Pitfalls and Caveats in Surface Light Hydrocarbon Surveying

WEISSENBURGER, KATE S., Conoco Inc., Ponca City, OK

The credible application of light hydrocarbon surveying in petroleum exploration requires an appreciation of the genesis, mobility, and alteration of light hydrocarbons in subsurface and surface environments. In this context, case studies and experimental work are discussed in this paper illustrating some of the pitfalls that beset light hydrocarbon surveying. These examples document a variety of spurious or "false" light hydrocarbon anomalies that are genetically related to soil and sediment properties (e.g., mineralogy, parent material, indigenous organic material, biological activity) rather than to seepage phenomena. Also discussed are instances in which the natural alteration of biogenically produced light hydrocarbons in surface environments has yielded hydrocarbons that mimic t ermogenically produced hydrocarbons. In addition, examples are shown in which light hydrocarbons produced during shale diagenesis (and that can be expelled via shale compaction) resemble thermogenically produced light hydrocarbons. Finally, data are presented that indicate that acid extraction, a popular analytical technique, can itself produce hydrocarbons from soil and sediment samples containing particular clay and organic constituents.

Thus, the challenge in light hydrocarbon surveying is to critically evaluate the presence of light hydrocarbons in surface materials in terms of a number of possible origins, not all of which imply the existence of subsurface reservoired hydrocarbons. The ubiquity of light hydrocarbons in soils and sediments must be acknowledged and reconciled with the data analysis procedures that are used. Most important, light hydrocarbon "highs" cannot, a priori, be assumed to represent seepage features. Finally, care must be taken to avoid the commonly used data analysis methods with which hydrocarbon "anomalies" can be manufactured through the use of arbitrary cut-offs and the misapplication of conventional statistical methods.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)