A California oil field thought to be one of the biggest in the U.S. has been producing much less oil than hoped, according to a report by Alliance Bernstein analyst Bob Brackett.

The field, a formation of rock known as the Monterey Shale, was thought to have 15 billion barrels of “technically recoverable” reserves, according to government estimates. That’s triple the amount of oil found in huge and newly prolific fields in North Dakota and Texas. The formation lies under the San Joaquin Valley in central California. Most of the locations probed so far have been northwest of Bakersfield.

Drillers trying to tap the formation include Occidental Petroleum Corp., Plains Exploration & Production Co., Venoco Inc. and Berry Petroleum Co.

But drillers haven’t been able to get the Monterey Shale to produce oil at high rates. Brackett suggests that there are a few characteristics of the geology that could make the field more difficult to develop. There are lots of natural faults in the rock, which means drillers can’t easily control the flow of oil through faults they create. Also, the rock is not under enormous pressure, so there is less force pushing the oil to the surface. And the oil may be relatively thick and sticky, which slows its flow.

If the Monterey had performed as well as the Bakken field in North Dakota or the Eagle Ford in Texas, it would be delivering an additional 300,000 barrels of oil per day by now, Brackett says. Instead, production in California is flat.

“We don’t expect a ‘Bakken Boom’ to strike the San Joaquin Valley,” Brackett wrote. “We expect California production to grow only modestly.”

Some drillers blame the permitting process in California, which Brackett noted has been slow. But his data suggests that oil production is tracking closer to oil prices than to the pace of permits. That could meant that permitting isn’t the problem, but rather that drillers must be assured of a very high oil price in order to risk trying to tap those difficult resources.

Still, Brackett said the field remains an attractive resource play, especially if oil prices rise, because there is so much oil there. And it is far from trading centers in the middle of the country that are flooded with oil from North Dakota and Canada, which is driving down prices there.