Airborne Spectral Survey Experiment over Hydrocarbon Basins

William Collins

Airborne surveys covering 3,000 line mi over ten hydrocarbon test sites were flown in a joint research program funded by nine major oil companies. The objectives of the program were to determine the visible and near-IR spectral properties of basin surfaces under the highest spectral and radiometric sensitivity.

The sites were flown with the Geophysical Environmental Research Corporation's airborne spectroradiometer system. This instrument takes measurements over 20-m square areas in a continuous line profile beneath the aircraft. The data acquired are spectral measurements of 512 channels between 400 and 1000 nm and 64 channels between 2.0 and 2.5 microns.

The 400 to 1000-nm spectral region yields information on iron mineralogy and vegetation stress. The premise is that gas seepage can result in surface anomalies such as iron oxide staining or bleaching. The gas can also adversely affect vegetation, leaving it in a "stressed" state. The vegetation stress is detected by a spectral anomaly in the infrared portion of the chlorophyll spectrum.

The 2.0 to 2.5-micron portion of the spectrum contains identifying spectral bands of many clay, carbonate, and other minerals. This data is used to map lithologies, spectral patterns of minerals within lithologic units, and possible clay and carbonate anomalies related to seepage.

The infrared mineral mapping has had some important results. (1) Lithologic mapping at fine detail in complex areas as Lisbon Valley and Comb Ridge can be accomplished with the very rapid airborne techniques. (2) Large mineral patterns occur within and across lithologic units. These are subtle variations in clay content and type. They may be related to basin processes such as paleodrainage. (3) Carbonates can be detected in lithologic units and as apparent soil "cements." They can also be differentiated and identified as limestone and dolomite. (4) Clay contents mapped in basins are on the order of 2 to 5% of the total mineral content. (5) Differences on the order of 2% in clays must be detectable to map surface mineral patterns.

As part of the program, soil gas sampling was done by Exploration Technologies, Inc., in the Overthrust belt and Velma/Cement. Sampling was done on zones of high clay, carbonate, iron staining, and vegetation stress as detected in the spectral data. The results show a promising correlation between the presence of these features in the airborne survey data and ground-measured soil-gas anomalies.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.