I'm not sure Unocal is worth $18.5 billion based on their proved reserves of oil and gas. But the amount of additional oil and gas that's likely to be discovered on their leased acreage is much harder to estimate. Chevron and CNOOC may believe that there's much more oil & gas to be found in Unocal's properties, and if that turns out to be correct then Unocal could be worth $25 billion. That kind of information isn't readily available in SEC reports, so I can't estimate how much more oil & gas will be found. But I have a high regard for Chevron's management, so there's probably a substantial amount of undiscovered "black gold" on Unocal's properties.
I agree. No oil company is valued based on proven reserves alone. You've got to take into account potential reserves, as well as the other businesses the company operates in. Stock market investors generally do that. And no, you cant estimate the value of potential reserves from SEC filings alone, but there are other sources, like the US Geological Survey.
The stock market definitely thinks Chevron is overpaying. They announced their bid on April 4. On that day stock opened at $59.08. It closed at 56.74. The next day it continued to drop to 55.44. That's a drop of 6.6% in just 2 days of trading, right after an announcment. That's no statistical fluke.
Obviously investors think the original bid was too high. And now Chevron wants an even higher bid? Bad news for Chevron shareholders.