Oil and gas formation begins with the accumulation of organics on the sea-floor; these are the dead remains of organisms living in the water column, such as microscopic plankton, which rain down on the sea floor below. This will only occur in rather unusual settings, where the sea floor is stagnant such that there is no oxygen present to break the organic remains down and no sea-floor dwelling organisms present that might feed on the organics. A high sediment accumulation rate of may also help to bury the organics before the action of decay can break them down. As the sediment pile becomes deeper the organics within it are subjected to heat and pressure which leads to formation of oil and then gas. For oil and gas extraction, it is important that the source rock is not 'over-cooked' or the hydrocarbons will be destroyed. There must be suitable reservoir-rock, such as a porous sandstone, into which the hydrocarbons can migrate and accumulate. This must be overlain by an impervious cap-rock, such as a clay, which prevents the hydrocarbons from escaping to the surface. Finally, the geometry of the reservoir and cap-rock bodies must be such that the hydrocarbons become trapped; usually folding will suffice.

Coal typically forms on land from vegetation in lowland, swampy, mire environments. Stagnant waterlogged soil prevents the accumulated plant debris from breaking down. The recognisable remains of plants are often visible within coals and associated shales, confirming their plant-origin. The picture above shows a piece of coal containing a network of fossilised fern leaves - clear evidence that it was formed from vegetable remains.