Exploration
Heavy oils are frequently found at the margins of geologic basins, according to the USGS, and are assumed to be residue from formerly light oil that has lost its light molecular weight components through degradation by bacteria, water flowing through it, and evaporation.

One of the most common methods of locating heavy oil and oil sand deposits is automated 2-D electrical imaging, which plots electrical conductivity variations in the earth. Resulting geo-electrical sections can be interpreted as geological cross sections. This technique is cost-effective for exploration because oil sands are highly resistive.

Reserves
Heavy oils are found around the world, with an estimated 69 percent of the world's technically recoverable heavy oil and 82 percent of the technically recoverable natural bitumen located in the Western Hemisphere. The Eastern Hemisphere, however, contains an estimated 85 percent of the world's light oil reserves.

Reserves are often labeled "technically recoverable" or "non-technically recoverable." This just means that technically recoverable reserves are known or estimated to exist and technologies exist to recover them. Non-technically recoverable heavy oils are those that are known to exist but require more advanced technologies to remove the oil than currently exist.

Among the more notable heavy oil reserves are: Venezuela's Orinoco Heavy Oil Belt; Canada's Athabasca Oil Sands; Russia's Volga-Ural Basin; Brazil's offshore Campos Basin; Alaska's Prudhoe Bay; and China’s Luda field in Bohai Bay.

The largest known extra-heavy oil accumulation is Venezuela's Orinoco heavy oil belt, reports the USGS. The reserve boasts 90 percent of the world's extra-heavy oil when measured on an in-place basis. The Canadian province of Alberta contains 81 percent of the world’s known recoverable bitumen. The two countries’ reserves account for approximately 3.6 trillion barrels of heavy oil and bitumen in place.

Of the 35 billion barrels of heavy oil estimated to be technically recoverable in North America, the USGS estimates that approximately 7.7 billion barrels are assigned to known producing accumulations in the Lower 48 States, and 7 billion barrels are assigned to the North Slope of Alaska.