THE CASE FOR EARLY GENERATION AND ACCUMULATION OF OIL

Uncertainties remain concerning the common assumption that economic oil pools result only from deep, catagenic oil generation. These uncertainties stem from the many geological criteria that point to early oil entrapment and, furthermore, from the failure to resolve problems of oil migration out of, and through, consolidated sediments. Early oil emplacement is indicated by the preferential charging of paleostructures, inhibition of diagenesis and compaction by reservoired oil, folded oil‐water contacts, and by evidence supporting the immaturity of huge heavy‐oil deposits. The uncemented and uncompacted nature of the Athabasca “tar sands”, the perfect preservation of fossil wood within them, and the tilted oil‐water contacts at the Athabasca, Peace River and Cold Lake accumulations, support geological deductions of very‐early oil emplacement, and geochemical criteria for its immaturity. If such huge volumes of oil are immature, this would be in harmony with geological observations which conclude that pools of mature oil most probably result from in‐reservoir maturation of early‐expelled, biogenically‐generated heavy oil and methane. Hydrocarbons that remain in source rocks are maturated during burial, but are immobilised by progressive loss of effective permeability. Copyright © 1990, Wiley Blackwell. All rights reserved