"Petroleum" is a broad term that includes both crude oil and natural gas. A. Petroleum formation is the result of biological as well as chemical activity. B. The difficulty of finding adequate sources of oil on land has resulted in a greater number of offshore drilling sites. C. Petroleum extraction can have a negative impact on the environment. D. Petroleum tends to rise to the surface, since it is lower in density than water. E. Current methods of petroleum extraction enable oil producers to recover about half of the world's petroleum reserves. F. Accidents involving oil tankers occur when tankers run into shore reefs or collide with other vessels.