Alan K. Burnham

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 .

Computational aspects of kinetic analysis. Part A: The ICTAC kinetics project-data, methods and results

Abstract Part A of this series of papers (Parts B to E follow) presents the data and methods used, as well as the results obtained by participants in the ICTAC Kinetics Project. The isothermal and non-isothermal data sets provided were based on a hypothetical simulated process as well as on some actual experimental results for the thermal decompositions of ammonium perchlorate and calcium carbonate. The participants applied a variety of computational methods. Isoconversional and multi-heating rate methods were particularly successful in correctly describing the multi-step kinetics used in the simulated data. Reasonably consistent kinetic results were obtained for isothermal and non-isothermal data. There is, of course, no ‘true’ answer for the kinetic parameters of the real data, so the findings of the participants are compared. An attempt has been made to forecast the tendencies for the future development of solid state kinetics.

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.

On the mechanism of kerogen pyrolysis

Abstract Aromaticities determined by 13 C n.m.r. are reported for five shale oil samples (Green River formation) prepared under widely different pyrolysis conditions. In the absence of high-pressure hydrogen, the total amount of aromatic carbon in the products is nearly twice that in the raw shale. This is true for a wide range of pyrolysis conditions, although the distribution of aromatic carbon between the oil and carbonaceous residue changes. High-pressure hydrogen appears to inhibit both the formation of additional aromatic carbon during pyrolysis and the coking of aromatic oil. An improved kerogen decomposition mechanism is reported that accounts for these effects and provides for changes in the aromaticity of the liquid product with pyrolysis conditions. Further work is necessary to make it quantitative and account for gas formation.

Pyrolysis kinetics for Green River oil shale from the saline zone

Abstract The nature of the organic and mineral reactions during the pyrolysis of Saline-zone Colorado oil shale containing large amounts of nahcolite and dawsonite has been determined. Results reported include a material-balanced Fischer assay and measurements of gas evolution rate of CH 4 , C 2 H x , H 2 , CO and CO 2 , Stoichiometry and kinetics of the organic pyrolysis reactions are similar to oil shale from the Mahogany zone. X-ray diffraction and thermogravimetric analysis results are used to help determine the characteristics of the mineral reactions. Kinetic expressions are reported for dawsonite decomposition, and it is demonstrated that the temperature of dolomite decomposition is substantially lower than for Mahogany-zone shale because of the presence of the sodium minerals.

Computational aspects of kinetic analysis. Part D: The ICTAC kinetics project : multi-thermal-history model-fitting methods and their relation to isoconversional methods

This paper is Part D of a discussion of the computational stage of solid-state reactions as applied to the data sets of the ICTAC Kinetic Analysis Project. This Part critically evaluates the results from the various participants and finds that kinetic analysis programs used by Burnham, Roduit, and Opfermann give very similar results. Isoconversional methods give kinetic parameters that agree qualitatively with those from subsequent nonlinear regression to appropriate models. Single-heating-rate methods work poorly and should not be used or published.

Development of a detailed model of petroleum formation, destruction, and expulsion from lacustrine and marine source rocks

Abstract A variety of laboratory experiments, including programmed micropyrolysis, isothermal fluidized-bed pyrolysis, oil evolution from a self-purging reactor, pyrolysis-mass spectrometry, and hydrous pyrolysis are analyzed to derive chemical kinetic expressions for pyrolysis of lacustrine and marine kerogens. These kinetic parameters are incorporated into an improved, detailed chemical-kinetic model which includes oil and gas generation from kerogen, oil degradation by coking and cracking, gas generation from residual kerogen, and hydrogen consumption reactions. Oil is described by eleven boiling-point fractions of two chemical types. The model includes equation-of-state calculations of vapor/liquid equilibria and PVT behavior. The model can simulate closed, open and leaky systems, and the open system can include an inert-gas purge. The porosity is calculated for both unconstrained conditions as well as conditions simulating natural compaction and fracturing during sedimentary burial. Model calculations are compared to results from a variety of laboratory experiments, including hydrous pyrolysis. Oil expulsion efficiencies and properties are also calculated for a variety of geological conditions. The relative amounts of water and hydrocarbon phase(s) expelled are governed by saturation-dependent relative permeabilities. Gas/oil ratios in the expelled petroleum are related to organic content and geological heating rate.

Temperature and pressure dependence of n-hexadecane cracking

The rates and product compositions of hexadecane cracking are reported for temperatures ranging from 300 to 370°C and pressures of 150 to 600 bar. The overall apparent activation energy at an intermediate pressure of 300–350 bar is about 74 kcal/mol. This is higher than the overall 60 kcal/mol energy consistent with higher temperature measurements, even though the rates from our highest temperatures overlap with those from earlier reports. The product composition is consistent with a free radical mechanism in which alkene intermediates react with primary and secondary radicals to form branched and normal liquid products both smaller and larger than the starting material. Pressure has a retarding effect on the rate of reaction, but the dependence is not measured precisely enough to say more than that the survival of oil is likely to vary by a factor of two or so under typical geologic conditions. A simple kinetic model having five first-order reactions is presented that predicts the lumped kinetic species of C1, C2–C4, C5–C9, C10–C15, C16, and C16+. The gas is depleted in methane compared to most natural gas.

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.

PMOD: a flexible model of oil and gas generation, cracking, and expulsion

A new computer program, PMOD, is used to develop and test a wide variety of global models of varied complexity for the generation and destruction of hydrocarbons from petroleum source rocks. Chemical reaction models are constructed interactively by supplying the empirical formula of the reactants and products, desired reactions, and reaction constraints. PMOD calculates stoichiometric coefficients that conserve elemental balance. The chemical reactions are integrated into two compaction and bulk-flow expulsion models in order to predict the amounts and compositions of products expelled from the source rock under geological conditions. A variety of published and unpublished experiments are used to show when various model types are required and to derive the parameters for the various models. This exercise provides a good means to compare the strengths and weaknesses of the various models and provides guidance on the types of information needed to calibrate them. Chemical properties calculated by PMOD for all models include apparent Rock-Eval parameters and other common geochemical measurements.