The time of inception and duration of oil expulsion from source rocks and its accumulation in trapped reservoirs remains a controversial problem. Many geochemists advocate that oil is expelled from source rocks only when crude oil-like hydrocarbons are formed in source rocks through the heat and pressure imposed by deep burial. Conversely, many experienced petroleum geologists point out that empirical data on the habitat of oil accumulations call for early, short-lived expulsion of hydrocarbons shortly after the burial of source and reservoir sediments.

Because structural and stratigraphic complexities in the subsurface generally prohibit unequivocal interpretation of origin and movement of hydrocarbons from source to reservoir, it is suggested that special attention be given to unfaulted lenticular traps which limit alternative interpretations to the minimum.

In a lenticular trap, geologic circumstances permit designation of enclosing pelitic sediments as source rock, and this allows the calibration of bona fide source-rock characteristics. Lenticular traps which never have been buried deeply and possess no avenues for long-distance migration demonstrate that hydrocarbons can be expelled and matured without the heat and pressure that geochemists advocate.

Maturation of hydrocarbons takes place with time and depth of burial in source rocks and entrapped accumulations alike. These circumstances do not necessitate late expulsion and can be satisfied by advocating early expulsion of proto-oil, which then takes the same maturation path in a reservoir as that which it follows in the source rock; the point of separation is early, and the catalytic effects on proto-oil in the reservoir then run parallel with those on oil remaining in the source rock, later expulsions being prevented by drastic losses in permeability through lithification, compaction, and diagenesis.

Late expulsion of hydrocarbons during the prolonged catagenic sequence in source rocks is believed by absence of accumulations in late traps, and because deep accumulations which should have benefitted from a longer expulsion sequence than shallower traps do not in fact provide supporting volumetric evidence.

Resolution of the time-of-expulsion controversy becomes increasingly important because of the role which advocation of late or early migration plays upon exploration-prospect assessment. As hydrocarbons become ever more difficult to discover, every effort must be made to sharpen the explorationist's analytic tools, not least by reducing uncertainties regarding basic petroleum-geologic problems.