Abstract The Afar Triangle offers an example of a passive continental slope which is undergoing formation at the present time. Its structural style of down-to-the-rift and antithetic faulting has produced a horst and graben topography and an interspersing of blocks of acidic and basic crust. Structural lows are filled with evaporite and clastic deposits while highs are often capped with carbonates. Basement structure and stratigraphy of the mature analogue, the United States Atlantic passive continental slope, is strikingly similar although it has undergone further considerable evolution in space and time. The structural style and sedimentary patterns provide numerous examples of good potential reservoir rocks and traps in both instances. These have been further enhanced on the Atlantic example by development of major reefal facies and through progradational and erosional patterns. What is lacking for an accurate assessment of hydrocarbon potential are easily identifiable source beds of sufficient magnitude and quality. The resulting question mark of the presence of economically viable deposits of hydrocarbons on the United States Atlantic margin points out the danger of formulating national policy on estimates of reserves in undrilled frontier areas based primarily upon sediment volumes. These estimates are not taken seriously by professional exploration people. The proper approach is to initiate an agressive drilling policy consistent with reasonable ecological considerations. Only in this way can a rational assessment of our national potential hydrocarbon position be determined.

Abstract It is commonly impossible to distinguish structure due to deposition and erosion from tectonic structure. Moreover, stratigraphic agents producing trap limits are usually dependent on tectonic influences. It is thus impractical to contrast stratigraphic versus structural traps with the intention of searching for one type and not searching for the other. This is most clear in the case of carbonate traps where tectonism controls development of both erosional and depositional structure and affects depositional and diagenetic facies distribution. In the Florida-Bahamas carbonate province, intraplatform straits and basins are sites of negative residual Bouguer gravity anomalies. A correctable refractor near the top of Lower Cretaceous rocks is depressed in these same areas. Thus, present topographic lows overlie structural lows in the platform’s foundation. Similar relations are indicated for the Tampico-Tuxpan and Scurry reef platforms and are markedly evident in the Central Basin platform and the Leduc-Rimbey trend. This relation is a potentially useful one, because the geophysical anomalies reflecting the structures which control the position of the platforms commonly exceed those stemming directly from carbonate masses. Depositional and, to a degree, diagenetic facies have consistent topographic settings in both recent and ancient platforms. Calcarenites predominate at the edge; calcilutites and evaporites are most common in the platform interiors. Bases of platform-edge slopes are typically sites of deposition of allochthonous shallow-water sands mixed with coarse rubble containing balls of pelagic mud. Elevated edges commonly are leached and dolomitized, and dolomite is present in many places within the platform-interior evaporites. Great quantities of hydrocarbons have been found in the leached and dolomitized platform edges, in porous and permeable platform-interior dolomites, in dolomitized conglomerates bordering bases of platform slopes, and in fractured reservoir rocks in adjacent basin facies. The requirements for an oil field are structure, reservoir, seal, and a commercial quantity of hydrocarbons. Geophysical tools are best suited to discern structure. Velocities and reflection character also provide some insight to lithologic variations. Outcrop and subsurface studies enable mapping of distribution of reservoirs and seals. Slabbed cores from ancient carbonate rocks reveal sedimentary structures identical with those observed in recent carbonate units. Thus, study of modern carbonate deposits is a valuable aid in interpretation of rocks, and slabbed cores are essential for a detailed understanding of carbonate depositional and diagenetic history. Temperature and hydrocarbon-generating history of source beds can be discerned from the nature of organic matter remaining after oil and gas are gone. Knowledge of this relation enhances the ability to predict types of hydrocarbons to be encountered in a given region. Prospects for testing must be chosen on the basis of areal extent of structure and the regional distributions of reservoirs, seals, and hydrocarbons. Finally, management and backers should be prepared to drill two or three evaluation wells following the completion of a successful wildcat in a carbonate reservoir.