The Assessment of Heavy Crude Oil and Bitumen Resources
Heavy oils and bitumens can be separated into three categories for assessment purposes: identified bitumen deposits, identified heavy oil deposits, and undiscovered bitumen and heavy oil deposits. Bitumen-bearing rocks and many heavy oil reservoirs are devoid of natural reservoir energy that would enable primary recovery, thus assessments are essentially made for oil in place. Resource estit-mates are volumetric calculations based on extent and thickness of the reservoirs and bitumen or heavy oil saturations. Primary recovery, however, enables the calculation of recovery factors and reserves. The economics of many phases of heavy oil recovery, transportation, and refining ultimately effect the reserve estimates.
Most heavy oils and bitumens probably represent the degraded remnants of conventional petroleum deposits that were 50-90% larger, a concept important in assessing undiscovered heavy oil and bitumen deposits. To predict accurately the occurrence of bitumen and heavy oil, assessments need to involve the formulation of models for oil generation, expulsion, migration, and degradation. Some assessments of undiscovered or poorly known heavy oil deposits simply use the geologic characteristics of known deposits to predict the presence and amount of undiscovered heavy oil* Others use the proportion of cumulative conventional oil production that was heavy to make rough estimates of reserves and undiscovered resources. The amount of undiscovered bitumen in most countries is probably extremely low, because most bitumen deposits crop out or were located during the search for conventional petroleum.
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There is a continual need to update estimates of oil and gas resources remaining to be discovered, and also to refine the methodologies for making these assessments. In 1974, AAPG sponsored a research conference dealing with the above topics, and many of the papers presented there were published in AAPG Studies in Geology 1. As a follow-up to that volume, a U.S. Geological Survey workshop was held in 1983, and many papers from talks presented there, in addition to several other papers, are contained within this volume. The 22 papers have been grouped into two types: those describing methodologies for evaluating resources and those presenting assessments of both conventional and unconventional resources.