California’s Monterey shale holds 21 MMbbl of recoverable oil: USGS


RESTON, Virginia -- The Monterey formation in the deepest parts of California’s San Joaquin basin contains estimated mean volumes of 21 MMbbl of oil, 27 Bcf of gas and 1 MMbbl of NGL, according to the first USGS assessment of unconventional, technically recoverable resources in the Monterey formation.

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 resources in 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.  

Source rock

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 MMbbl of oil recoverable from the Monterey. In addition, in 2012, USGS assessed the potential volume of oil that 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 Bbbl 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 MMbbl of oil per year.

Undiscovered resources

The estimate of undiscovered continuous oil in the deep Monterey ranges from 3 MMbbl to 53 MMbbl (95% to 5% probability, respectively). The estimate of natural gas ranges from 5 Bcf to 72 Bcf (95% to 5% probability, respectively), and the estimate of NGL ranges from 0 MMbbl to 3 MMbbl (95% to 5% probability, respectively).

These 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.

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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