Figure 1--General scheme of hydrocarbon gas history from its genesis to its sampling. In the source rocks, three different gas steps of formation may be defined: gas generation associated with the oil, late gas generation from the mature kerogen, and secondary cracking of the oil remaining in the source rock. In the reservoir, after expulsion and migration, two other possible gas inputs may be distinguished: secondary cracking of the reservoired oils and bacterial contamination. Segregation due to transport may occur in the source rocks, in the drains, and from the reservoir.