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<p>Abstract</p> <p>It is broadly accepted that so-called 'thermal' gas is the product of thermal cracking, 'primary' thermal gas from kerogen cracking, and 'secondary' thermal gas from oil cracking. Since thermal cracking of hydrocarbons does not generate products at equilibrium and thermal stress should not bring them to equilibrium over geologic time, we would not expect methane, ethane, and propane to be at equilibrium in subsurface deposits. Here we report compelling evidence of natural gas at thermodynamic equilibrium. Molecular compositions are constrained to equilibrium,</p> <p><display-formula><graphic file="1467-4866-10-6-i1.gif"/></display-formula></p> <p>and isotopic compositions are also under equilibrium constraints:</p> <p><display-formula><graphic file="1467-4866-10-6-i2.gif"/></display-formula></p> <p><display-formula><graphic file="1467-4866-10-6-i3.gif"/></display-formula></p> <p>The functions [(CH₄)*(C₃H₈)] and [(C₂H₆)²] exhibit a strong nonlinear correlation (R²= 0.84) in which the quotient Q progresses to K as wet gas progresses to dry gas. There are striking similarities between natural gas and catalytic gas generated from marine shales. A Devonian/Mississippian New Albany shale generates gas with Q converging on K over time as wet gas progresses to dry gas at 200°C.</p> <p>The position that thermal cracking is the primary source of natural gas is no longer tenable. It is challenged by its inability to explain the composition of natural gas, natural gases at thermodynamic equilibrium, and by the existence of a catalytic path to gas that better explains gas compositions.</p>