The deepest hole in a sedimentary section is currently 31,441 ft (9,583 m), but the deepest production is only 26,536 ft (8,088 m); the depth gap between deepest hole and deepest producer is the largest in the history of the petroleum industry. This prompts a critical reevaluation of methane stability in deep potential reservoirs.

We developed a computer program to calculate the equilibrium composition of gases in deep reservoirs of various lithologies. The program establishes the assemblage with minimum free energy for specified conditions of temperature and pressure corresponding to conditions down to 40,000 ft (12,192 m) and can handle up to 70 components, 25 elements, and 20 phases. It does not assume ideal gas behavior but does assume ideal mixing. Calculations have been made for average, high, and low geothermal gradients; hydrostatic and lithostatic pressures; and with and without graphite. Calculated equilibrium compositions show that methane alone in an inert reservoir has considerable stability, and 99.4% survives to 40,000 ft (12,192 m). Even in the geologically more realistic system with water, 99.1 survives.

The full capability of the program is demonstrated for a sandstone reservoir with graphite, calcite cement, and a range of minor minerals. There, methane shows considerable stability for average geothermal gradients with both normal and abnormal pressures. For high geothermal gradients, part of the methane is lost by reaction, and significant amounts of carbon dioxide are added to give a gas composition at 40,000 ft (12,192 m) that contains only 9% methane. The program shows that, in general, temperature is much more important than pressure in controlling gas composition, crude oil is not thermodynamically stable at any depth, and a few percent of hydrogen is frequently present in deep gases.