Oil fields of the world with recoverable reserves more than 500 million bbl and gas fields with reserves more than 3.5 Tcf are analyzed to determine what characteristics they have in common and, on the other hand, reasons why some of these fields have unusual characteristics.

Giant hydrocarbon accumulations require that there be a giant trap, formed more or less concurrently with generation of the hydrocarbons from the organic source materials, and abundant source beds rich in organic matter. Marine sediments are dominant as source beds, but nonmarine beds also preserved the organic materials which supplied the hydrocarbons to many giant fields. Although argillaceous sediments generally trapped more organic matter, carbonates also can be sources.

The reservoir rocks of most giant fields are very porous and permeable, but there are notable exceptions; in the exceptions, total reservoir volume compensates for deficiency in reservoir quality. The reservoir rocks must be interconnected with channels of migration, or carrier beds, from the source beds. An effective seal must be present to prevent escape of hydrocarbons from the reservoir; the most efficient seal is provided by evaporites. Evaporites, in addition to sealing many important reservoirs, also may be the primary agent responsible for the development of the structures of many large fields--generally through either diapirism or decollement. Unconformities have had an important role in some fields in aiding trap development and in bringing carrier beds into juxtaposition wi h others through which hydrocarbons may migrate. Giant traps caused by lateral facies changes, changes in reservoir matrix, postdepositional diagenetic changes, and paleogeomorphic factors are few, but their absence does not mean that they are scarce; most exploration has been directed toward the structural types of trap, and searches for other types of traps have been few.

Though most giant-field reserves are in Mesozoic and Tertiary rocks, there is no preferred age of source beds or reservoir rocks. The important factor is the time when, during the sedimentational cycle of a basin, the largest amounts of organic matter were buried and preserved. It makes little difference if this time was Cambrian or Pleistocene.

Higher than normal geothermal gradients probably resulted in greater efficiency of hydrocarbon generation in certain basins.

The sources of gas include the same types of materials that generated liquid oil, but they also include other materials of vegetal origin that do not contribute significantly to the formation of oil. Hence gas can be derived from a greater variety of source materials. Volumes of gas and oil generally are inversely related to increasing depth. Downward increase of temperature results in the "phasing out" of oil and the dominance of gas. Liquids disappear and are replaced by gas in the range 5,640-8,380 m (18,500-27,500 ft); the actual depth depends on the geothermal gradient present in the field under study.

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The depths given are not necessarily the present over-burden thicknesses; many reservoirs are affected by weight of overburden of an earlier geologic time, which today may no long exist because of subsequent uplift and erosion.

We present for consideration a basin classification having different spatial relations with cratons and continental borders, and having different evolutionary development. The type of basin having the largest number of giant fields is the basin which is, or was, a downwarp toward an oceanic area. Intermontane basins, developed either as geosynclinal-type basins between rising geanticlines or in transverse downwarps, generally are smaller in size, but many such basins contain clusters of very large fields. Interior cratonic areas exposed to long periods of erosion and tectonic and/or epeirogenic activity also contain numerous large fields, but a smaller percentage of giant fields.

During the past 15 years, deeper drilling and exploration of new basins have led to the discovery of an increasing percentage of Mesozoic vs. Tertiary giant fields. The greatest future prospects for discovery of giant fields are (1) in the continental shelves of the world; (2) onshore in Asia; (3) unexplored basins containing mainly continental strata; and (4) all areas of the world where deliberate exploration for obscure traps has not been carried out--i.e., stratigraphic, unconformity-associated, and paleogeomorphic traps.