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GOR-Isotopes - A New Tool for the Quantitative Assessment of Gas Generation and Gas Typing in Petroleum Systems |
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Introduction
Despite the fact that gas formation is a more complex process than oil formation, the geochemical techniques employed for natural gas exploration are still relatively rudimentary and primarily dominated by empirical models of gas formation. In contrast to oil which is generated in the so-called "thermal oil window", natural gas is generated throughout the entire thermal evolution of sedimentary basins. A new technique is now available that addresses quantitatively many issues of natural gas formation. This model is based on direct closed-system pyrolysis measurements of quantities and isotope fractionations for gases generated from specific source rocks. With the application of first principle calculations of hydrocarbon generation we can extrapolate the high-temperature pyrolysis measurements to any geologic heating rate. Using experimental data of different source rock types (Type I and II shales and Type III coal) we are able to generate output files that display the most critical properties for specific source rocks for any thermal history of a petroleum system e.g., isotope fractionation patterns, temperature and maturity of the gas source rock, gas quality (e.g., wetness), gas maturity, and the gas to oil ratio (GOR). Additionally, integration of our gas isotope model results with those of basin models allows for prediction of the geologic conditions pertaining to gas generation within a basin (e.g., time of gas formation, depth of gas kitchen, amount of gas formed in target areas, etc.) These results can provide critical information for mapping gas migration pathways, determining reservoir filling history, characterizing reservoir connectivity, and identifying multiply sourced gas. |
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Figure 1A: Dependence of bond energies on isotopic composition |
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Table 1A: Typical calculated kinetic parameters at various different temperatures
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Figure 1B: Determination of the activation energy and the ratio of the frequency factors |
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Figure 1C: Kinetic isotope fractionation generally increases with increasing bond energy |
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| Figure 1D: A linear correlation between the ratios of the frequency factors and the fractionation constant DDE is generally observed |
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Model Constraints
Although the distribution of activation energies associated with gas generation is quite broad, there is a general correlation between bond dissociation enthalpy related to isotope fractionation and the bond dissocaition energy (Figure 1C). The bond dissociation enthalpy also correlates quite closely with the ratio of the isotopically substituted and unsubstituted frequency factors (Figure 1D). This relationship provides and important simplifying constraint for isotope modeling based on the Arrhenius equation. |
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Integration of Experiment and Theory
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Vacuum Line for Gas Transfer |
Quantum chemistry theory clearly demonstrates that although there is a close relationship between bond dissociation energies and isotope fractionation factors, there is a broad distribution of activation energies associated with bond breaking in hydrocarbons. This fact is reflected in the high variability of hydrocarbon generation rates observed for different kerogen types (Figure 2E). Consequently, no one model can be applied to explain the generation and chemical composition of natural gas dervived from various different kerogen types. Moreover, even for an individual kerogen type the rate of gas generation and its chemical composition is quite variable and is strongly dependent on the geology and thermal history of the basin (Figure 2F). Therefore, in order to accurately predict gas generation potential and the chemical composition of the product, the results of laboratory experiments must be combined with kinetic theory. One simple example of this concept is the temperature dependence of kinetic isotope fractionations. Isotopic compositions measured in the laboratory derived from high temperture accelerated reactions can not be directly applied to geologic settings. The application of experimental isotopic results to actual geologic environments requires extrapolation to geologic heating rates via kinetic modeling (Figure 1G).
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High Temperature Pyrolysis Ovens |
Gas Chromatographs |
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Figure 1E: Hydrocarbon Generation Rates are Highly Variable for Different Types of Kerogen |
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Figure 1F: Hydrocarbon Generation Over Time is a Complex Process |
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Figure 1G: Temperature Dependence of Kinetic Isotope Fractionation |
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