Complex sentence: Heavy oils and oil sands occur world-wide, but the two most important deposits are the Athabasca Oil Sands in Alberta, Canada and the Orinoco extra heavy oil deposit in Venezuela.
Split into simpler sentences: Heavy oils and oil sands occur world-wide. The two most important deposits are the Athabasca Oil Sands in Alberta, Canada and the Orinoco extra heavy oil deposit in Venezuela.