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Definition of Heavy Oil and Natural Bitumen

Richard F. Meyer

Definition and categorization of heavy oils and natural bitumens are generally based on physical or chemical attributes or on methods of extraction. Ultimately, the hydrocarbon's chemical composition will govern both its physical state and the extraction technique applicable. These oils and bitumens closely resemble the residuum from whole-crude distillation to about 1,000°F; if the residuum constitutes at least 15% of the crude, it is considered to be heavy. In this material is concentrated most of the trace elements, such as sulfur, oxygen, and nitrogen, and metals, such as nickel and vanadium. A widely used definition separates heavy oil from natural bitumen by viscosity, crude oil being less, and bitumen more viscous than 10,000 cp. Heavy crude then falls in the ange 10°-20° API inclusive and extra-heavy oil less than 10° API. Most natural bitumen is natural asphalt (tar sands, oil sands) and has been defined as rock containing hydrocarbons more viscous than 10,000 cp or else hydrocarbons that may be extracted from mined or quarried rock. Other natural bitumens are solids, such as gilsonite, grahamite, and ozokerite, which are distinguished by streak, fusibility, and solubility. The upper limit for heavy oil may also be set at 18° API, the approximate limit for recovery by waterflood.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.