The document discusses formations, injection drilling, and different rock types found in formations. It defines a formation as a number of rock strata with similar properties, and notes they can vary widely in thickness. Formations are typically named for where first described. It also discusses three main types of sedimentary rock - limestone, sandstone, and shale. Source rocks are organic-rich sediments capable of generating hydrocarbons. Main formation types include crude oil reservoirs and non-conventional reservoirs like shales and sands. Basins are collections of formations that hold oil and gas reservoirs. Injection wells, which make up 30% of oil extraction, use gas and water injection to maintain reservoir pressure and increase oil recovery.