Petroleum Expulsion Part 3. A Model of Chemically ... - ACS Publications

Energy & Fuels 2006, 20, 309-319

309

Petroleum Expulsion Part 3. A Model of Chemically Driven Fractionation during Expulsion of Petroleum from Kerogen S. R. Kelemen,* C. C. Walters, D. Ertas, and H. Freund ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801

D. J. Curry ExxonMobil Exploration Company, Houston, Texas 77252 ReceiVed June 7, 2005. ReVised Manuscript ReceiVed October 12, 2005

The expulsion of hydrocarbons from kerogen is the initial step in the primary migration process, during which the composition of the expelled petroleum is enriched in saturated and aromatic hydrocarbons while the retained bitumen is enriched in polar compounds. The physical and chemical principles responsible for this chemical fractionation are not well understood, and numerous theories have been proposed to explain the expulsion process. A multicomponent equilibrium likely exists between the kerogen matrix and the expelled fluid during petroleum generation and expulsion. To test whether such equilibrium can explain the nature and extent of chemical fractionation, an extended Flory-Rehner Regular Solution Theory model was developed and applied to a series of kerogens of varying structure, generative potential, and maturity. Thermodynamic parameters for immature Type II and IIIC kerogens (solubility parameter, cross-link density, and native swelling) were derived experimentally and extended to higher maturity. Mixtures of model compounds with well-defined properties were created to reflect the composition of primary generated products of kerogen thermal decomposition. Multicomponent equilibrium then was calculated under closed system conditions. The amount and composition of modeled expelled products are most sensitive to the generative potential and cross-link density of the kerogen. In general, lower source richness and cross-link density is associated with bitumen retention and a relative enrichment of the aliphatic components in the expelled fluid. Higher source richness and cross-link density result in earlier expulsion of fluids that are enriched in polar components. The predicted compositions of expelled fluids correspond well with the compositional range observed for produced petroleum. The predicted bitumen (kerogen-retained, soluble organic matter) compositions are uniformly >50% C14+ NSOs at all levels of maturity for all modeled kerogens. A chemically driven equilibrium mechanism based on Regular Solution Theory can explain almost completely the nature and extent of chemical fractionation that takes place during expulsion.

1. Introduction Tissot and Welte define primary migration as the release of petroleum compounds from kerogen and their transport within and through the capillaries and narrow pores of a fine-grain source rock.1 Expulsion of hydrocarbons from kerogen is the initial step in the primary migration process, during which the composition of the expelled petroleum is enriched in saturated and aromatic hydrocarbons while the retained bitumen is enriched in asphaltene and polar compounds. The physical and chemical principles responsible for petroleum expulsion and chemical fractionation are not well understood. Examining expulsion based solely on geological evidence is difficult because of the large number of independently changing variables, few of which can be sufficiently constrained in natural systems. Laboratory simulations of expulsion suffer because of the inherent link to petroleum generation, which must be accelerated using substantially higher temperatures than normally occur in basins. The uncertainty in constraining these processes in petroleum system models stands in stark contrast with the considerable advances made in our understanding of * Corresponding author. Phone: (908) 730-2389. Fax: (908) 730-3232. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984.

source rock deposition, kerogen compositions, kinetics and mechanisms of petroleum generation, and reservoir alteration processes.2 Models that attempt to account for the chemical fractionation observed between expelled oil and retained bitumen have been reviewed recently.3-5 These expulsion models can be divided between those that emphasize one of two different rate-limiting processes: (1) The movement of hydrocarbons within a source rock or (2) The release of generated hydrocarbons from the kerogen. There are many expulsion models that target the chemical or physical processes of oil moving within the source rock mineral matrix as the rate-determining step. Some considered the amount and type of organic matter as being critical to generating (2) Makhous, M.; Galushkin, Yu. I. Basin Analysis and Modeling of the Burial, Thermal and Maturation Histories in Sedimentary Basins; Editions Technip: Paris, 2005. (3) Pepper, A. S.; Corvi, P. J. Mar. Pet. Geol. 1995, 12, 417. (4) Mann, U. In Geofluids: Origin, Migration and EVolution of Fluids in Sedimentary Basins; Parnell, J., Ed.; Geological Society Special Publication 78; Geological Society: London, 1994; pp 233-260. (5) Mann, U.; Hantschel, T.; Schaefer, R. G.; Krooss, B.; Leythaeuser, D.; Littke, R.; Sachsenhofer, R. F. In Petroleum and Basin EVolution; Welte, D. H., Horsfeld, B., Baker, D. R., Eds.; Springer: Heidelberg, 1997; pp 397-515.

