Heavy crude oil
or Extra Heavy oil
is any type of crude oil
which does not flow easily. It is referred to as "heavy" because its density
or specific gravity
is higher than of light crude oil
. Heavy crude oil
has been defined as any liquid petroleum with an API gravity
less than 20°,
meaning that its specific gravity is greater than 0.933.
Production, transportation, and refining of heavy crude oil present special challenges compared to light crude oil. The largest reserves of heavy oil in the world are located north of the Orinoco river in Venezuela, the same amount as the conventional oil reserves of Saudi Arabia, but 30 or more countries are known to have reserves. Heavy crude oil is closely related to tar sands, the main difference being that tar sands generally do not flow at all. Canada has large reserves of tar sands, located north and northeast of Edmonton, Alberta.
Physical properties that distinguish heavy crudes from lighter ones include higher viscosity and specific gravity, as well as heavier molecular composition. Extra heavy oil from the Orinoco region has a viscosity of over 10,000 centipoise and 10° API gravity . Generally a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow.
Some petroleum geologists categorize bitumen from tar sands as extra heavy oil although bitumen does not flow at ambient conditions.
Heavy crude oils provide an interesting situation for the economics of petroleum development. The Resources of Heavy oil in the world are more than twice of those conventional light crude oil. On one hand, due to increased refining
costs and high sulfur content, heavy crudes are often priced at a discount to lighter ones. The increased viscosity and density also makes production more difficult (see: reservoir engineering
). On the other hand, large quantities of heavy crudes have been discovered in the Americas including Canada
and Northern California
. The relatively shallow depth of heavy oil fields (often less than 3000 feet) contributes to lower drilling
Heavy oil is asphaltic
. It is "heavy" (dense and viscous) due to the high ratio of naphthenes
). Heavy oil has over 60 carbon atoms and hence a high boiling point and molecular weight. For example, the viscosity
of Venezuela's Orinoco extra-heavy crude oil lies in the range 1000-5000 cP
, while Canadian extra-heavy crude has a viscosity in the range 5000-10,000 cP, about the same as molasses, and higher (up to 100,000 cP for the most viscous commercially exploitable deposits).
A definition from the Chevron Phillips
Chemical company is as follows:
The "heaviness" of heavy oil is primarily the result of a relatively high proportion of a mixed bag of complex, high molecular weight, non-paraffinic compounds and a low proportion of volatile, low molecular weight compounds. Heavy oils typically contain very little paraffin and may or may not contain high levels of asphaltenes.
As a rule, heavy crudes have a more severe environmental impact than light ones. With more difficult production comes the employment of a variety of enhanced oil recovery
techniques, including steam flooding and tighter well spacing, often as close as one well per acre. Heavy crudes also carry contaminants. For example, Orinoco extra heavy oil contains 3.5% sulfur
as well as vanadium
Heavy crude oils contain more carbon in relation to hydrogen, thus releasing more carbon dioxide (a greenhouse gas
) per amount of usable energy when burned.
Advanced technologies are mitigating the environmental impact via horizontal wells and increased energy efficiency, but, barrel for barrel, heavy crudes will likely always be more environmentally damaging than light crudes.
Most geologists agree that crude becomes "heavy" as a result of biodegradation
, in which lighter ends are preferentially consumed by bacterial activity in the reservoir, leaving heavier hydrocarbons behind. This hypothesis leans heavily on the techniques of petroleum geochemistry