistry of petroleum source rocks.

INTRODUCTION

Surface geochemistry provides a tool to map the surface expression of oil or gas accumulations, regardless of the type of trap or type of hydrocarbon present.

Surface geochemistry generally cannot discriminate between shows or marginal or economic quantities of petroleum. However, surface geochemistry represents a powerful petroleum indicator technique.

More than 20 different surface geochemical methods are known by which surface expressions can be measured. Further complicating their understanding, the protocols of each of these methods are perhaps equally variable.

Surface geochemistry is becoming more widely used because of its low cost, ease of application, and improvements in technology in the last 10 years. Individuals and companies often prefer to locate positive indications of petroleum on their land. An American Association of Petroleum Geologists-affiliated society has published an extensive bibliography on the subject, and relevant studies on drilling successes of geochemical prospects are now available.2 3

Use of surface geochemistry is premised upon no reservoir being ideally sealed, leading one to deduce that imperfect seals are always present. The conclusion could be drawn that petroleum reservoirs leak (seepage). Then, from the time of field origin, the soils overlying a field represent natural, long-term, concentrating media of petroleum. This concentration often exists in parts per million although still within the detection limit of the new generation laboratory instruments. Surface geochemistry is successful because empirical study about microseepage shows that hydrocarbons are displayed in many forms directly (vertically) above oil and gas fields.4

Surface geochemistry should be used only if it can reduce the risk or finding costs of oil and gas. Surface geochemistry is a tool, like seismic, gravity, basement magnetics, geologic mapping, and drilling and logging technologies.

Beyond data assembled about the six attributes that explorers commonly use to describe petroleum occurrences - source rock, reservoir rock, structure, seal, thermal history, migration, and entrapment - surface geochemistry contributes two additional attributes.

Table 1 represents the relative contributions by geology, seismic, source rock studies, and surface geochemistry to describe the petroleum environment. It further represents that only surface geochemistry reveals important information about hydrocarbon charge, hydrocarbon type, and seepage of hydrocarbons to the surface.

Surveys involve the following stages:

objective-setting and planning (design),
orientation (information gathering about the sampling environment) and choice of methods,
survey sampling (field, laboratory) and follow-up,
data presentation and analysis, and
interpretation.

Total time involves about one to four months. Companies and individuals who begin by selecting contractors solely on cost criteria may unknowingly forgo participating in the first two stages and sacrifice flexibility during the process.

CHOICE OF METHODS

Selecting surface geochemistry methods normally occurs after objective-setting.

Table 2 lists many direct and indirect techniques used in the past. For each geochemical (or geophysical) survey method used, various media are available for sampling as shown (second column). The third column shows the geochemical target, a petroleum-induced signature suspected to exist near the surface.

Techniques are termed direct if they identify one or more component hydrocarbon gases or liquids present in the media sampled. Direct methods employ techniques that suggest the nature of aromatic, naphthenic, or paraffinic compounds in the soils, and employ variations of gas chromatography, fluorescence spectroscopy, and mass spectrometry.

Sample media available include gases or liquids in the following: air, soil or subsoils, ground and surface water, drainage sediments, and lake and ocean sediments. Hydrocarbons detected by direct survey methods may include methane, ethane, and heavier alkanes, paraffins, alkenes, and multiple-ring aromatic compounds that include bitumens, and sulfur compounds.

Bitumens are high molecular weight components of oils, Their aromatic components fluoresce when exposed to ultraviolet light. Fluorescent compounds are detected in small quantities from soils and drainage sediments. Aromatic compounds can be used to "fingerprint" oil. Specific fluorescence spectra are matched with those data known to characterize oils produced nearby to establish oil "fingerprints." The technique is applicable for identifying multiple pays or multiple oil sources. In the opposite sense, fluorescence can suggest environmental contamination such as agricultural pesticides.

Techniques are termed indirect hydrocarbon sensing techniques if they are designed to sense a secondary product caused by the upward movement of hydrocarbons. Indirect techniques cannot sense hydrocarbons directly. As an example, radiometric "low" anomalies may occur where carbon dioxide has contributed to calcite recementation in the soil zone. 5 Induced polarization methods5 can detect resistivity and chargeability contrasts caused by abundant calcite and/or pyrite in the subsoil area. Magnetic surveys can detect magnetite-like mineralization thought to be related to leaking oil field brines. The microbiological indicators of oil and gas might be termed quasi-direct sensing methods.

Alkali-earth elements, potassium, calcium, manganese, and magnesium, detected by assays can be deposited in some soils and leached from others by unusual geochemical environments. These environments are suspected to exist above and laterally-adjacent to the surface projection of oil pools. Arguably, situations exist where some indirect indicators could act as quasi-direct indicators. Trace elements and associated gases are examples.

