Jerry J. Sweeney

Evaluation of a simple model of vitrinite reflectance based on chemical kinetics

We present a simplified version of a vitrinite maturation model, complete with sample spreadsheet, based on changes in vitrinite composition with time and temperature. The simplified model, called EASY%R[o], uses an Arrhenius first-order parallel-reaction approach with a distribution of activation energies. EASY%R[o] has been calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. With EASY%R[o], a profile of vitrinite reflectance vs. time can be obtained for a given stratigraphic level if the time-temperature history for that level has been estimated. When applied to multiple stratigraphic levels, EASY%R[o] can be used to compute profiles of the percent of vitrinite reflectance with depth for comparison with borehol data and to optimize thermal history models. EASY%R[o] can be used for vitrinite reflectance values of 0.3 to 4.5%, and for heating rates ranging from those in the laboratory (1 degree C/week) to those in slowly subsiding geologic basins (1 degree C/10 m.y.). Examples of model applications range from sedimentary rocks heated by an igneous intrusion to a variety of burial histories. Vitrinite maturation predicted by EASY%Ro is compared to other methods currently being used, such as the Lopatin time-temperature index, level of organic maturity, and other approaches using a single activation energy. Our model successfully estimates vitrinite reflectance due to thermal metamorphism of sedimentary rocks heated by igneous intrusions, geothermal fluids, and burial in a variety of basin setting .

A chemical kinetic model of vitrinite maturation and reflectance

Abstract A chemical kinetic model is presented that uses Arrhenius rate constants to calculate vitrinite elemental composition as a function of time and temperature. The model uses distributions of activation energies for each of four reactions: elimination of water, carbon dioxide, methane and higher hydrocarbons. The resulting composition is used to calculate vitrinite reflectance via correlations between elemental composition and reflectance. The correlations are derived from published measurements. The model is valid for %Ro values from slightly less than 0.3 to slightly greater than 4. Model calculations are compared to published vitrinite data from both laboratory experiments and sedimentary columns where adequate thermal histories are available. Calculated and measured %Ro values generally agree within 0.1 at low rank and 0.4 at high rank, which is comparable to uncertainties in the experimental values. This confirms our starting premise that vitrinite reflectance is properly described by standard chemical kinetics with activation energies that extrapolate from laboratory to geological maturation temperatures. The model indicates that the relationship between the extent of oil generation and vitrinite reflectance is nearly independent of heating rate.

A Model of Hydrocarbon Generation from Type I Kerogen: Application to Uinta Basin, Utah

We have developed a computer model that can predict when and how much oil and gas are generated from a source rock during its burial and later uplift. Kinetic parameters for the oil and gas generation reactions are obtained from high-pressure pyrolysis experiments carried out over a wide range of heating rates and temperatures. In our kinetic model, which applies only to Green River shale, we use a single activation energy of 52.4 kcal/mole and different pre-exponential factors for different products of primary pyrolysis, which allows us to extrapolate laboratory-derived kinetics to geologic heating rates. This model is in contrast to the wide distributions of activation energies or artificially low apparent activation energies used in some models of petroleum formation. hen extrapolated to geologic heating rates on the order of 10°C/m.y., our kinetics show that the temperature of the maximum rate of oil generation (Tp) changes by about 15°C when the heating rate is changed by an order of magnitude. Changes in pressure have relatively minor effects on the kinetics of oil generation but are important for gas generation reactions. We used geophysical data from oil fields in the Uinta basin of Utah to develop a thermal history model of Green River Formation source rocks. This time-temperature history was used to predict the maturation level of the kerogen at a given depth and to predict changes in the compositional characteristics of the oil. The shape of calculated oil generation rate curves, as a function of depth in the basin, mimics the shape of the overpressure curves; this similarity suggests that oil-gas generation may be an important cause of overpressuring. Maturation levels and compositional characteristics of the oil predicted by our model agree very well with characteristics of the oil recovered from the basin.

Chemical Kinetic Model of Hydrocarbon Generation, Expulsion, and Destruction Applied to the Maracaibo Basin, Venezuela

This paper describes the development and application of a compositional chemical model of hydrocarbon generation, expulsion, and destruction for the Cretaceous La Luna Formation source rock of the Maracaibo basin, Venezuela. Applications include both laboratory and geological settings. Laboratory pyrolysis experiments were used to study bulk oil generation, expulsion, and associated changes in composition of the kerogen, extractable organic matter, and expelled and unexpelled hydrocarbons. The laboratory experiments were also used to determine kinetic parameters to quantitatively describe organic reactions, via a computer model that also includes simulation of pressure-driven primary expulsion, over widely varying conditions. We show that the chemical model accurately sim lates the experimental results. Thermal history models for wells in the Maracaibo basin were used to simulate hydrocarbon generation and pore pressure development in the La Luna Formation and expulsion into nearby Cretaceous reservoirs. Results of the modeling indicate that both compaction disequilibrium and organic maturation play important roles in the development of excess pore pressure in the La Luna Formation. The model simulation of the variation of indicators such as Rock-Eval parameters and extract and oil compositions shows generally good agreement with measurements from remaining kerogen, oils, and extracts recovered from the La Luna Formation and from nearby Cretaceous reservoirs.

Pyrolysis kinetics applied to prediction of oil generation in the Maracaibo Basin, Venezuela

Abstract We use chemical kinetic parameters for oil generation derived from modified Rock-Eval and Pyromat instruments, coupled with thermal history models, to predict the timing and extent of oil generation in the Maracaibo Basin of Venezuela. The vitrinite reflectance model developed at Lawrence Livermore National Laboratory is used to calibrate thermal history models with measured vitrinite reflectance profiles. We examine the way differences in the kinetic parameters affect predictions of maturation in several parts of the basin with different thermal histories. Maturity indicators, such as H/C atomic ratio of residual kerogen and API gravity of reservoired oil, are compared to the calculated extent of oil generation. We use the comparison to check the accuracy of the coupled oil generation and thermal history models.

Application of maturation indicators and oil reaction kinetics to put constraints on thermal history models for the Uinta Basin, Utah, U.S.A.

Abstract I test the ability of a rapid pyrolysis oil reaction kinetic model to predict characteristics of oil generation in the Uinta Basin of Utah, U.S.A. I use kinetic coefficients, determined by rapid pyrolysis of core samples obtained from the Altamont and Redwash fields, to calculate the amount of oil generated at different depths in several wells and compare that amount with measures of the extent of reaction, such as production index and hydrogen index, obtained from core and cutting samples. In making the calculations, I use different values of maximum burial depths of the sediments and heat flow to study the effect of variations in these thermal history parameters on the oil reaction predictions. I find that by using a constant heat flow in the model of 57 mW/m2 over the past 58 million years with about 1800 m of erosion in the last 10 million years the resulting calculations of oil maturation levels are consistent with maturation indicators obtained from the wells. These values are not unique, however; a 300-m decrease in the amount of erosion can be compensated by increasing the heat flow by about 4 mW/m2. I discuss how variations in the input parameters affect the results of both the basin thermal models and the kinetic models.