Case Studies - Oil & Gas Exploration

Paleochannel Mapping for Shallow Gas Exploration

Airborne electromagnetic surveys (AEM) have proven useful for exploring Quaternary paleochannels and deeper incised valley systems to depths of 300 metres for natural gas. AEM systems which were originally developed for mining exploration have been used to map paleochannels in North America and Australia for groundwater, aggregate and environmental applications for many years. AEM systems measure the resistivity of the earth. Sand and gravel filled paleochannels, sandstones and conglomerates, are mapped with AEM as resistivity highs in contrast to clay rich sediments and shale indicating the location and some information about the character of paleochannels and possible geological traps.

Very shallow gas wells are producing natural gas from Quaternary aged glaciofluvial channels in Northern Alberta. Wells in the Sousa area have been completed at only 100 m and have achieved production from reservoirs as shallow as 45 m. Figure 1 depicts a glaciofluvial channel in cross section. The trap may be derived from glaciolacustrine clay which has been compacted by the weight of subsequent ice build up.

Figure 2 is an example of a resistivity map from an AEM survey. This map depicts a depth slice at approximately 65m. The fluvial system is obvious as a red resistivity high. Much detail can be seen in the apparent extensive dendritic system in this area which measures 10 km by 3 km. The wells at 10-11-112-1W6 and 10-2-112 -1W6 are believed to have had initial production of 2.4 mmcf/d and 1.4 mmcf/d respectively from channels ranging in depth from 45 to 90 m.*

Fugro offers the RESOLVE helicopter frequency domain electromagnetic (HEM), MEGATEM and GEOTEM fixed wing time domain electromagnetic (FTEM) systems which are suitable for exploration of paleochannels. The RESOLVE system is more sensitive to subtle resistivity contrasts in the top 50m and the MEGATEM and GEOTEM systems offer greater depth penetration, typically to a maximum of 300m in the Western Canadian Sedimentary Basin. The actual maximum depth penetration will depend on the conductivity of the soil and rock type and water content.

Airborne electromagnetic surveys, typically costing about $100 per line kilometer for large volumes, are much less expensive than ground based methods such as seismic for prospecting for shallow gas and yield high resolution 3-dimensional datasets reflecting the resistivity variations of the earth’s layering.

*Canadian Discovery Digest, Exploration Review, Sousa 111-2W6 Quaternary Gas, April 2001

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