Geologic Insights Gained from Studying the Total Petroleum Systems of the World

By

 Thomas S. Ahlbrandt¹

(1) U.S. Geological Survey, Denver, CO

 Challenging insights were gained by analyzing the 149 Total Petroleum Systems and their constituent 246 Assessment Units that contain more than 95 percent of the known conventional hydrocarbon resources during the recent USGS World Petroleum Assessment 2000. Ten significant insights related to the elements of the petroleum systems are: 1) Petroleum is trapped in many ways, less that half of known petroleum occurs in exclusively structural traps. 2) Type II source rocks are by far the dominant source rock type and source rocks occur throughout the sedimentary rock record. 3) Mesozoic source rocks (particularly Jurassic- Cretaceous) are the most important volumetrically. 4) Young Cenozoic petroleum systems are volumetrically dominant, and much petroleum has clearly been lost from older petroleum systems. 5) The key elements of petroleum systems are cyclic. 6) Despite enormous recent success with deepwater reservoirs, volumetrically they are currently the least significant of those considered; continental reservoirs are dominant. 7) Future discoveries will be dominantly from clastic reservoirs. 8) Salt is a very effective long-term seal, and salt seals are a critical preservational component of older Paleozoic petroleum systems. 9) Most of the petroleum systems in the world are dominated by vertical migration or limited lateral migration (less than 20 kilometers) from the mature source rock area. 10) Many major conventional natural gas systems are closely linked to large unconventional (continuous) resources.