First, hydrocarbons, or compounds that could have been converted into hydrocarbons at the intense pressures of the earth’s depths, must have been a common constituent of the primordial materials out of which the earth was formed. Second, in the four and a half billion years since the earth accreted, the primordial hydrocarbons must not subsequently have become dissociated and fully oxidized to carbon dioxide and water by exposure to the significant amounts of oxygen bound in the rocks of the earth’s crust.