| Description |
Petroleum is considered to originate from marine and terrestrial organic matter (composed principally of hydrogen and carbon) buried along with sediment. The organic molecules are eventually broken by thermal stress encountered during progressive burial. The product of this heating can be liquid and(or) gas, which then moves out of the rock from which it originated and into nearby porous rock (reservoir) where further movement is curtailed by some type of trapping mechanism, such as an impenetrable rock layer. There the petroleum accumulation waits for discovery by exploratory drilling, or perhaps is subjected to additional geologic forces, such as faulting, and once again migrates closer to the surface of the Earth. Proximity to the surface increases the petroleum's susceptibility to interaction with circulating oxygenated waters and(or) bacteria, which may cause radical changes in the chemical makeup of the petroleum deposit and commonly results in heavy oil. Heavy oil makes up 9 percent of oil in the United States. California is the largest producer of heavy oil: 70 percent of California oil production is heavy oil. Heavy oil, sometimes referred to as low-gravity oil, is defined here as oil with an API gravity of 10j to 20j, inclusive. Physical and chemical properties, which differentiate it from higher gravity or light oil include viscosity, pour point, and chemical composition. They affect the producibility and the products that can be refined from it. "Heavy oil" often is applied to petroleum with a sulfur content higher than 2 percent by weight (Speight, 1991). Heavy oil is often produced conventionally, (i.e., California); however, heavy oil is often unconventional in that the characteristics of the deposit may require special recovery methods which affect the cost of production. The recoverability of heavy oil is constrained by many factors. These characteristics include API gravity, transportation required, geological aspects, size of the deposit, depth of the deposit, technology required, and environmental impact. Biodegradation, resulting in heavy oil, ultimately has financial implications for an oil deposit because of the physical changes it imparts to the product. Bitumen in the reservoir can cause production problems by filling the reservoir porosity and preventing natural water drive from maintaining reservoir pressure. Where production of heavy oil is possible, it may be expensive or uneconomic. When all other geologic and production conditions are equal, the reservoir size may be a more important factor for a deposit of heavy oil than it is for a deposit of lighter oil. This is because heavy oil incurs extra expenses in raising it up the well pipe and keeping it liquid during transport. The price of a barrel of heavy oil is $8.00 at this time (1994), (Toal, 1994), several dollars below that of lighter gravity oil. |
