Satellite radar guides exploration in deepwater offshore Brazil
Radar from satellites has mapped floods, geological structures, three-dimensional topography, and surface movement associated with earthquakes and sub-sidence. It has also been used to map oil slicks on the sea over the entire Brazilian Shelf in a 1998-99 survey, and as part of a worldwide Offshore Basin Screening™ project. These slicks originate in naturally seeping petroleum or result from oil discharged from ships and oil rigs.
The focus of offshore oil exploration is moving inexorably into ultra-deepwater, as exploitation of the accessible parts of the world's continental shelves reaches saturation point. Development in 1000 meters water depths is now industry standard, and Brazil's Petrobras is helping lead the movement into depths of 2000 meters. Radar from satellites can offset the high cost of deepwater drilling by establishing the presence of an active oil source and charge at a very early stage in exploration. It also provides an unbiased eye on environmental performance of explorers.
Satellite radar
There are two main civilian radar satellites - the Canadian Radarsat and the European ERS. Both comprise active microwave radar or SAR (synthetic aperture radar), which now offers the industry wide-swath, multiple-repeat imagery over all of the world's continental margins, at any water depth. Satellite SAR images the ocean day and night, and through clouds. It maps slicks (flat patches of the sea surface) that can be related by analysis to petroleum seepage or pollution. The continuously imaging radar data are provided as individual scenes of 10,000 sq km (ERS) and 29,000 sq km (Radarsat). One hundred and fifty of these scenes have been analyzed over the Brazilian continental shelf, resulting in at least double coverage of the entire shelf to 3000 meters depth.
The basic principle for detecting petroleum seepage or pollution by satellite radar has been known to generations of mariners as "pouring oil on troubled waters." Part of the emitted radar energy (5.6 cm wavelength) directed at the ocean is returned to the satellite due to the roughness of the sea surface and is imaged as a grey speckle (Figure 1). However, when the sea is smoothed by the visco-elastic properties of an oil slick, whatever the origin, the radar backscatter is reduced, producing a dark area on the image.
As the prime control of sea surface morphology is wind, slick formation is controlled by wind shear over the sea. In order to observe slicks on radar, data has to be acquired within a precise low wind and swell envelope. Proprietary global hindcast meteorological models of wind speed and swell are used to select weather-compliant scenes in archive, or following a specific acquisition program, before data purchase.
Seepage slicks
Satellite SAR systems are very effective at observing slicks, both man-made and natural, but few slicks result from hydrocarbon seepage (rarely more than 5% of slicks detected). Most slicks are neither seepage nor pollution, but result from the behavior of long-chained organic molecules derived from the decay of plankton and land-based inputs to the sea. These form a natural film that creates widespread slicks at certain low wind speeds. A consistent and systematic analysis scheme to analyze ocean features (such as the effects of the frequent dense rainstorms off Brazil) and to discriminate between seepage-slicks, pollution, and natural film slicks, has been developed by NPA-TREICoL. The scheme, developed in a two-year research program in 1992-94, has been validated in more than 340 offshore basins from 68 countries.
Seepage-slicks offshore Brazil tend to be small (100 meters to 2 km) and many have a characteristic "blob" formed by surface tension as the seeping bubbles reach the sea surface (Figure 1C). The seepage sites, located to plus or minus 300 meters by the satellite, are characterised by ultra-thin "pancake-shaped" slicks that can be located at sea using GPS and sampled from small boats (Figure 2). Many slicks are biodegraded, although sufficient concentrations of biomarkers are present to enable the seepage to be characterized by GC-MS carbon isotopes and other tools to prove seepage-source correlation and type.
Brazil's deepwater oil
It is generally accepted that almost all traps in basins containing a mature oil source will leak small but detectable amounts of oil to the surface as petroleum seepage. The rate of seepage (and therefore, the density of seepage-slicks) depends on geological conditions and source type. For example, seepage-slicks over gas sources are rare.
Seepage increases, for example, with high subsidence rates, salt and mud diapirism, and tectonic compression. The relative seepage densities observed offshore Brazil are consistent with the contrasting geological controls on seepage operating in each basin. Seepage was mapped, for example, in the Foz do Amazonas, Potiguar, Espirito Santo, and Pelotas Basins. Major seepage was indicated for parts of the Santos Basin, as well as the Campos Basin.
As seepage represent the ends of migration pathways, the value of detecting seepage in these high-risk basins confirms the presence of an active charge, thus eliminating basin source risk. Brazil seepage mapping indicates that the Campos Basin is by far Brazil's most prolific basin, accounting for 80% of the country's oil reserves and has an impressive success rate of 60%.
Turbidite sands within both the Upper Cretaceous and Tertiary post-Rift sections are now the primary exploration target along the Brazil margins, almost exclusively in deepwater. Many show a pattern of repeating surface seepage detectable by satellite radar.
Benchmarking pollution
The largest (often several km long) and most obvious slicks offshore Brazil occur adjacent to some of the producing fields of the Campos Basin (Figure 3). Pollution appears to be discharging directly from platforms (Figure 1A), but the largest slicks are remnant pollution, where oily slicks have been moved by current and tide (Figure 1B).
Most of the pollution is likely to be have been produced formation water or from discarded drilling fluid. Because the satellites are global ranging, they offer a good method of benchmarking pollution over the entire shelf. Pollution can be linked directly to an offending platform, and environmental performance of each operator can be monitored.
Acknowledgement
The development of these techniques was partially funded under the British National Space Centre's application development program during 1992-94. The GEC Marconi Research Laboratory provided theoretical models and validation of SAR imaging of slicks.
Authors
Maria de Faragu Botella (BSc, MSc) graduated in geology from Madrid's Complutense University and worked as a geologist for several years in Spain. She joined NPA Group after obtaining a MSc in applied remote sensing from Imperial College, London. She is Offshore Basin Screeningtrademark (OBS) Operations Manager in Edenbridge, Kent, UK.
Geoff Lawrence (BSc, PhD, FGS) has worked on geophysical, geochemical, and remote sensing projects for Hunting Surveys and Consultants, on satellite radar for NASA's JPL, and was Manager of Satellite Remote Sensing for BP Exploration in 1985-92. He formed the The Really Easy Imaging Company Ltd (TREICoL) to develop offshore basin screening jointly with NPA. He was a founding member and first president of the Geological Remote Sensing Group.