Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Anomalous amounts of hydrocarbon gases in soil pores have long been used in search of commercial accumulations of petroleum by surface geochemical explorationists. Sufficient new field discoveries are made on the basis of soil gas surveys to maintain interest, but the results are too inconsistent to warrant routine use of the method. We hypothesize that the main reason for the unconvincing track record of soil gas surveys and of other surface geochemical exploration techniques is a poor understanding of the mechanisms and pathways of hydrocarbon migration from petroleum deposits to the surface. Thus, these techniques are often used when their application is inappropriate.

The intensity and pathway geometry of the hydrocarbon vertical leakage is strongly affected by groundwater flow between a deposit and the surface. To test this hypothesis, we conducted a comprehensive study of the dynamics, major ion chemistry, halogen ratios, d¹⁸O and d²H, and ¹⁴C ages of groundwaters; the distribution of groundwater recharge and discharge areas; and a survey and MS analysis of hydrocarbon components in soil gas in an area with known oil fields in the Lower Cretaceous Mannville Group in east-central Alberta. We found that the groundwater flow pattern is regionally downward and into the underpressured Mannville reservoirs. Shallow flow systems are associated with local topographic highs and depressions. Soil gas anomalies, shown best by benzene and toluene, were observed but are ill defined. Their geographic association seems to be with local groundwater discharge areas rather than with subsurface petroleum accumulations.

Our interpretation is that descending groundwater flow (underpressuring of the reservoirs) prevents the formation of well-defined soil gas anomalies, regardless of the mechanism of migration from pool to surface. A slight increase in soil gas signatures in local discharge areas is probably due to lateral transport of gas by shallow groundwater flow. Our findings provide an explanation for the unsuccessful attempts to apply surface geochemical exploration in Alberta, where most of the near-surface petroleum fields are strongly underpressured, thus inducing downward flow of groundwaters and obliterating near-surface geochemical signatures.