The Effective Use Of Electromagnetic Methods To Delineate A Fluvial Paleochannel System Controlling Oil Migration Near Glenrock, Wyoming

A combination of three different electromagnetic methods was effectively used to map the shallow lithologic<br>variation interpreted to control the subsurface migration pattern of hydrocarbons near Glenrock, Wyoming.<br>Petroleum hydrocarbons were seeping from the south bank of the North Platte River approximately one-half mile<br>west of the Township of Glenrock, Wyoming. Product was moving along the top of the water table through coarse<br>grained sediments and discharging into the river. Initial investigations, resulting in the excavation of three trenches,<br>were unable to determine the source and migration pathways that the hydrocarbons were following. A geophysical<br>survey was performed that provided a map of the subsurface geology that controlled free and dissolved product<br>migration.<br>The geophysical methods used included terrain conductivity, using two instruments that allowed two different coil<br>separations (the Geonics EM-31 and EM-34), and transient electromagnetics (TEM). The portability and data<br>collection efficiency of the EM-31 guided its use as a reconnaissance tool. Data from the EM-31 provided a map of<br>near-surface conductivity patterns indicative of shallow lithologic variation between coarse-grained paleochannel<br>deposits and tine-grained fluvial overbank sediments. Correlation of the EM-31 response with known geology<br>observed in test pit excavations resulted in some concern as to whether the mapped lithologic variation was too<br>shallow to have controlled groundwater migration. To address this, the EM-31 was operated at ground, and at 3 and<br>7 feet (ft) heights above ground over a high conductivity zone (interpreted to be fine-grained material). Data<br>obtained indicated that the high conductivity material was not a thin surficial veneer, but instead represented finegrained<br>deposits at depths of 0 to greater than 10 ft. The EM-34 instrument was also used at a lo-meter coil<br>separation along selected traverses to confirm the vertical extent of the interpreted lithologic variation. In addition, a<br>profile of central loop TEM sounding provided enough information to allow one-dimensional computer modeling,<br>which provided a quantitative depth extent of fine-grained (fluvial overbank) deposits. Therefore, preferential<br>groundwater migration pathways (fluvial paleochannels) were not only detected, but quantitatively delineated using<br>this effective combination of electromagnetic methods.