Experts estimate that ten times as much petroleum exists in sources like tar sands, heavy oil, and perhaps even in shale than in conventional reservoirs.
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Asources like tar sands, heavy oil, and perhaps even in shale than
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Bsources like tar sands, heavy oil, and perhaps even in shale than are
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Csuch sources as tar sands, heavy oil, and perhaps even in shale as are
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Dsuch sources as tar sands, heavy oil, and perhaps even shale as
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Esuch sources as tar sands, heavy oil, and perhaps even shale than
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