SATELLITE DETECTION OF NATURAL HYDROCARBON SEEPAGE: PALM VALLEY GAS FIELD, AMADEUS BASIN, CENTRAL AUSTRALIA

Abstract

Digital image processing of advanced aircraft and Landsat Thematic Mapper (TM) satellite remotely sensed data over sandstones of the Palm Valley Gas Field, central Australia, showed a distinct colour anomaly about 6 km long by 1.5 km wide which is not obvious in visible wavelength imagery. Field inspection showed that the colour anomaly was characterised by different rock- weathering colour, a geobotanical anomaly, calcium carbonate precipitation within rock fractures, and different soil pH. Inorganic rock geochemistry indicates significant chemical differences in some major elements. A limited number of soil gas samples were analysed and within the remotely sensed colour anomaly some had above- threshold concentrations of methane, ethane, propane and butane. Preliminary processing of airborne magnetic and gamma spectrometric data over the anticline did not indicate any significant values that suggested abnormal development of magnetite or clay minerals within the colour anomaly. Carbon and oxygen isotope analyses on calcrete from within the colour anomaly suggest, somewhat inconclusively, that hydrocarbons have not contributed significantly to the formation of the calcium carbonate component of the calcrete. Consideration of all available information suggests that the colour anomaly detectable by aircraft and Landsat TM satellite remote sensing corresponds to a zone of surface alteration resulting from long- term seepage of hydrocarbon gases. This colour anomaly, the first of its type reported from Australia, was detected because of spectral reflectance differences resulting from a combination of increased soil carbonate and different geobotanical characteristics from those of the surrounding terrain.