The Complementary Roles of Kinetics and Thermodynamics in the Generation and Preservation of Oil and Gas
Colin Barker, Professor Emeritus, University of Tulsa
1001 Camino Rancheros, Santa Fe, NM 87505, USA
For all reactions, including the generation of petroleum, thermodynamic parameters control the composition of reaction products at equilibrium and kinetic parameters control how fast this equilibrium assemblage is approached. At low temperatures reaction rates are less favorable and reactions move towards thermodynamic equilibrium at a rate that can be relatively slow, even on a geologic time scale. This is clearly shown by the occurrance of thermodynamically unstable materials such as diamonds, kerogen, and crude oil. All these can survive indefinitely. When thermodynamically unstable materials, such as the complex organic matter produced by organisms, are heated on burial, reactions occur. These vary from subtle structural changes (e.g., a to b orientations) to more substantial changes (e.g., cleaving long alkane chains). The progress of these reactions is predictable by first order chemical kinetics and a large body of information, consistent with observed trends in Nature, has been accumulated using kinetic models. Kinetically-controlled reactions lead ultimately to thermodynamically stable products.
When reactions are thermodynamically controlled, geologic age (time) is no longer a factor but the rock matrix becomes an essential part of the reacting system. All commercial petroleum reservoirs are shallower than 8 km, so that if petroleum-like hydrocarbons are generated deeper they must have migrated upwards to their present locations. Thermodynamic equilibrium implies that the system is capable of responding rapidly to changes in temperature, pressure, and composition. Hydrocarbons formed at, say, 100 km will be in thermodynamic equilibrium with their surroundings at that depth, but as they move up to steadily shallower environments temperature and pressure decrease and hydrocarbon composition will change to reflect equilibrium under these new conditions. The hydrocarbon composition continues to change and remains in thermodynamic equilibrium appropriate to the local temperature/ pressure/composition conditions. However, at some point the temperature will become low enough that the rate at which thermodynamic equilibrium is approached will be controlled by reaction rate, i.e., at lowered temperatures kinetics will become significant and the composition will be “frozen in”. What controls the composition will be the shallowest depth at which the hydrocarbon assemblage is in equilibrium. Original source depths are essentially irrelevant. At present the depth of the transition from thermodynamic to kinetic control is not well known but is probably at approximately 10 km.
If petroleum accumulations are produced by upward migration, vertical compositional profiles should reflect this. The trend from shallow oil to deep gas is well-documented. If abiogenic oil is introduced at the bottom of the sedimentary column why is there no deep oil? If it is introduced as gas how can it generate thermodynamically unstable materials like crude oil? Also, many subtle compositional changes are not consistent with an abiogenic origin. For example, readjustments at chiral centers in biomarkers (steranes, hopanes, etc) are consistent with biological values shallow (R>S) moving towards thermodynamically controlled values (R=S) with increasing depth. If petroleum is introduced deep it should start with the R=S equilibrium ratio, but there is no reason why the value should change away from equilibrium as it moves to shallower depths and lower temperatures. The trend cannot be explained by leaching from organic- rich shales because these are typically of low permeability and would not be the migration route favored by upward moving fluids which would follow faults or other high permeability zones such as sandstones.