Precisely. :^) Ages ago, or rather, even ages ago, when I was in college, one of the econ profs (he was a big deal back then, although he only worked in the Carter White House) mentioned in lecture that cartels don't normally last as long as OPEC had by then (it was formed around 1960 I think), and for their duration they tend to "leak" because members want to cheat, whether they are national entities or individual companies.
OPEC was formed in the 1950's. It's purpose was to break the Anglo-American cartel known as the Seven Sisters. It was completely successful in that endeavor. Afterwards, it suffered the same problems all cartels suffer, mainly internal cheating. For instance, during the oil embargo the Iraqi oil minister, Saddam Hussein, made an under the table deal to sell oil to the US.