Unconventional Targets Give San Juan New Life

American Association of Petroleum Geologists (AAPG)

The time-worn phrase "everything old is new again" is an apt description for much of the revved-up activity in the oil patch these days.

Old producing zones thought to have died long ago are being resuscitated via advanced technologies, including horizontal drilling and innovative seismic techniques.

New zones also are being tapped and produced successfully in a variety of approaches, including commingling production from a previously unproduced interval with that of a known producer.

And perhaps the best known "new again" region is the Permian Basin in west Texas, where activity is at a near-fevered pitch.

An active yet lower profile revitalization is under way in the San Juan Basin in the Four Corners area, which straddles the borders of New Mexico, Colorado, Arizona and Utah.

The renewed action is the result of completions in unconventional reservoirs in the Cretaceous-age Mancos shale and Gallup sandstone formations, according to Sophie Berglund, senior geologist at WPX Energy.

Sophie Berglund

WPX has been active in the basin for a number of years.

Berglund noted that legacy drilling and completion operations focused on conventional reservoirs in these formations for decades. Vertical drilling has achieved much successful production in the basin, but it's the more progressive technology being applied to previously bypassed pay zones that is driving success today.

The characteristics of the Mancos and Gallup can make drilling a challenge for the operators.

"The Mancos shale and Gallup sandstone reservoirs were deposited relative to the same shoreline in the Cretaceous Western Interior Seaway," Berglund said. "But they differ in environment and reservoir characteristics.

"The San Juan is a big area, and there are some interesting changes in how the reservoir changes and the maturity of the source rock changes to make distinct plays," she commented. "Serendipitously, Mancos source rock changes phase from volatile oil to dry gas coincident to changes in the nature of the reservoir."

Wet and Wild

The Mancos traditionally has been viewed as a source rock of oil and natural gas and as a seal for conventional reservoirs, according to AAPG member Ron Broadhead, senior petroleum geologist at the New Mexico Bureau of Geology and Mineral Resources.

He provided some highlights:

  • Mancos shales are organic-hydrocarbon source rocks.
  • The oil window is in the shallow southern part of the basin.
  • The thermogenic gas window is in the deeper northern part of the basin.
  • Its maturation is influenced by the depth and proximity to the Tertiary San Juan volcanic field of southern Colorado.
  • A contribution to the New Mexico Geological Society Field Trip guidebook in 1974, courtesy of the late AAPG member C.M. Molenaar at Shell Oil Co., offered insight on the Gallup sandstone:

    "Generally, the Gallup sandstone consists of northeastward prograding coastal barrier strand plain or delta front sandstones that grade seaward into offshore marine mudstones of the Mancos shale … The Gallup sandstone on the west side of the San Juan Basin displays considerable lithofacies variation owing to a major fluvial system, which contributed sediments."

    WPX Energy made a significant natural gas find in the Mancos in 2010, but dry gas was so yesterday at the time.

    So the company opted to explore the wet gas/condensate and oil windows in the central and southern parts of the San Juan Basin.

    "While the wet gas/condensate window didn't appear prospective, we found a unique body of sand encased in the Mancos shale along the southern margin of the basin, which the industry refers to as the Gallup sandstone," Steve Natali, senior vice president of exploration at WPX, said at the time.

    Four vertically drilled exploratory wells later, they had confirmed a major oil discovery.

    Including the original four wells, the company has drilled 16 Gallup oil wells effective February 2014, with 29 wells planned for this calendar year.

    Following the pilot holes, horizontal drilling and hydraulic fracturing became the technologies-of-choice for this success in the Gallup.

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