A new regional exploration method for detecting hydrocarbon alteration plumes: the ALTREXÔ method

Abstract

Recent work has shown that in some cases hydrocarbon reservoirs are overlain by an associated alteration plume. If it were possible to detect these plumes with an airborne geophysical technique, then the technique could be used on reconnaissance surveys to identify prospective areas. More detailed geophysical (eg, seismic) or geochemical surveys would be required to further develop these prospects. Hydrocarbbn alteration plumes have been detected using the following geophysical techniques: magnetic; electrical; induced polarisation; and radiometric methods. Of these methods only magnetic and radiometric methods can be acquired from an airborne platform. However, the electrical method responds to resistivity or conductivity and this quantity can also be mapped with an electromagnetic method ? which can be mounted on an aircraft. The ALTREXÔ method therefore consists of an electromagnetic system, a magnetic system and, optionally, a radiometric system installed on an aircraft. The method has been tested (with electromagnetic and magnetic installations only) in three onshore basins in the southwestern United States of America. Only one of the seven fields shows a near-surface magnetic anomaly interpreted to be associated with the alteration zone. The other six fields, however, do have magnetic responses which can be associated with deeper structural features, which may control the hydrocarbon traps. The electromagnetic data, which respond to the conductivity in the top 300 m of the subsurface, show an anomalous response in most cases. This anomalous response generally correlates with the surface projection of the producing field (which are generally at 1 km to 2.5 km depth). Results suggest that a combination of airborne magnetic and electromagnetic data will provide a useful reconnaissance tool for regional scale exploration for alteration zones. Whether these zones are indicative of producing fields or not would require further work.