The nature and interpretation of geophysical and geochemical anomalies over oil and gas fields

Abstract

Geochemical prospecting seeks surface or subsurface evidence of vertical migration of hydrocarbons. One application is to directly determine the presence of oil and gas in the reservoir. The basic principles of geochemical methods were recently reviewed by Colin Barker ( TLE , June and July 1999) and by Dietmar “Deet” Schumacher ( TLE , March 2000). However, Barker's articles did not discuss the location/interpretation of surface geochemical anomalies that, for the explorationist, is a most interesting and important subject, and Schumacher's article mainly discussed the case of “apical” anomalies over oil and gas fields, that is not always the occurrence. Surface geochemical exploration for hydrocarbons began in the 1930s. It has had great successes and publicized failures. In some cases geochemical anomalies were reported directly over oil and gas accumulations (apical), but in many cases the anomalies appeared annular and formed a ring (“halo”) above the margins of the field. This ambiguity has been restricting the use of this technique for many years. Examples of both cases are abundant in scientific journals. However, seismic and magnetic anomalies have also been observed over the edges of hydrocarbon accumulations. I suggest that combining the methods can improve exploration success. Absorption has been relatively ignored as an exploration tool in the United States. One characteristic of hydrocarbon accumulation is an increase in attenuation and changes in the frequency content of waves passing through the accumulation. Thus, absorption can be an independent source of information about the presence of hydrocarbons. Modern seismic data can detect the frequency absorption caused by hydrocarbon accumulation. As with geochemical anomalies, these absorption anomalies are in …