‘Stoplights’ managed infill plan

Greenland Gets 3-D Look

Working on the back deck in Arctic conditions can be cold and tough – but crews were well prepared with modern seismic equipment designed to withstand the wear and tear. Technical downtime was limited to only a few hours for the whole three-month arctic campaign. Photos, graphic courtesy of PGS
Working on the back deck in Arctic conditions can be cold and tough – but crews were well prepared with modern seismic equipment designed to withstand the wear and tear. Technical downtime was limited to only a few hours for the whole three-month arctic campaign. Photos, graphic courtesy of PGS

When it comes to hydrocarbon exploration, challenges are a given.

And when it comes to a list of daunting regions for exploration, the Arctic ranks about as high on the list as anything in the world.

But the potential for possible big finds in certain locales, e.g., offshore West Greenland, is a powerful incentive for the companies to figure out whatever it will take to keep the exploration process moving along.

West Greenland has a sketchy history of exploration. Two-dimensional seismic data were acquired as far back as the 1970s and as recently as 2008. Several wells have been drilled thus far.

(At press time, Greenland’s offshore potential was underlined when Cairn Energy announced it had tested pockets of oil and evidence of gas in the Baffin Bay Basin.)

Even so, a commercial discovery has long remained elusive.

“The potential for hydrocarbon discovery remains high,” said Per Eivind Dhelie, chief geophysicist at PGS.

“Upper Cretaceous sedimentary sections outcropping along the coast of the nearby Nuussuaq Peninsula reveal excellent quality reservoir, source and seal rocks overlain by Paleocene age volcanics,” he said. “These offer excellent analogs to sections potentially buried deep beneath the ocean floor.

“Additional positive hydrocarbon indicators include numerous oil seeps discovered along the coast in the Disko-Nuussuaq-Svartnhuk Halvo region,” Dhelie said. “These have been typed by the Geological Survey of Denmark and Greenland to five different source intervals dating from Cretaceous to Paleogene.

“Four-way structural closures have been mapped based on vintage seismic data in the West Disko area, despite sub-basalt data quality issues,” he added. “And interpreted gas clouds along with amplitude anomalies, some with favorable AVO signatures, can be observed in the shallower Tertiary section, hinting at potential deeper hydrocarbon charges.”

Unfriendly Regions

To the chagrin of the interested companies, this is a downright unfriendly area for seismic data acquisition.

As PGS geared up to acquire two 3-D seismic surveys for Husky Oil Ltd. in the Arctic waters offshore West Greenland in 2009, the company approached the task with eyes wide open.

Dhelie, outlined some of the formidable challenges to seismic exploration there, including:

  • Short three-four month ice-free season.
  • Avoiding icebergs during acquisition.
  • Hard water-bottom notorious for strong multiple energy.
  • Volcanic flows with associated dikes and sills within and overlying prospect objectives.

It was determined that PGS’s dual-sensor 3-D GeoStreamer technology with deep-tow capabilities was the method-of-choice to overcome the data acquisition hindrances indigenous to the area – namely the weather and the seismic imaging challenges. The more than 2,200-square-kilometer 3-D surveys took place in West Disko Blocks 5 and 7, which are operated by Husky.

Dhelie noted that dual-sensor streamer technology allows for streamers to be towed at greater depths –15 meters for this project – than conventional (hydrophone-only) seismic cables without compromising the high frequency spectrum. With all six seismic streamers at tow depth of 15 meters, surface noise is minimized and the operational window can be increased, allowing acquisition to proceed in inclement weather conditions.

Over the course of the Arctic program, downtime for weather was 3 percent and 12 percent for Blocks 7 and 5, respectively, which was significantly lower than expectations if streamers were towed at a more conventional depth.

“The deep-towed streamers also enhance the natural frequency response below 20 hertz, which is important for sub-basalt imaging,” Dhelie said. “We used the 2-D vintage data from the area to analyze and optimize acquisition parameters, and special attention was focused on source optimization for increased low frequency penetration.

“The increased energy recorded in the amplitude spectrum lower than 20 hertz was found more appropriate to penetrate primary energy through attenuative volcanic rocks than what conventional tow depths have achieved historically,” he noted.

Warning: Iceberg Crossing

Even though the survey was implemented during the area’s annual ice-free period, weather conditions were far from summer-like.

“The water temperature was below zero,” Dhelie said, “so when you take things out of it they freeze instantly, which we knew in advance.

“In the water, these things are moving around, so they don’t freeze,” he said. “Also, this is salt water, which helps prevent freezing.”

Special consideration in program operations included radar imaging for iceberg detection to avoid striking the icebergs, given that the survey spread made it impossible to turn quickly to avoid obstacles.

Because the survey had to be accomplished during the brief ice-free season and the icebergs likely would obstruct sail lines, project success required efficient infill management.

Dhelie noted that a proprietary infill method commonly referred to as the “stop-light system” proved to be invaluable. Stop-light plots were constructed and reviewed online by Husky and PGS to determine the need for extra infill lines.

Survey operations boasted no negative incidents.

“There were thumbs up everywhere,” Dhelie said.

“The dual-sensor technology proved invaluable to the survey efficiency as well as to the very good data quality,” he emphasized.

The project was the first 3-D survey in this locale where other companies also hold blocks. If a successful well should come in, it most likely would open up a whole new area to exploration, Dhelie suggested.

Per Eivind Dhelie presented the paper "Successful 3-D Seismic Exploration Offshore West Greenland Using Dual-Sensor Streamer Technology" at the recent AAPG International Conference and Exhibition in Calgary, Canada.

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At ICE 2010 ...

Per Eivind Dhelie presented the paper "Successful 3-D Seismic Exploration Offshore West Greenland Using Dual-Sensor Streamer Technology" at the recent AAPG International Conference and Exhibition in Calgary, Canada.

Emphasis: Geophysical Review