BRISBANE, Australia—To yield the best information, 3-D seismic surveys need to be designed specifically for unconventional targets, said Mark Stanley, regional geophysicist, CGG, speaking at Hart Energy’s DUG Australia Conference & Exhibition in Brisbane. And special considerations are needed for work in Australia, where very large areas are currently being explored for tight sand and shale reservoirs.

Conventional seismic survey designs commonly used in Australia for 3-D acquisition include wide station intervals, short offsets, conservative line intervals and moderate sweep frequencies. These surveys are designed to identify small, shallow targets that are structurally controlled.

However, unconventional reservoirs require different approaches because identification of fractures and rock properties are important, and the targets are often deep and cover large areas. Indeed, success in shale-gas exploration often depends on the ability of seismic methods to quantify the orientation and density of natural fractures in the reservoirs.

Stanley proposed that unconventional surveys should be designed with narrow station intervals, long offsets, wider line intervals and broader sweep frequencies.

The finer spatial sampling allows for better attenuation of coherent noise, so interpolation can work.

“We need to go to significantly longer offsets to make the most of our seismic data,” said Stanley. The longer offsets help image deeper targets and help detect fractures, and are also needed for velocity and AVO analyses. “Seismic waves ‘see’ the fractures, and longer the offsets the more of these fractures they ‘see’,” he said. “Long offsets are much more sensitive than near offsets.” Mapping the variations in azimuths can allow prediction of fracture density—where best to drill, fracture orientation and how best to frac.

Seismic surveys are expensive, and large offsets require a lot of equipment. But, if the offset-line spacing ratio is kept at 10:1, then the actual number of channels required will only mean a modest increase in cost for capital equipment. “If we can use long offsets with wide line spacing we can make these surveys economic,” he said. The wider spacing helps pare down costs because it requires less layout work, allows for faster acquisition, and has fewer environmental impact and cultural heritage issues. All of these factors are important in Australia.

Finally, broader bandwidths improve penetration for deeper targets and improve inversion for rock properties. “We should be trying to get down to 2 Hz,” Stanley said.

“We need to learn from the marine seismic experience,” he added. “In the 1990s it was typically small two-streamer efforts focused on known fields. Now we routinely acquire thousands to tens of thousands of square kilometers as our first step of exploration. Sail-line spacing has been the key to seismic productivity. It’s time for us to do this in our search for unconventional reservoirs.”

Contact the author, Peggy Williams, at pwilliams@hartenergy.com.