Monterey Formation, San Joaquin Basin: Little Continuous Oil Left to Find (USGS 2015)

The U.S. Geological Survey estimates a mean of 21 million barrels of oil, 27 billion cubic feet of gas and 1 million barrels of natural gas liquids of undiscovered, technically recoverable continuous resources in the Monterey Formation of California's southern San Joaquin Basin: 14 million barrels in the Monterey Buttonwillow assessment unit and 7 million in the Monterey Maricopa unit.

What was assessed

The Miocene Monterey Formation is the source rock for most of the oil in the San Joaquin Basin's structural and stratigraphic traps, including several fields holding billions of barrels of recoverable oil. Recent drilling aimed at oil still inside the source rock itself appears to have been unsuccessful so far.

Earlier USGS work estimated the basin's undiscovered conventional oil and gas (Gautier and others, 2007) and likely additions to reserves in known fields, among them Monterey reservoirs at Elk Hills and Lost Hills (Tennyson and others, 2012). This assessment covers Monterey rock that may hold a continuous accumulation, oil that formed in or near the zone of generation and never migrated up into conventional traps. Two assessment units were defined, excluding known conventional fields:

1. the Monterey Buttonwillow Continuous Oil unit;
2. the Monterey Maricopa Continuous Oil unit.

Figure 1: The San Joaquin Basin Province and its two Monterey assessment units, whose boundaries approximate the 10,000-foot depth to the top of the Monterey. Base from the U.S. National Park Service. USGS.

The geology

The basin's rocks record 120 million years of deposition beside a converging continental margin, which changed about 20 million years ago to sideways motion along the San Andreas Fault system. The Monterey was laid down after that, from about 17 to 5 million years ago, when the coast lay well inland of today's and little coarse sediment reached the sea: fine-grained, organic-rich rock full of diatoms and foraminifera, with deep-sea sandstone in the far south. It is typically about 2,000 feet thick, up to about twice that in the deepest parts.

A structural high near Bakersfield splits the basin into the Buttonwillow depocentre to the north and the Maricopa to the south, where the top of the Monterey lies more than 14,000 and 15,000 feet down. Oil began forming only in the last few million years, as the rock was buried to about 12,000 feet, and most of it moves readily upward along fractures, faults and sandstone layers into traps. More than 80 wells have reached the Monterey where it is deep enough to have made oil, and none has produced more than small amounts outside conventional traps.

Inputs and results

For oil, the total ranges from 3 million barrels (a 95% chance of at least that much) to 53 million (a 5% chance).

Sources

• Marilyn E. Tennyson, Ronald R. Charpentier, Timothy R. Klett and others, Assessment of undiscovered continuous oil and gas resources in the Monterey Formation, San Joaquin Basin Province, California, 2015, U.S. Geological Survey Fact Sheet 2015–3058. https://pubs.usgs.gov/publication/fs20153058