The Monterey Formation’s source rock in the deepest parts of California’s San Joaquin Basin likely has relatively little recoverable unconventional oil or gas, the U.S. Geological Survey (USGS) said.

The USGS assessment team concluded that most of the petroleum from the formation’s shale has migrated from the source rock, likely leaving little excess hydrocarbons behind. The assessment was the first for unconventional resources in the Monterey. Previous conventional studies have found far more oil potential.

“Most exploratory wells in the deep basin are unlikely to be successful,” the agency said in an Oct. 6 report.

The mean estimate of the formation’s unconventional, technically recoverable resources are 21 million barrels of oil (MMbbl), 27 billion cubic feet of gas (Bcf), and 1 MMbbl of natural gas liquids, according to the USGS assessment.

By contrast, conventional resources in the basin are estimated to be in the billions of barrels.

Vito Nuccio, acting USGS energy resources program coordinator, told Hart that additional assessments of unconventional hydrocarbons will be conducted in the same area and released by the next fiscal year.

For mean volumes of unconventional oil in the Monterey, the USGS used two assessment units or areas. In the Monterey Buttonwillow, the agency assessed 14 MMbbl of oil and in the Monterey Maricopa 7 MMbbl of oil.

This USGS analysis, based on drill­ing results to date, concluded that some recoverable oil probably remains in the source rock, but the success rate for drilled wells that produce at least 2,000 barrels will be low. The estimated ultimate recovery (EUR) will be low for even for successful wells.

Previous USGS estimates were far higher but considered existing production and potential new conventional oil finds.

In 2003, conventional oil and gas in the San Joaquin Basin had an estimated mean recovery of 121 MMbbl of oil in the Monterey.

In 2012, USGS assessed potential oil volumes that could be added to reserves through increased recovery. The results suggested 3 Bbbl of oil might be added to reserves from Monterey reservoirs in conventional traps.

New estimates for unconventional oil are meager by comparison. Geological data from more than 80 older wells that penetrated the deep Monterey Formation indicate that retention of oil or gas in the shale source rock is poor, probably because of natural fracturing, faulting and folding.

The range of oil recovery in the Monterey ranges from 3 MMbbl to 53 MMbbl, based on probability calculations.

“Understanding our domestic oil and gas resource potential is important for many reasons, including helping policy makers to make informed decisions about energy policy, leasing of federal lands and impact on other resources such as water,” Nuccio said. “That’s why the USGS maintains a strong oil and gas assessment team whose goal is to assess new domestic and global areas and continually update previous assessments as warranted.”

The new estimates are for technically recoverable oil and gas resources, which are those quantities of oil and gas producible using available technology and industry practices regardless of economic or accessibility considerations.

Darren Barbee can be reached at dbarbee@hartenergy.com.

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