石油是由1，000萬(wan)至6億(yi)年前古代海洋里死(si)亡的(de)微小動植物（浮游生(sheng)物）殘體形(xing)成(cheng)的(de)。這些生(sheng)物死(si)后，便會沉入海底的(de)沙里或泥里。

隨著(zhu)歲(sui)月流(liu)逝，生(sheng)物(wu)(wu)有(you)(you)機(ji)(ji)體在沉(chen)積層(ceng)(ceng)(ceng)內腐爛了。這些(xie)地層(ceng)(ceng)(ceng)內的氧氣(qi)很少(shao)或根(gen)本就(jiu)沒有(you)(you)氧氣(qi)，因(yin)此(ci)殘(can)體被(bei)(bei)微生(sheng)物(wu)(wu)分解(jie)為富碳化(hua)合(he)物(wu)(wu)，最(zui)終形(xing)成(cheng)有(you)(you)機(ji)(ji)層(ceng)(ceng)(ceng)。這些(xie)有(you)(you)機(ji)(ji)物(wu)(wu)質(zhi)與沉(chen)積物(wu)(wu)混和，形(xing)成(cheng)了細(xi)密的頁巖(yan)(yan)(yan)(yan)或源巖(yan)(yan)(yan)(yan)。隨著(zhu)新(xin)的沉(chen)積層(ceng)(ceng)(ceng)不斷沉(chen)積，源巖(yan)(yan)(yan)(yan)被(bei)(bei)施加了巨大的壓(ya)力和熱(re)量(liang)，這些(xie)熱(re)量(liang)和壓(ya)力使(shi)得(de)有(you)(you)機(ji)(ji)物(wu)(wu)質(zhi)成(cheng)為了原油(you)和天然(ran)氣(qi)。石油(you)從源巖(yan)(yan)(yan)(yan)內流(liu)出，積聚在厚度更(geng)高、孔隙更(geng)多(duo)的石灰巖(yan)(yan)(yan)(yan)或沙巖(yan)(yan)(yan)(yan)（稱為貯油(you)巖(yan)(yan)(yan)(yan)）中。地殼運動使(shi)得(de)石油(you)和天然(ran)氣(qi)被(bei)(bei)截留在不滲透巖(yan)(yan)(yan)(yan)層(ceng)(ceng)(ceng)或蓋巖(yan)(yan)(yan)(yan)（例如(ru)花(hua)崗石或大理石）之間的貯油(you)巖(yan)(yan)(yan)(yan)內。