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heavy oil and tar sand, crude oils below 20° on the American Petroleum Institute (API) gravity scale that require mining or thermal recovery. Although the lighter conventional crudes are often waterflooded to enhance recovery, this method is essentially ineffective for heavy crudes between 20° and 10° API gravity, and thermal recovery becomes necessary. Heavy crude oils have enough mobility that, given time, they will be producible through a well bore in response to thermal recovery methods. Tar sands, which are less than 10° API gravity and contain immobile bitumen, will not flow into a well bore even under thermal stimulation and thus require mining.
History of use
Discovery
In ancient times the Elamites, Chaldeans, Akkadians, and Sumerians mined shallow deposits of asphalt, or bitumen, for their own use. Mesopotamian bitumen was exported to Egypt where it was employed for various purposes, including the preservation of mummies. The Dead Sea was known as Lake Asphaltites (from which the term asphalt was derived) because of the lumps of semisolid petroleum that were washed up on its shores from underwater seeps.
Bitumen had many other uses in the ancient world. It was mixed with sand and fibrous materials for use in the construction of watercourses and levees and as mortar for bricks. It was widely used for caulking ships and in road building. Bitumen also was employed for bonding tools, weapons, and mosaics and in inlaid work and jewel setting. In various areas it was used in paints and for waterproofing baskets and mats. Artistic and religious objects were carved from bitumen-impregnated sands, and the mining of rock asphalt was an important industry.
Centuries later, during the age of exploration, Sir Walter Raleigh found the famous “Pitch Lake” deposits in Trinidad. The Dutch made similar discoveries in Java and Sumatra.
Potential as a crude oil source
Of the world’s total oil resources, about 21 percent are heavy oils and about 30 percent are tar sands, though not all of these resources are considered recoverable. The development of heavy oil and bitumen reserves is increasing around the world. The increasing volume of cheaper heavy oil in the supply mix has provided an incentive for refiners to upgrade their equipment to process the poorer-quality heavier crudes. The upgrading investments have helped to maintain a demand for heavy oil in spite of the declining price of conventional crudes since the early 1980s. As the demand for heavy oil and crude from tar sands remains strong, heavy-hydrocarbon development projects are being initiated in several parts of the world. In addition, unsuccessful attempts to find new giant conventional oil fields in recent years has caused some producers to turn to the marginally economic heavy hydrocarbons to replace depleted reserves.
Composition and origin
Chemical composition
Geochemical analyses indicate that the heavy hydrocarbons are composed primarily of asphaltenes, resins, and metals (most commonly vanadium and nickel). The nature of individual heavy oil deposits varies widely as they are rarely chemically hom*ogeneous. Bitumen distribution in a deposit also varies, depending on the permeability and porosity of the reservoir rock.
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Formation
Nearly all the deposits of heavy hydrocarbons are degraded remnants of accumulations of conventional oils. Degradation begins when oil migrates toward the surface and encounters descending meteoric water (rainwater or any other water of atmospheric origin) containing oxygen and bacteria at temperatures below 93 °C (about 200 °F). A tarlike material is formed at the oil-water contact, and it eventually invades the entire oil accumulation. A process known as “water washing” removes the more water-soluble light hydrocarbons, particularly the aromatics. Biodegradation preferentially removes the normal paraffins. Heavy hydrocarbon accumulations may represent as little as 10 percent of the original conventional oil. They contain asphaltenes, resins, sulfur, and such metals as vanadium and nickel, which results in an increase in density. These apparently are the residues of a natural concentrating process and were not contributed by other sources. Thus, the deposits were emplaced as medium-gravity crudes, which later became immobilized by degradation in the reservoir. Some of the heavy oils, however, appear thermally immature and therefore may be unaltered.
The geologic environment
Almost all the heavy hydrocarbon deposits have been found in formations of Cretaceous, Paleogene, and Neogene age (about 145 million to 2.58 million years old). The exceptions include some deposits in Alberta, Canada, and in Russia. In Alberta bituminous Paleozoic carbonates unconformably underlie Mesozoic rocks (the Paleozoic Era began about 541 million years ago and lasted until the beginning of the Mesozoic Era, roughly 252.17 million years ago). In Russia most of the heavy hydrocarbons occur in strata dating back to the Paleozoic Era and earlier (i.e., the late Precambrian, which ended about 541 million years ago). Some heavy hydrocarbons are found in Paleogene and Neogene rocks in Central Asia.
The most prolific heavy hydrocarbon reservoir sediments are sandstones that were originally deposited in fluvial and deltaic, nearshore environments. The exceptions are the bituminous carbonate rocks of Alberta, Russia, and Central Asia. Smaller deposits of asphaltic carbonate rocks are common, notably in the Middle East and in Italy. Many heavy oil reservoirs have been found offshore beneath the continental shelves of Africa and North and South America. In addition, heavy hydrocarbons have been discovered beneath the Caspian, Mediterranean, Adriatic, Red, Black, North, Beaufort, and Caribbean seas, as well as beneath the Persian Gulf and the Gulf of Mexico.
