As the easiest, most obtainable oil will get pumped out, the remaining oil is tougher to reach, dearer to make, and sometimes demands new systems or approaches. This is just a resource depletion difficulty. High quality: WTI is taken into account a "light sweet" crude oil, that's easier to refine, https://directoryhere.com/listings767357/about-book-a-massage-to-your-hotel