USGS Estimates 21 Million Barrels at California's Monterey Formation

The Monterey Formation in the deepest parts of California’s San Joaquin Basin contains estimated mean volumes of 21 million barrels of oil, 27 billion cubic feet of gas, and 1 million barrels of natural gas liquids, according to the first USGS assessment of continuous (unconventional), technically recoverable resources in the Monterey Formation.

“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 ,” said Vito Nuccio, Acting USGS Energy Resources Program Coordinator. “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 volume estimated in the new study is small, compared to previous USGS estimates of conventionally trapped recoverable oil in the Monterey Formation in the San Joaquin Basin. Those earlier estimates were for oil that could come either from producing more Monterey oil from existing fields, or from discovering new conventional resourcesin the Monterey Formation.

The area of the potential continuous accumulation assessed in this study is limited to where the Monterey Formation is deeply buried, thermally mature, and thought to be generating oil.

The assessment team concluded that most of the petroleum that has originated from shale of the Monterey Formation in the assessment area has migrated from the source rock, so there is probably relatively little recoverable oil or gas remaining there, and most exploratory wells in the deep basin are unlikely to be successful.  

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

The oil and gas readily migrates from the deep Monterey Formation to fill the many shallower conventional reservoirs in the basin, including some in fractured Monterey Formation shale, and accounts for the prolific production there.

Although the data suggest that there is apparently not a large volume of unconventional oil and gas resources in the Monterey in the deep part of the basin, there are still substantial volumes of additional conventional oil and gas resources oil in the Monterey Formation in the shallower conventional traps in the San Joaquin Basin, as indicated by earlier assessments.

In 2003, USGS conducted an assessment of conventional oil and gas in the San Joaquin Basin, estimating a mean of 121 million barrels of oil recoverable from the Monterey. In addition, in 2012, USGS assessed the potential volume of oilthat could be added to reserves in the San Joaquin Basin from increasing recovery in existing fields. The results of that study suggested that a mean of about 3 billion barrels of oil might eventually be added to reserves from Monterey reservoirs in conventional traps, mostly from a type of rock in the Monterey called diatomite, which has recently been producing over 20 million barrels of oil per year.

The estimate of undiscovered continuous oil in the deep Monterey ranges from 3 million to 53 million barrels (95 percent to 5 percent probability, respectively). The estimate of natural gas ranges from 5 to 72 billion cubic feet (95 percent to 5 percent probability, respectively), and the estimate of natural gas liquids ranges from 0 to 3 million barrels (95 percent to 5 percent probability, respectively).

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

USGS is the only provider of publicly available estimates of undiscovered technically recoverable oil and gas resources of onshore lands and offshore state waters. The USGS Monterey Formation assessment was undertaken as part of a nationwide project assessing domestic petroleum basins using standardized methodology and protocol.


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