Assuming these two staments are true:

"The Monterey Shale formation contains about two-thirds of the nation's shale oil reserves."

"Just 600 million barrels of oil can be extracted with existing technology, far below the 13.7 billion barrels once thought recoverable"

My calculations come out to a huge drop from (assuming my calculations are correct)

20.5 Billion Barrels -> 7.4B Billion Barrels (Seems like a significant drop IMHO)

However, 7.4B and 20.5B both seems trivial compared to Saudi Arabia and Venezuela and the other top reserves as listed on wikipedia.