Besides being direct indicators in some settings, hydrogen and helium also act as structural indicators.7 Because of their small molecular size, workers speculate these elements more easily escape through faults and fractures than paraffinic and aromatic hydrocarbons, suggesting that a compound's relative geochemical mobility should be considered. Helium's ultimate source is from radioactivity decay of earth radionuclides, particularly uranium. Noble gases, helium, neon, and radon act as structural indicators because of their nonreactivity.

Table 3 compares several groups of geochemical methods used for recent onshore surveys against pertinent characteristics of the method. At least nine methods represent direct hydrocarbon indicators. Three others provide a semblance of the "fingerprinting" ability described. Thirteen techniques that display integrative advantages include seven direct and six indirect techniques. Because of their low cost per unit of area, several indirect methods are used for wide-area or reconnaissance surveys. Many methods perform well in both reconnaissance surveys (wide-area) or detail surveys (small area).

An important distinction for direct methods - instantaneous or integrating - is used here to classify methods. By design, integrating methods collect data over longer periods, or remove any short-term fluctuations. In the opposite sense, free soil gases as helium and hydrocarbons record significant fluctuations over the short periods. Techniques that purposely detect short term fluctuations are termed instantaneous techniques.

Do not overlook the advantage of large sampling volume. A large volume is better than a small volume when sampling earth phenomena. Magnetic, resistivity, and induced polarization geophysical surveys are good examples of geophysical techniques that measure potential-field responses for large volumes of rock to varying depths.

PROJECT DESIGN

The design of an effective program means making decisions about:

sample material,
sampling pattern (spacing, sample media, depth),
sample preparation,
analytical procedure, and
criteria for interpretation.8

To make these decisions requires some assumptions about the following factors:

Dispersion, or mobility characteristics of hydrocarbon compounds,
Local environmental constraints that influence dispersion processes,
Target size and its effect on sample density and sample spacing,
Availability of sample material,
Analytical capability and logistical considerations.

For example, target size can influence the program cost. The larger the target, the fewer samples are needed to detect it. As a rule of thumb, an anomaly cannot be confirmed with fewer than six samples occurring within the anomaly limits. Conversely, to establish higher confidence in the results, at least 75% of the samples should be located outside the anomaly to define what constitutes the local and regional anomaly threshold limits.

Evaluate logistical constraints. In high cost areas, it may be an advantage to "double or triple" sample but analyze only one third of the samples. Other samples can be retained for backup purposes only for anomaly follow-up, if necessary. In this example, a fill-in survey has been possible without remobilizing to the field.

ORIENTATION SURVEYING

Orientation studies consist of a series of preliminary experiments to decide optimum survey procedures and are completed in one of four ways:9

by traveling to the field area to systematically collect samples and pertinent information;
by performing a literature search;
by performing a theoretical orientation;
by performing no orientation (should be avoided).

The field orientation survey.

A classic orientation survey comprises field sampling and analysis around a known field. A scenario consists of preliminary field sampling and observation aimed at determining the existence and characteristics of hydrocarbon anomalies. This information allows selecting adequate prospecting techniques and for deciding the factors/criteria for interpreting geochemical data. Evaluate those factors to determine several factors:

contrast between anomaly and background,
the most suitable survey method(s),
optimum depth of sampling,
shape and extent of an anomaly,
reproducibility of results,
possibility of contamination, and
sampling schemes, sampling media, and between-sample spacing.

Decisions about orientation surveys should be made by the company and not a contractor or laboratory. To complete or omit the orientation procedures is a first critical milestone for application of geochemical surveys.

The orientation survey is generally simple. It consists of one or two traverses and one or more vertical soil profiles. The work is conducted over a known oil field that is geologically similar to the proposed survey area (or possibly connected to it) and then continued outside the known productive area into background. Traverse lengths may be 2-10 miles. The spacing must be 1/8 to 1/10 mile between samples, or closer in the absence of data to indicate width of production. Fully-developed recent discoveries are good examples for orientation study. New, shut-in discoveries are best.

The taking of shallow, deep (12 ft), and shot-hole samples is ideal; sampling both on-field and off-field is critical. The use of multiple techniques during the orientation is imperative. Expenditures ranging from $2,000-40,000 and the taking of up to 800 samples in large orientation surveys are reasonable. As a rule, over-surveying during orientation study ensures sufficient data to organize reconnaissance and detail surveys properly. Often the orientation survey area can be made a part of the larger survey anticipated.

The literature (orientation) study.

If it is impractical to visit the field area to conduct an orientation survey ahead of the main survey, significant information may be obtained by review of previous work, communications with contractors, and professionals familiar with this or similar areas.

Theoretical orientation.

