Summary

The combination of ground-based magnetic horizontal gradient intensity (HGI) and radiometric surveys has proven cost-effective for hydrocarbon exploration in Western Canada. Using surveys in a region where seismic exploration is ineffective, the author has made three different new field discoveries with the first well drilled in each case, and the discovery wells were placed on production. Overall, the author has had better than 85% success using these surveys for predicting hydrocarbons will be encountered upon drilling, a figure commensurate with that achieved by the few other users of this technology.

The theory that underpins this survey technology is that hydrocarbon microseepage by near vertical ascent of coloidal-sized gas bubbles of light hydrocarbons through a network of interconnected, groundwater-filled joints, fractures and bedding planes instigates diagenetic changes in the lithologic column above oil and gas fields. Near-surface chemical and/or bacterial degradation of these hydrocarbons enhances magnetic mineralization (in zones an average of 120m below the surface in the areas surveyed) and alters the balance of uranium and potassium minerals (at the surface). These effects can be measured rapidly at the surface with magnetometers and gamma radiation spectrometers. By computing and mapping the magnetic HGI, and by computing the difference between the thorium-normalized uranium and thorium-normalized potassium radiation (DRAD), a useful picture of the hydrocarbon potential of the mapped area emerges.

The author has found HGI and DRAD surveys valuable as guides for exact positioning of subsequent seismic surveys, especially the considerably more expensive 3-D surveys, and useful as standalone surveys in areas where seismic exploration is ineffective, as happens in the areas of two of the case histories presented here. In general, the HGI survey creates apical anomalies that are congruent with the reservoir that caused them, while the DRAD anomalies tend toward being haloes around the causative reservoir. In the case of channel sands, DRAD anomalies clearly outline the channels. The two types of surveys are easily conducted together and are most often complementary.

HGI/DRAD survey costs vary for many of the same logistic reasons that seismic costs do; typically, the cost is 20% the cost of 3-D seismic surveys for the same area. Because survey data axe collected entirely with sensors above ground, and low ground-pressure vehicles are used, these surveys are environmentally benign.

Although it has generally been thought that all any surface geochemical exploration technique can do is determine if there is a reservoir somewhere below, research by the author suggests that the combination of numerical value of HGI and the azimuth of total magnetization of near-surface diagenetic anomalies can identify the geologic formation of the reservoir below that caused the anomaly. This observation has significant economic ramifications. Survey theory and observations of magnetic azimuth correlated with different reservoir ages are discussed. Seven case histories are presented to illustrate application of the survey technology.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah