It's got a "bathtub" shape that reflects the initial lack of extraction technology, then a long period of gradually declining price (in constant dollars), and then a sharp increase that starts in about 1970. Even ignoring the wild price swings in the 1970s caused by OPEC price manipulation, it looks pretty clear that by then the costs were starting to increase.
Really, I'm clueless with this stuff, so a nice little basic graph like that is where it's at :)