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310 Energy & Fuels, Vol. 20, No. 1, 2006

sufficient bitumen to exceed a saturation threshold.6-10 The establishment of effective and continuous migration pathways within the source rocks may be deemed critical.11-15 Other models have considered pressure buildup from generation and compaction and the failure of the rock fabric forming microfracturing as a key element in expulsion.16-19 Still others have evoked gas availability and movement of oil in a gas or supercritical phase20-22 or movement of oil in an aqueous phase.16 These elements are controlled mostly by the sedimentary conditions during source rock deposition and by secondary diagenetic processes that occur during the evolution of sedimentary basins.4 Consequently, the mechanisms that define oil movement will differ according to the lithofacies of the source rock. Competing alternative models involve the concept that the rate-limiting factor for expulsion is the release of oil as it is being generated from its source kerogen. This hypothesis attributes little importance to movement of petroleum within the mineral matrix. Rather, it postulates that the expulsion of oil from kerogen is controlled by absorption or adsorption of the products onto the surface of the kerogen and/or diffusion of the hydrocarbons through the kerogen.3,23-29 The efficiency of the release of oil is controlled primarily by the amount of organic carbon and its composition.25 A foundation for these concepts can be traced to observations made on source rocks33 and to the recognition of the absorptive capacity of solid organic matter as revealed by solvent swelling experiments.25,31-37 Type (6) Tissot, B. In Migr. Hydrocarbons Sediment. Basins, IPF Explor. Res. Conf., 2nd; Doligez, B., Ed.; Editions Technip: Paris, 1987; pp 1-19. (7) Durand, B. Org. Geochem. 1988, 13, 445-459. (8) Ungerer, P. Org. Geochem. 1990, 16, 1. (9) Burnham, A. K.; Braun, R. L. Org. Geochem. 1990, 16, 27-39. (10) Lafargue, E.; Espitalie´, J.; Broks, T.; Jacobsen, T.; Nyland, B. Org. Geochem. 1994, 22, 575-586. (11) Palciauskas, V. V.; Domenico, P A. AAPG Bull. 1980, 64, 927937. (12) Palacas, J. G.; Anders, D. E.; King, J. D. In Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks; Palacas, J. G., Ed.; AAPG Studies in Geology 18; American Association of Petroleum Geologists Studies in Geology: Tulsa, OK, 1984; pp 71-96. (13) Sassen, R.; Moore, C. H.; Meenden, F. C. Org. Geochem. 1987, 11, 379-383. (14) Talukdar, S.; Gallango, O.; Vallejos, C.; Ruggiero, A. In Migr. Hydrocarbons Sediment. Basins, IPF Explor. Res. Conf., 2nd; Doligez, B., Ed.; Editions Technip: Paris, 1987; pp 59-78. (15) Mann, U. In Sediments and EnVironmental Geochemistry; Heling, D., Rothe, P., Fo¨rstner, U., Stoffers, P., Eds.; Springer: New York 1990; pp 152-178. (16) Barker, C. AAPG Bull. 1972, 56, 2068-2071. (17) Ungerer, P.; Behar, F.; Discamps, D. In AdV. Org. Geochem., Proc. Int. Meet. 10th; Bjorøy, M., Ed.; Wiley: Chichester; 1983; pp 129-135. (18) Hunt, J. M. AAPG Bull. 1990, 74, 1-12. (19) Du¨ppenbecker, S. J.; Dohmen, L.; Welte, D. H. In Petroleum Migration; England, W. A., Fleet, A. J., Eds.; Geological Society Special Publication 59; Geological Society: London, 1991; pp 47-64. (20) Price L. C. J. Pet. Geol. 1989, 12, 289-324. (21) Leythaeuser, D.; Poelchau, H. S. In Petroleum Migration; England, W. A., Fleet, A. J., Eds.; Geological Society Special Publication 59; Geological Society: London, 1991; pp 33-46. (22) Yalc¸ in, N. M.; Inan, S. In 2nd International Symposium on the Petroleum Geology and Hydrocarbon Potential of the Black Sea Area, SileIstanbul, Turkey, Sept 22-24, 1996; pp 24-25. (23) Ritter, U. Org. Geochem. 2003, 34, 319-326. (24) Ritter, U. J. Geochem. Explor. 2003, 78, 417-420. (25) Sandvik, E. I.; Young, W. A.; Curry, D. J. Org. Geochem. 1992, 19, 77. (26) Thomas, M. M.; Clouse, J. A. Geochim. Cosmochim. Acta 1990, 54, 2775-2779. (27) Thomas, M. M.; Clouse, J. A. Geochim. Cosmochim. Acta 1990, 54, 2781-2792. (28) Thomas, M. M.; Clouse, J. A. Geochim. Cosmochim. Acta 1990, 54, 2793-2797. (29) Stainforth, J. G.; Reinders, J. E. A. Org. Geochem. 1990, 16, 61.

