THE DISTRIBUTION OF MAJOR OIL AND GAS RESERVES IN REGIONAL BASIN STRUCTURES – AN EXAMPLE FROM THE POWDER RIVER BASIN, WYOMING, USA

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.