The location and geographic distribution of oil and gas reserves within a producing basin often indicate that a remarkable concentration of reserves has taken place in a small area. The concentration of reserves is the result of secondary hydrocarbon migration, itself governed by the basin‐wide regional structure which was present at the time of migration. These factors permit the definition of the most prospective areas of a new basin or play at an early stage in the exploration cycle. Basic geochemical data are required for the definition of critical details of the effective generative depocentre, and regional geological‐geophysical data are required to establish regional structures near particular source‐bed levels, or near top Basement as the deepest prospective level. Other data commonly utilized in the early stages of exploration, such as basin classification schemes, reservoir bed studies and tectonic analyses, are rather less important in forward basin evaluation. A particularly clear example of the importance of the regional basement structure in the concentration of oil reserves exists in the Powder River Basin, Wyoming, USA. Cumulative oil production here was over 1.9 B brls** in 1984 from more than 460 oilfields and pools. However, 1.5 B brls (79% of the basin output) were produced from only 22 fields (4.7% of the number of fields and pools in the basin). It is also remarkable that all 22 major oilfields in the Powder River Basin are located on positive Bouguer gravity anomalies, which are expressions of deep basement irregularities. Regional lateral oil migration in the Powder River Basin appears to have utilized routes resulting from Laramide deformation (Late Cretaceous ‐ Paleocene). The resultant basin structure was accentuated by regional structural‐topographic anomalies, which became sites of major structural and stratigraphic traps. In this case regional gravity maps seem to show sufficient details of the deep structure for the accurate definition of major exploration targets. The Powder River Basin is thus an excellent example of the importance of regional geological and geophysical reconnaissance data aimed at, and available for, the definition of major migration pathways at an early stage of hydrocarbon exploration. When combined with geochemical data, such an approach will become the basis for forward exploration in all known basins.

The observable concentration of the major deep gas accumulation areas in the NW German Basin is the result of focused secondary migration. Gas generated in several effective depocenters (regional structural lows) from Late Carboniferous source beds migrated into adjacent regional structural highs. The preferred migration paths are qualitatively predictable on the basis of present basin geometry. Optimal conditions for gas accumulations exist where the presently trapping regional highs have also been sites of favorable reservoir development. On‐going and future deep‐gas exploration efforts in the basin can be regarded as quite hopeful.

In all producing basins, most of the oil and/or gas reserves are concentrated in a minority of fields. This distribution of oil and gas reserves is the result of “focusing” during lateral and vertical basin‐wide migration. Although the physico‐chemical and geological processes involved in hydrocarbons migration are not yet fully clarified, lateral migration directions can be established by integrating geological, geophysical and geochemical data. The Petroleum Systems concept is of use here. When combining Petroleum Systems data with well‐controlled regional structure maps, areas of hydrocarbons concentration (i.e. migration focusing) can be defined as areas of the highest exploration potential. Some basic petroleum‐geological data are reviewed from the Eastern Venezuela Basin and the Llanos Basin region in Colombia. By using published Bouguer gravity maps as regional structural form‐line maps, areas of preferred lateral migration are shown to be areas of maximum hydrocarbon accumulations. This leads on to an exploration approach in which Petroleum Systems data and regional (gravity or other) structure maps are used to evaluate acreage for its exploration value with a minimum expenditure of time, manpower and related funds.

The PreRif Basin in northern onshore Morocco may contain major oil and gas reserves. This foreland basin is located between the Tertiary‐Recent Rif Orogen to the north and the stable Moroccan Meseta to the south; for the purposes of this paper, it includes the Rharb Basin, an area of strong Neogene subsidence which extends offshore to the west. Prospective sandstone and carbonate reservoirs are present at a number of stratigraphic levels, ranging in age from Triassic and Jurassic to Miocene and Pliocene. Lower and Middle Jurassic shales and carbonates constitute potential source beds for “thermal” oil and gas; biogenic gas is generated within Miocene/Pliocene shales. Potential source beds are expected to have reached thermal maturity in the deepest portions of the present basinal area since the Late Miocene. Seals are provided by intraformational shales, transgressive shales of Tertiary age, Triassic evaporites, and by a thick, complex glidemass (or “olistostrome”) of late Miocene (Tortonian) age. Multiple traps can be expected; they will include anticlines, fault closures, salt‐tectonic features, and structural‐stratigraphic traps. Gravity‐defined low areas in the northern portion of the basin are probably the main generative depocentres. Lateral oil and gas migration has occurred on a regional scale since the Late Miocene, and has continued to the present day. A Jurassic Petroleum System includes the surface oil seeps and shut‐in oilfields of the Sidi Cacem area along the basin's southern flank. A Miocene Petroleum System supplies gas production in the Rharb Basin. The prospective area covers some 15,000 sq. km. Available reconnaissance data include published gravity, magnetic, seismic, and surface‐geological surveys. Modern airborne surveys of prospective areas should be followed by detailed exploration, including 3‐D seismic surveys. New recoverable field reserves may lie in the range of 50 to 150 million barrels oil‐equivalent per discovery; several new discoveries can be expected, leading to new total reserves possibly totalling 0.5 billion barrels oil‐equivalent.

Continental margins, the border zones between continents and oceans, are preferred sites for hydrocarbon accumulations throughout geologic history. Passive Atlantic‐type margins are found worldwide. Active Pacific‐type margins exist together with passive margins where endogenous vertical crustal mobility took place. Vertical mobility is preferred over lateral mobility. Plate‐tectonic hypotheses are doubted. Exploration for gas and oil must be based on sound data. Kontinentalränder, die Grenzen zwischen Kontinenten und Ozeanen, sind durch die geologische Geschichte hindurch bevorzugte Räume für Ansammlungen von Gas und Öl. Passive atlantische Kontinentalränder sind weltweit verbreitet. Aktive pazifische Kontinentalränder erscheinen zusammen mit passiven Kontinentalrändern, wo endogene vertikale Krustenmobilität auftritt. Vertikalmobilität wird einer Lateralmobilität vorgezogen Plattentektonische Hypothesen werden in Zweifel gestellt, sie werden ohnehin zunehmend abgeändert. Exploration auf Gas und Öl muss auf einer gesunden Datenbasis beruhen, nicht auf wissenschaftlicher Spekulation.