Kelemen et al. Table 1. Thermodynamic Parameters for Kerogen Used in Model Calculations cross-link density (mol/cm3)

native swelling fraction

structure

solubility parameter (J/cm3)1/2

II II II II

A1 A2 A3 A4

22.1 22.1 22.6 23.1

0.12 0.93 0.52 0.93

0.76 0.87 0.82 0.87

IIIC IIIC IIIC IIIC

B1 B2 B3 B4

22.6 22.6 23.1 23.6

0.16 0.70 0.43 0.70

0.83 0.87 0.90 0.97

kerogen type

I, II, III, and IIIC kerogens have sufficient sorptive properties to explain residual oil concentrations in mature source rocks and coals. Expulsion models based on the interactions of generated products with the source kerogen have gained favor in recent years, primarily for their potential to account for chemical fractionation and expulsion efficiencies in terms of kerogen chemistry rather than the additional complexity imposed by lithological controls. In the latest publication to examine this concept, Ritter23,24 concluded that polymer solution theory based on the relative solubility of petroleum compounds in kerogen predicts the nature, but not the magnitude, of the observed fractionations. He concluded that selective solubility is only one contributing factor to chemical fractionation during expulsion, though it may be the dominant process in specific circumstances. A Regular Solution Theory model for petroleum expulsion from kerogen was constructed to more accurately test the nature and extent of chemical fractionation due to selective partitioning.36,37 The theory postulates that: (1) the kerogen/bitumen/ oil system remains in thermodynamic equilibrium, (2) kerogen behaves as an elastomer network upon swelling, and (3) solvent-solvent and solvent-kerogen interactions are largely nonspecific. From these assumptions, a theoretical framework is constructed that incorporates concepts from the Flory-Rehner theory of rubber elasticity38 and the Regular Solution Theory of Hildebrand et al.39 to explain the swelling behavior of kerogen by different solvents and solvent mixtures. We proved that an extended Flory-Rehner and Regular Solution Theory framework could explain the average swelling data for Type II and Type IIIC kerogens in different solvents.37 Thermodynamic parameters (solubility parameter, cross-link density, and native swelling) are determined for Type II and IIIC kerogens by optimizing the match between the experimental data and regular solution predictions (Table 1). These parameters can then be used to calculate the extent of swelling of kerogen in solvents and complex mixtures of solvents. In this article, we utilize this framework to model equilibrium between kerogen and specific molecules that are representative of those produced during thermal decomposition of kerogen in a closed chemical system (30) Pepper, A. S. In Petroleum Migration; England, W. A., Fleet, A. J., Eds.; Geological Society Special Publication 59; Geological Society: London, 1991; p 9. (31) Larsen, J. W.; Li, S. Org. Geochem. 1997, 26, 305. (32) Larsen, J. W.; Li, S. Energy Fuels 1997, 11, 897. (33) Larsen J. W.; Shang, L. Energy Fuels 1994, 8, 932-936. (34) Ballice, L. Fuel 2003, 82, 1317-1321. (35) Larsen, J. W.; Parikh, H.; Michels, R. Org. Geochem. 2002, 33, 1143-1152. (36) Ertas, D.; Kelemen, S. R.; Halsey, T. C. Energy Fuels 2006, 20, 295-300. (37) Kelemen, S. R.; Walters, C. C.; Ertas, D.; Curry, D. J. Energy Fuels 2006, 20, 301-308. (38) Flory, P. J.; Rehner, J., Jr. J. Chem. Phys. 1943, 11, 521. (39) Hildebrand, J. H.; Praudnitz, J. M.; Scott, D. L. Regular and Related Solutions; Van Nostrand Reinhold: New York, 1970.

