Giant oil accumulations and their areal concentration efficiency

Abstract

The ratio of the areal concentration (barrels of oil equivalent;km2) of accumulated oil in the richest sedimentary basin to that in the poorest basin, among those basins having giants and supergiants, is more than 500 to 1. On the contrary, the ratio of the richest to the poorest petroliferous basin in terms of the total organic concentration may be less than 20 to 1. Such a large discrepancy between these two ratios suggests that the organic concentration alone cannot account for oil concentration in commercial reservoirs, because significant quantities of oil may have been lost to the surface and also disseminated throughout the sedimentary sequence in the geological past. Most of the disseminated oil cannot be produced economically by the present-day technology. For a better petroleum assessment, the author stresses the importance of the petroleum accumulation! preservation efficiency in commercial reservoirs; this may be evaluated as combining, 1) the organic concentration of the source rock, 2) the duration of oil generation, 3) the thickness of the oil-generating window, 4) the total geologic age involved, and 5) the fluid expulsion efficiency and the presence or absence of undercompacted intervals. Other parameters such as the trap, reservoir and cap rock, and also the types of organic matter in the source rock, would have a great influence on the efficiency of oil concentrations in commercial reservoirs as well. However, most of them are related to the geologic conditions of each specific area or basin, and thus are more difficult to generalize than those mentioned above in the context of the global distribution and concentration of oil reservoirs. Petroleum reserves of two sedimentary basins from different parts of the world, whose geologic settings and reservoirs and source rock potentials resemble to each other, can be greatly different from each other. In such a case, a simple volumetric method based on thickness and areal distribution of matured source rocks will not provide a reasonable oiVgas reserve figure, because for one thing the expulsion efficiency of generated petroleum differ from area to area. Difficulties are not only on the expulsion efficiency of hydrocarbons, but also on the assessments of hydrocarbons dispersed in source and low-grade reservoir rocks, as well as hydrocarbons lost to the surface. Even if the geologist is lucky enough to be able to establish a volumetric relationship among them in a basin or area, he would definitely face a serious difficulty in applying the same relationship to another basin in a different part of the world with a similar or different geologic setting. In summary, petroleum is extremely unevenly distributed in the world. According to McDowell's (1975) study of four major sedimentary basins of the world, the percentage of reservoired oil compared to the total volume generated in source rocks varies so widely: only about 2% in the West Siberian Basin, about 3% in the Permian Basin, west Texas, about 9% in the Arabian Gulf Basin and about 30% in the Los Angeles Basin, U.S.A. This study suggests that a