Source - Oil only forms in marine sedimentary rocks with high organic content, namely shale. There must have been high biologic productivity in the ancient sea and it must be preserved. Trap - Oil migrates through reservoir rocks until it reaches a point where it can travel no further. Such traps include anticlines and domes, faults, old coral reefs, and salt domes. Well - An average of one well in ten drilled in known oil fields finds oil.
Primary production consists of drilling a well and pumping out the available petroleum.
Secondary recovery techniques prolong a well's productivity once primary production runs out by forcing more oil out by water injection. It requires a community effort between all the well owners operating over the same structural trap when secondary production is economically feasible. Well owners over low parts of the structure pump water into the reservoir rock, floating oil up through the pore space to where well owners over high parts of the structure pump it out. World use is 56 million barrels per day. At that rate, the known reserves will last for about 35 years.