This highly speculative approach is based on application of theoretical models, some basic principles of exploration geochemistry, and assumptions as to the surficial geology/ geomorphology and climate in the survey area. This approach is not recommended.

RECONNAISSANCE AND DETAIL SURVEYING

Reconnaissance and detail refer to the study area size and the nature of the objectives set. Some practitioners refer to four categories: frontier, reconnaissance, detail, and development.

The design stage involves setting of objectives, tentative choices of field and laboratory techniques anticipated, and requires customer input on the following:

type and areal dimensions of largest and smallest trap (field) anticipated,
number of oil or gas objectives expected,
their depths,
known fields in area if any,
general geology,
extent of area under study - large (basin size), medium (several townships, a county, a fairway or trend), small (one to a few square miles),
possible sources of contamination,
input from orientation study,
objective(s) sought, and
level of detail and confidence limits demanded from results.

Reconnaissance surveys.

Size (or area) and level of detail are considerations for production surveying. Reduce very large basin-sized or fairway-sized areas to manageable units by reconnaissance surveying. Therefore, the use of indirect methods can exceed the use of direct methods.

The purpose of reconnaissance work is to search a relatively large area to call attention to local interest areas where eventual follow-up (detail) work will occur. The spacing and types of samples are chosen to detect, but not necessarily outline the favorable provinces or fairways, etc., at a low cost compared to detail surveys.

Systematic sampling across unknown ground should accomplish two objectives:

eliminate unprospective ground, and
identify prospective areas or targets sought.

In reconnaissance surveys, the first objective is most important and the other is of secondary importance; in detail surveys, the opposite applies.

Specifically, objectives for most reconnaissance surveys correspond to one of the objectives set out in Fig. 1 (A, 1-3). Reasons for surveying are numerous and reconnaissance surveys may also be used to accomplish one or more of the several additional goals listed below:

To determine if unexplored basins (regions) are petroliferous in character and subsequently rank areas by this data;
To define fairways and rank prospective areas before seismic surveys;
To eliminate areas of low or no potential before land acquisition;
To identify areas for further follow-up by detail surveys; and
To evaluate prospective geochemical characteristics along seismic lines.

Detail surveys.

The purpose of detail surveying is to identify or confirm drillable objectives.

The confidence in drawing conclusions ties at a higher level than those of reconnaissance surveys. Between sample spacing is usually less for detail work than reconnaissance work and, if unknown, can be decided using the semi-variogram calculation10 from previous work. Background, local, and regional anomaly thresholds (geochemical anomaly contrast) must be maximized and readily identifiable. Anomaly limits must be seen.

Results must distinguish between false and valid anomalies at low levels of contrast. Sinclair's probability graphs11 may be an aid over simple statistics to establish these limits. Objectives of most detailed surveys are outlined in Fig. 1 (B. 4 7).

REFERENCES

Groth, P.K., and Groth, L.W., Bibliography for surface and near-surface hydrocarbon prospecting methods, APGE, 2nd edition, Denver, Colo., 1994.
Northeast Research Institute Inc., The Petrex fingerprint technique, Sales and informational brochure, NERI, Denver, Colo., 1992, 18 P.
Potter, R.W., Harrington, P.A., Silliman, A.H., and Viellenave, J.H., The significance of geochemical anomalies to hydrocarbon exploration, abs. AAPG Hedberg Research Conference, Apr. 24-28, 1994, Vancouver, B.C.
Klusman, R. W., Soil gas and related methods for natural resource exploration, John Wiley & Sons, 1993, 483 p.
Hansen, D.A., Radiometrics, in Practical Geophysics for the Exploration Geologist, R. Van Blaricom, ed., Northwest Mining Assn., Spokane, Wash., 1980, pp. 2-38.
Oehler, D.Z., and Sternberg, B.K., Seepage-induced anomalies, "false" anomalies, and implications for electrical prospecting, AAPG Bull., Vol. 68, No. 9, 1984, pp. 1,121 45.
Jones, V.T., and Drozd, R.J., Predictions of oil and gas potential by near-surface geochemistry, AAPG Bull., Vol. 7, No. 6, 1983, pp. 932-952.
Rose, A.W., Hawkes, H.E., and Webb, J.S., Geochemistry in mineral exploration, Academic Press, 2nd edition, London, 1979, 657 p.
Thomson, I., Getting it right, in Exploration geochemistry, design and interpretation of soil surveys, J.M. Robertson (ed.), Vol. 3, Reviews in Economic Geology, Economic Geology Publishing Co., El Paso, Tex., 1987.
Clark, I., Practical geostatistics: Applied Science Publishers, London, 1980, 129 P.
Sinclair, A.J., Probability graphs, Assn. of Exploration Geochemists, Rexdale, Ont., 1976, 76 P.