USGS downgrades oil, gas estimates for Monterey formation, San Joaquin basin
A federal revision of the estimated reserves in California's Monterey shale in the San Joaquin basin is small compared with previous estimates, said the US Geological Survey.
An assessment showed the Monterey formation in the San Joaquin basin contains estimated mean technically recoverable unconventional resources of 21 million bbl of oil, 27 bcf of natural gas, and 1 million bbl of natural gas liquids.
The latest assessment, released Oct. 6, lowered the estimate compared with a 2011 federal energy estimate of 13.7 billion bbl of recoverable oil overall, including previous USGS estimates of conventionally trapped recoverable oil in the Monterey formation.
The assessment team concluded most of the petroleum that originated from Monterey shale in the San Joaquin basin has migrated out from the source rock, meaning relatively little recoverable oil or gas probably remains in the Monterey shale.
The Monterey share covers 1,750 sq miles and reaches three other basins in addition to the San Joaquin basin. Future USGS reports will estimate technically recoverable resources in the other basins.
"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.
Most exploratory wells in the deep basin are unlikely to be successful, USGS researchers said.
Geological data from more than 80 older wells that penetrated the deep Monterey formation indicate that retention of oil or gas in source rock is poor, probably because of natural fracturing, faulting, and folding.
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 shale, USGS said.
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 in the shallower conventional traps.
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 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 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 recently has produced more than 20 million bbl per year.
The estimate of undiscovered continuous oil in the deep Monterey ranged at 3- 53 million barrels (95% to 5% probability, respectively). The estimate of natural gas ranged at 5-72 bcf (95% to 5% probability, respectively), and the estimate of NGLs ranged at 0-3 million bbl (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 currently 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.
