Organic rocks were formed from ancient marine sediments containing the bodies of plankton and other organic debris. Under high temperatures and pressures those sediments turned to rock. Some organic rock - like oil shale - contains a high percentage of carbon.
But even these carbon-rich rocks are not an economically useful fuel. Instead oil companies rely on a quirk of geology. Once formed, oil can ooze out of the rocks it formed in. It migrates upward through tiny cracks and fissures until it eventually reaches the impermeable rock. Trapped beneath the rock the oil slowly accumulates.
Where do you find the carbon?
Rock containing organic carbon, and fossil fuels
What form of carbon?
How long will the carbon remain?
Tens- to hundreds- of millions of years
How much carbon is there?
About 10 million x 10 12 kg