Petroleum geochemistry

Sulphur greases the wheels

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What, in detail, are the fundamental chemical reactions that generate petroleum? Quite simply, we don't know. Such information is essential in developing predictive models for locating and exploiting deposits of oil and natural gas, but has remained elusive due to the complexity of the sedimentary systems in which petroleum forms. In his paper on page 164 of this issue1, Lewan takes a large step in the right direction — he presents a new hypothesis, based on laboratory experiments, that gives organically derived sulphur species a pivotal role in regulating the rate of petroleum generation.

Lewan proposes that sulphur radicals initiate the breaking of C-C bonds within kerogen (insoluble sedimentary organic matter) to produce the many hydrocarbon fragments that constitute oil, the sulphur radicals being derived from the thermal decomposition of kerogen itself. This view compares with the conventional wisdom that holds that the decomposition of kerogen to form oil is strictly a thermally driven, kinetic process, dominated by the strength of molecular bonding2,3. Thus, in the past, the inherent weakness of C-S and S-S bonds has been seen as being responsible for the faster generation of petroleum from kerogens that are enriched in sulphur.

In contrast, Lewan suggests that although structural weaknesses are responsible for the generation of sulphur radicals, it is the activity of these radicals, once formed, that facilitates cleavage of C-C bonds during petroleum generation, not the formation process itself. If this idea holds up, it is highly significant — the implication is that more rapid petroleum generation from sulphur-rich kerogen stems from the immediate geochemical environment, and not from a structural characteristic of the kerogen macromolecule.

In support of his hypothesis, Lewan provides compelling evidence from laboratory experiments in which 1-phenyldodecane, a model compound intended to represent kerogen, was heated in the presence of varying concentrations of sulphur radicals. The results show a strong positive correlation between the rate of 1-phenyldodecane cracking and the abundance of sulphur radicals. Lewan concludes that because sulphur-rich kerogen can release more sulphur radicals during maturation, initiation of cracking reactions involving C-C bonds is enhanced and petroleum generation occurs at lower thermal stress. An important feature of Lewan's experiments is that they were conducted in a closed system, allowing secondary reactions to take place between initial reactants and products.

Those who employ relatively simple models of petroleum occurrence, which consider only irreversible reactions, time and temperature2,3, may be disturbed by the idea that the geochemical environment surrounding organic compounds influences chemical reactions. Many of these models rely on kinetic data obtained from open-system pyrolysis, where organic matter is heated under dry conditions at ambient pressure in a stream of inert carrier gas. Rapid removal of reaction products minimizes secondary reactions. As Lewan emphasizes, these experiments do not replicate the geochemical environment surrounding kerogen in sedimentary basins, where reactants are confined at high pressure (several hundred bars). So it could well be that reaction mechanisms and model predictions based on open-system pyrolysis experiments have little relevance to processes occurring in natural systems.

Yet the implications of the geochemical environment having a large part in regulating thermal maturation of sedimentary organic matter go well beyond the design of laboratory experiments. Kerogen is just one component of sedimentary rocks, which may contain a grab bag of inorganic minerals and aqueous fluids of extremely diverse composition. Theoretical, field and experimental studies have delivered increasing evidence to support the contention of some researchers, including myself, that the inorganic components of sediments are highly reactive and may participate directly in organic geochemical processes. These components may act as catalysts or regulate key chemical variables such as redox state and pH, which in turn will control thermodynamic drives that influence the direction and extent of chemical reactions, both organic and inorganic4,5,6. In addition, aqueous pore fluids may represent a reactive source of hydrogen and oxygen for the creation of hydrocarbons and oxygenated organic compounds6,7.

Sulphur is ubiquitous in natural systems, and occurs in its elemental form and an entire range of oxidation states in minerals and aqueous pore fluids. It is intriguing to consider the effect these sulphur sources may have during the thermal maturation of sedimentary organic matter because the presence of various inorganic sulphur species greatly enhances the rates and specificity of numerous organic reactions. These effects are not limited to free-radical reaction mechanisms. For example, the decomposition of aqueous alkanes and alkenes to form methane and carbon dioxide proceeds more rapidly in the presence of aqueous sulphur species in intermediate oxidation states8. Other studies have shown increased rates for organic-matter oxidation by aqueous sulphate in the presence of elemental sulphur9,10.

Given the high reactivity of sulphur species with organic molecules, such species are also likely to influence the stability of petroleum after its expulsion from its source rock. In particular, elemental sulphur produced during thermochemical sulphate reduction in petroleum reservoirs may be an effective source of sulphur radicals; these radicals could enhance the cracking of oil to produce natural gas by the same mechanism that Lewan has proposed for the generation of oil from kerogen. The presence of water and aqueous sulphur species in sediment pore fluids may also represent a medium through which the degradation of oil can proceed by mechanisms other than thermal cracking.

Petroleum persists in some geological environments to much higher temperatures than existing models predict11, and a probable explanation is the switching on and off of specific reaction mechanisms in response to the availability of organic and inorganic reactants and catalysts. As petroleum resources become increasingly scarce, the need for predictive models that include explicit provision for the chemical reactions involved will necessarily increase. Well-designed laboratory experiments, such as those presented by Lewan, represent a powerful means to identify and quantify the relevant geochemical processes to include in these models.


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