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LABORATORY SIMULATIONS OF OIL AND NATURAL GAS FORMATION A major goal for the petroleum geochemist is to understand the origin of petroleum in relation to maturation of organicrich sediments. Thermal maturation involves numerous complex chemical and physical processes that modify the structure and composition of sedimentary organic matter during burial. Owing to the complexity of natural sedimentary basins, however, factors that regulate these processes are difficult to determine from field studies alone. Consequently, a variety of laboratory simulation techniques have been developed to provide information regarding the rates of reactions as a function of temperature and time, the absolute amounts of products generated, reaction mechanisms, and sources and sinks for individual species involved. For example, results of laboratory experiments demonstrating that some 'biological marker' compounds are initially attached to kerogen by chemical bonds but are subsequently released to crude oils during heating provide strong evidence for the biologic origin of petroleum (Eglinton and Douglas, 1988). Other experiments have shown that inorganic species such as minerals and transition metals may directly participate in organic reactions in addition to playing a catalytic role during organic transformations (Tannenbaum and Kaplan, 1985; Mango et al., 1994; Seewald, 1994). Oil and natural gas generation in natural systems is generally believed to occur at temperatures of 80-120°C and > 120°C, respectively (Tissot and Welte, 1984). Because natural maturation processes occur on time scales of millions of years, it is common practice to conduct laboratory simulations at temperatures higher than those in natural systems to enhance reaction rates so that transformations can be observed on laboratory time scales. A key premise underpinning this practice is the assumption that petroleum generation is primarily a kinetic process in which time can be directly traded for temperature. Extrapolation of results obtained at relatively high temperatures in the laboratory to low temperature natural systems is commonly achieved through use of the Arrhenius equation: k = Aoe-(Ea!RT)
where k is the rate constant, A 0 is the pre-exponential constant, Ea is the activation energy, R is the ideal gas constant and Tis temperature. Because the thermal maturation of sedimentary organic matter to produce oil and gas is an inherently complex process involving numerous sequential and parallel reactions, even the most complex models based on the Arrhenius equation represent gross simplifications of naturally occurring processes. A number of researchers have questioned whether it is appropriate to substitute temperature for time during laboratory experiments since significant changes in the relative rates of reaction, the relationship between diffusion and reaction rates, thermodynamic drives and even reaction mechanisms may occur at low temperatures. Thus, there are important limitations regarding the extent to which artificial maturation experiments can be
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