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Within the vast unconventional resources category, there are several different types of hydrocarbons, including very heavy oils, oil sands, oil shales, and tight oils. By the early 21st century, technological advances had created opportunities to convert what were once undeveloped resource plays into economic reserves. Very heavy crudes have become economical. Those having less than 15° API can be extracted by working with natural reservoir temperatures and pressures, provided that the temperatures and pressures are high enough. Such conditions occur in Venezuela’s Orinoco basin, for example. On the other hand, other very heavy crudes, such as certain Canadian crude oils, ...(100 of 9031 words)