The dearth of oil-prone marine source rocks (organic facies B) in the Tertiary-Mesozoic deposits underlying the Holocene shelves is a primary reason for the low success ratio when exploring for oil in these deposits. The predominant organic facies are C, which is gas-condensate prone and is usually dominated by terrestrial organic matter (OM) deposited at an oxic sediment-water interface, and D, which has a negligible generating capacity and is either dominated by highly oxidized OM from any source or by reworked OM deprived of its liquid generative capacity in a previous thermal event. The east and southeast coasts of the United States illustrate this problem in both a clastic and carbonate realm.

Oil-prone source rocks are rare under the shelves, but do exist. They are apt to be rather restricted in areal extent and reflect rather local conditions that permitted the development of anoxia at the water-sediment interface. These conditions are preferentially achieved during the structural developments that occur in the rifting stages. The lacustrine source beds that are the source of much oil in offshore Brazil, Gabon, and Cabinda are prime examples. Might such organic facies exist elsewhere under thick sedimentary piles? As shown by the Kimmeridgian in the North Sea, marine transgressions during the rifting stage are clearly an opportunity for the deposition of oil-prone marine source rocks (organic facies B). Although drifting usually ultimately leads to oxic deposition in an o en ocean, anoxic conditions can be preserved at least episodically for a long time as indicated by the Cretaceous of the Atlantic. The correct projection of the organic facies and maturation of the Cretaceous "black shales" under the continental shelves will clearly be rewarded.

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