H.D. Hedberg
March , 1981

Geochemical prospecting for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons as clues to the location of oil or gas accumulations. It extends through a range from clearly visible oil and gas seepages at one extreme to the identification of minute traces of hydrocarbons determinable only by highly sophisticated analytical methods at the other. There is no question in principle about the value of the method for petroleum exploration if properly applied. Historically, most of the world's major petroleum bearing areas and many of its largest oil and gas fields had attention first called to them because of visible oil and gas seepages. The mere presence of higher hydrocarbons in a region is encouraging in that it is usually proof that conditions in that region have been suitable for at least some petroleum generation. Often seepages are in close proximity to commercial oil and gas pools, but the absence of seepages does not at all negate prospects because it may only indicate that there has been little escape from such pools due to good sealing rocks.

Oil and gas are mobile fluids and rocks are more or less permeable; and surface oil and gas seeps primarily reflect avenues of migration (or escape) from deeper and sometimes laterally distant locations. Moreover, because subsurface oil and gas accumulations vary considerably in the degree to which they are sealed, the quantitative size of a seepage has little relation to the size of the accumulation.

Some small accumulations are marked by, very strong visible seepages, whereas some of the largest accumulations are so well sealed as to show no visible seepages and only microscopic seepages or none at all. The value of seepages (visible or microscopic, is thus-largely a matter of the accuracy with which they can be interpreted geologically. In some cases (e.g. Burgan Field) a well drilled vertically at the site of seepage would have discovered the field; in other cases where escape of hydrocarbons has been along low dipping fault planes or low dipping carrier beds, surface seepages may be many miles laterally from vertical superposition over the oil or gas accumulation. Again, the value of the information on the seepage, visual or microscopic, is always there, but it is only the geological interpretation that allows cashing in on this value.

On land, most visible seepages have already been recorded and the nature of their relationship to subsurface petroleum accumulation has been at least studied if not always successfully determined. The main task now for geochemical prospecting is the identification of the invisible or less clearly manifested "seepages" which can be determined only by detailed chemical analysis of fluids in surface and near-surface rocks.

The problems are not whether there is any value to the data, but rather, are (1) the techniques of identification, (2) the geological interpretation, and (3) the question of whether the interpretation can be good enough and useful enough to justify the cost.

Offshore, the situation is somewhat different. There, visual observation of seepages has been impeded by the water cover and reliance must be placed very largely on chemical analysis of the water column and the interstitial waters filling the pores of the blanket of young sediment covering much of the sea floor. Again, there seems to me no question of the innate value of the geochemical information, positive or negative. Again, the problems are in techniques of identification, and in geological interpretation, and in whether the geological interpretation can be good enough and useful enough to justify its cost. There is nothing wrong with the concept; it is only a question of our ability to adequately collect the data and correctly interpret the results, at a reasonable cost.

A geochemical survey should be thought of not as a black magic means of spotting the location of oil and gas pools but only as a simple common-sense method of gathering data on hydrocarbon occurrences too dilute to make visible seeps or impregnations -- data which if collected reliably, interpreted wisely, and used intelligently along with all other lines of evidence will always be helpful in the petroleum exploration of any area.