The Case for Early Generation and Accumulation of Oil

Abstract

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.

Timing of oil expulsion from source rocks and a revitalization of the pre-1970 model

“…The model presented for timing of expulsion of oil and lateral migration via horizontal fractures into nearby traps lends support to the pre-1970 model by providing possible explanations to some of its unresolved issues and implies that many of the enigmas associated with today's model (Wilson, 1990;Wilson, 2005) can be put aside, notably the long-distance sub-horizontal (up to 400km) secondary oil migration required to explain the major accumulations in Eastern Venezuela and Western Canada for example (Hunt, 1995).…”

“…Although wells might not be optimally positioned to detect migration route, we would expect to encounter permeable rock with residual oil saturation of 10-20% more often than we do (Wilson, 1990). k) Van Tuyl and Parker (1941) stated that kerogen (organic matter) buried to depths greater than 2km was unable to generate petroleum unless exposed to temperature well above those normally encountered at depths at which oil has been found in commercial quantities (p. 156).…”

“…Likewise, Teas and Miller (1933) concluded that in the Gutoskey lenticular sand zone, Raccoon Bend Filed of Texas expulsion was completed prior to 600m of burial suggesting that the 14° to 34° API oil variability (and other differences) was due to differences in local source rock material (see also Wilson (1990) for a review on early oil expulsion).…”

Potential contributions of extremophiles to hydrocarbon resources in marine extreme environments: A review

“…The abundant organic masses of extremophiles themselves could become good hydrocarbon resources in sediments. The associated macrobiota could also be conserved completely and further transferred into sedimentary organic materials due to less chance of being consumed by other active macrobiota in extreme environments, and finally to undergo the processes of the hydrocarbon formation (Wilson, 1990;Horita and Berndt, 1999;Brasier et al, 2002;Lollar et al, 2002;Wang and Tao, 2005). Although most of these kinds of hydrocarbon resources would disappear after their formation in marine extreme environments, some of original oil inclusions and bituminous residues were found in ancient Archean and Proterozoic strata (Rasmussen and Buick, 2000), which record the original richness of various organics and therefore have a great significance in the exploration of potential hydrocarbon resources in the early history of the Earth.…”

Reconsideration of the pre-1970 model for the timing of oil expulsion from source rocks

“…These differences could, according to Weeks (1956), only be explained by different source materials for the oil. Likewise, Teas and Miller (1933) concluded that, in the Gutoskey lenticular sand zone of the Raccoon Bend field in Texas, expulsion was completed prior to 600 m of burial, suggesting that the 14° to 34° API oil variability (and other differences) was due to differences in local source rock material (see also Wilson, 1990 for a review on early oil expulsion).…”