The rising temperature that accompanies increasing burial depth converts oil in a reservoir into thermal gas. A consideration of hydrogen balance during this cracking shows that approximately 3000 cubic ft (85 cubic meters) of gas (at standard temperature and pressure) is generated from each barrel of oil. In addition, a graphitic residue is precipitated. If the volume relationships among oil, thermal gas, and the graphitic residue are combined with data for gas solubility in pore water and gas nonideality (Z factor), then pressure can be calculated for any degree of thermal cracking. These calculations show that in an effectively isolated system, pressures would become very high and could considerably exceed the rock load, so that fracturing must occur causing pressure b eed off and loss of gas. The lithostatic gradient (1.0 psi/ft or 22.6 kPa/m) is reached after only about 1.0% of the oil is cracked. If the reservoir system remains open (i.e., at hydrostatic pressure) and is initially filled with oil that is subsequently cracked to gas, then roughly 75% of the gas will be lost or the reservoir volume must effectively increase in size, for example, by moving the gas-water contact downward.