Misuse of Surface Geochemical Tools
In the past, surface geochemical methods have been treated as tools to use experimentally, or out of curiosity, and are often discarded as a legitimate oil finding tool after the first dry hole is drilled. This results m the general distrust of all surface geochemical methods. What contributes to this distrust? It can usually be traced to:

• inadequate sampling for the method(s) used
• univariate surveys
• misinterpretation of faults
• poor analytical results.

Inadequate sampling
Always make sure the final data set actually covers the aerial extent of the prospect. The prospect should be thoroughly sampled after the reconnaissance phase to delineate any anomalies detected. A number of previously surveyed locations should be resampled if more than one survey is to be used in the interpretation.

Univariate surveys
Not only are the number of samples taken important but the number of methods used for reconnaissance and detailed surveys is as equally important. As the amount of data available for interpretation increases the greater the reliability of the anomaly detected. Data stacking is essential in validating the existence of a geochemical anomaly.

The simplest form of presentation is the plotting of data on a map. If sufficient data are available then the values can be contoured showing relative highs and lows. Single variable indirect methods are commonly presented in this way. Bubble or dot maps are also very useful in determining anomalous points, allowing a visual interpretation without the data interpolation associated with machine contouring.

Multivariate data sets can also be presented in map view using contours or symbols. However, comparison of the different variables using data of different levels of magnitude is sometimes difficult. Data normalization can resolve these differences by setting the mean to a constant for all the variables. The resulting values provide a method of determining how anomalous one variable is relative to another.

Misinterpretation of faults
Faults and fractures act as conduits for mobile fluids and hydrocarbons migrating to the surface. Often the surface signature is linear but discontinuous. These narrow trends may be delineated using a very close sample spacing of 330 feet or less.

Poor analytical results
The interpretation of a geochemical survey hinges on the quality of the data collected. For instance, when using gas chromatography for hydrocarbon surveys, are the saturated hydrocarbons separated from the unsaturated hydrocarbons? Is iso-butane separated from normal butane? Is there adequate separation between the increasing number of compounds represented in the heavier hydrocarbons? Poor separation can yield ambiguous information regarding hydrocarbon composition, making it difficult to discern the validity of possible anomalies.

With adequate sampling, multivariate surveys can yield anomalies that delineate hydrocarbon seepage. The correct analytical method and proper interpretation will also help determine the validity of seepage anomalies.