The oceans cover about two-thirds of the earth's surface and huge volumes of sediment and sedimentary rocks occur beneath the ocean basins and their margins Various authors have suggested that about 40% of the world's undiscovered hydrocarbon reserves may be offshore (e.g. Hedberg et al., 1979). However, meaningful estimates of producible deep-water hydrocarbon reserves are probably not possible at the present time, given the great uncertainties in the economic, engineering and geological factors which have to be taken into account in their calculation.
This paper will attempt to summarize the geological factors which could influence hydrocarbon prospectivity in deepwater areas. In essence, these are the same geological factors which control hydrocarbon occurrences in any area-namely source rocks, maturation, reservoir rocks and traps. However, the unique depositional and sedimentological environments which characterize the deep-ocean basins and their margins pose unique problems in their analysis in terms of these hydrocarbon-controlling factors. It is not simply a case of seaward extrapolation of our understanding of land and continental shelf petroleum geology.
In terms of their gross geological character, the oceanic areas can be divided into two main categories, namely ocean basins and continental margins (Fig. 1). The latter category can be further subdivided into active (or convergent) and passive (or rifted) margins. Much of this paper will be devoted to continental margins, and passive continental margins in particular, since these areas appear to have the best hydrocarbon
The deep ocean basins, beyond the foot of the continental slope, account for three-quarters of the world oceanic area (Hedberg et al., 1979) or about 50% of the total earth's surface. However, sediment thickness over almost all of this area is less than 1 km (often less than 500m) and it can be dismissed immediately in hydrocarbon prospectivity terms (e g. Hedberg et a l, 1979; Hedberg, 1983; St. John et al., 1984; Tucholke, 1986) In addition, numerous holes drilled by the Deep Sea Drilling Project (DSDP) and its successor the Ocean Drilling Project (ODP) have shown the sediments of the ocean basins to be generally fine-grained and poor in reservoir quality, very poor in organic carbon, and near flat-lying in attitude, which would prevent migration and trapping of any hydrocarbons formed (Table I, e.g. Hedberg, 1983; Katz, 1986)
Active or convergent plate boundaries occur where one of the earth's lithospheric plates is being subducted beneath another. Such boundaries can occur in a continental margin setting, with oceanic lithosphere being subducted beneath the edge of a continent, or in an intra-oceanic setting, with one oceanic plate subducting beneath another (e g. Dickinson and Seely, 1979). In terms of hydrocarbon prospectivity, active margins are undoubtedly the least well-understood of all deepwater areas. Few studies have been undertaken and conclusions vary widely, although a vague pessimistic attitude predominates.
Fig. 1 Worldwide distribution of passive and active continental margins.(available in full paper)
Table I Geological factors controlling hydrocarbon prospectivityin the major deep-ocean environments(Available in full paper)