Deep Gulf Targets Get New Tactics

Evergreen Interpretation

Seismic technology continues to drive offshore activity offshore for the deep gas play in the Gulf of Mexico.

Late last year the Minerals Management Service increased its estimates for deep gas in the Gulf of Mexico shelf by 175 percent, to 55 trillion cubic feet of gas. This change was based on:

  • Production from several deep gas fields.
  • Recent new discoveries.
  • New seismic data acquired and processed using the latest technology to improve imaging at greater depths.

That new seismic, along with gravity and magnetic data, have resulted in new conceptual plays that bode well for the deep gas play's future, according to the MMS.

Recent technological developments have improved imaging at depth, allowing oil companies to begin probing the potential of deeper targets — and with the number of deep gas successes on the rise, seismic contractors are pushing the limits for higher quality imaging of these deeper reservoirs.

And it's not surprising, given the growth in the deep gas play on the shelf, that a growing list of geophysical contractors are targeting the play as a part of their business plan.

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Seismic technology continues to drive offshore activity offshore for the deep gas play in the Gulf of Mexico.

Late last year the Minerals Management Service increased its estimates for deep gas in the Gulf of Mexico shelf by 175 percent, to 55 trillion cubic feet of gas. This change was based on:

  • Production from several deep gas fields.
  • Recent new discoveries.
  • New seismic data acquired and processed using the latest technology to improve imaging at greater depths.

That new seismic, along with gravity and magnetic data, have resulted in new conceptual plays that bode well for the deep gas play's future, according to the MMS.

Recent technological developments have improved imaging at depth, allowing oil companies to begin probing the potential of deeper targets — and with the number of deep gas successes on the rise, seismic contractors are pushing the limits for higher quality imaging of these deeper reservoirs.

And it's not surprising, given the growth in the deep gas play on the shelf, that a growing list of geophysical contractors are targeting the play as a part of their business plan.

For example, late last year, TGS-NOPEC completed the acquisition of its newest long-offset 2-D seismic survey, which spans a large part of the Gulf of Mexico with more than 28,000 line miles of data acquired with 8,000-meter offsets and record lengths of up to 13 seconds. The project was specifically designed to image targets as deep as 30,000 feet.

According to Karen El Tawil, vice president for TGS-NOPEC, the program's two-mile by two-mile grid is intended to aid companies in identifying the large deep structures capable of holding large quantities of natural gas.

The 2-D data is packaged into five stack sections, including four evenly distributed corridor stacks. The sections enable analyses of amplitude variations with offset, he said, which is another critical technology for companies searching for deep structures missed by older 2-D and 3-D seismic surveys.

In conjunction with the 2-D regional survey, the company initiated a regional evergreen interpretation project based on the new data coupled with biostratigraphic data. "The interpretation will be updated quarterly and is geared toward aiding explorationists to better understand the deep shelf's regional geology," said AAPG member, John Adamick, TGS-NOPEC's vice president of business development.

"Mapped surfaces include Lower Pliocene, Miocene and upper Oligocene," he said. "Isochron maps created between horizons will show locations of depocenters and fairways within stratigraphic intervals."

Twenty-five analog field studies are included in the initial interpretation project, and more analog studies will be added as well control data from new fields becomes available. The analogs could provide a valuable catalog of what deep gas fields look like across the Louisiana shelf.

3-D Activity

Last September TGS-NOPEC also began acquisition and processing on a 3-D survey that integrates multi-pass streamer data with 9,000-meter offsets and ocean bottom cable seismic with up to 10,000-meter offsets. Although all offset data will be acquired, those greater than 8,000 meters will not be infilled.

The project will cover more than 250 blocks in the Ship Shoal Addition and South Timbalier planning areas.

To create a seamless, integrated survey, both streamer and OBC receivers have been used to simultaneously acquire seismic signal data with two different crews. The survey is using 40-meter cross line spacing and 20 meter by 25 meter bins.

Unlike the 2-D survey that is geared toward identifying prospective subsurface structures, 3-D long offset seismic data is designed to define drilling prospects in the deep gas play's sweet spots and it covers many of the wells that are currently being drilled.

In addition to AVO analysis, seismic contractors point to pre-stack time migration and pre-stacked depth migration technology as important advancements for the deep gas play.

While long offset and record length seismic is a key to unlocking the secrets of buried reservoirs, there are real hurdles to overcome on the surface. The shelf's extensive infrastructure makes developing deep gas targets more economic, but it makes acquiring long offset seismic extremely difficult.

For example, TGS-NOPEC resorted to acquiring streamer data for its 3-D long offset survey in multi-pass mode — recording data in two passes with a 4,500-meter streamer.

Emphasizing the difficulties of acquisition is the fact that this effort falls in the middle of a heavily obstructed offshore area. The TGS-NOPEC vessel carries six streamers side by side that extend over a half-a-mile wide and each streamer is almost three miles long. In fact the total area towed is equivalent to 672 football fields.

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