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Energy & Fuels, Vol. 20, No. 1, 2006 311

Table 2. Composition of the Primary Generated Hydrocarbons Used in Model Calculations weight percent composition

kerogen type

C1

C2

C3

C4

C5

C6-C14 Sat

C6-C14 Aro

C14+ Sat

C14+ Aro

C14+ NSOs

P1 P2 P3 P4 P5 P6 P7 P8

II II II II IIIC IIIC IIIC IIIC

7.5 7.5 7.5 7.5 14.5 14.5 14.5 14.5

1.875 1.875 1.875 1.875 3.125 3.125 3.125 3.125

1.875 1.875 1.875 1.875 3.125 3.125 3.125 3.125

1.875 1.875 1.875 1.875 3.125 3.125 3.125 3.125

1.875 1.875 1.875 1.875 3.125 3.125 3.125 3.125

16.75 16.75 16.75 16.75 16.75 16.75 16.75 16.75

8.25 8.25 8.25 8.25 8.25 8.25 8.25 8.25

21.00 12.00 6.00 3.00 12.50 10.00 5.00 3.00

6.00 6.00 12.00 6.00 7.50 5.00 10.00 5.00

33.00 42.00 42.00 51.00 30.00 35.00 35.00 43.00

as a function of kerogen maturity. Using these compounds as surrogates for broad chemical groupings, the composition of bitumen and expelled fluid can be calculated and compared to the composition of petroleum. 2. Methodology 2.1. Yield and Composition of Primary Generated Oil. Within the Regular Solution Theory modeling framework, the amount and composition of the primary products generated by the thermal decomposition of kerogen will influence their partitioning between retained bitumen and expelled petroleum. The specific compositions of the primary generated products selected for study were not chosen to exactly match any specific set of open or closed system pyrolysis data. Instead, models were constructed that were believed to be representative of total primary products generated by Type II and Type IIIC kerogens. No attempt was made to model changes in fluid composition as a function of kerogen maturation. The lumped compositions of the calculated primary generated hydrocarbons are shown in Table 2. C1 through C5 hydrocarbons are listed as individual components, with methane accounting for half of the C1-C5 fraction in both Type II and IIIC kerogen product compositions. The C2-C5 component molecules appear in equal amounts and collectively account for 7.5 and 14.5 wt % of Type II and IIIC product compositions, respectively. The C6-C14 compounds are lumped into saturated and aromatic fractions, and their proportions are constant. In contrast, the relative amounts of C14+ saturated and C14+ aromatic hydrocarbons vary considerably and each organic matter type contains at least one composition where one predominates over the other. C14+ NSOs are the most abundant fractions in all modeled compositions and vary within a kerogen type. Variation in the composition within a kerogen type is restricted to the C14+ fraction and is designed to test the influence of product composition on the extent of chemical fractionation between polar and nonpolar components. The compositions of primary generated products in natural systems can never be known with certainty. There have been few pyrolysis studies of kerogen where complete mass balances were obtained.41 The compositions of the modeled primary generated products are somewhat arbitrary; however, they appear generally consistent with available mass balanced data from both open and closed systems. Figure 1 shows the model C4+ composition of primary generation together with open and closed system results for Type II and III kerogens calculated at 100% transformation for primary cracking of kerogen.41,42 Additional unpublished data are included in this figure using the identical experimental and analysis protocol.41,42 The compositions were calculated from experimental data assuming that the C6-C14 fraction is aliphatic. The model C4+ compositions are encompassed by the experimental results. The experimental compositions (e.g., open versus closed system) have (40) Van Krevelen, D. W. In CRC Handbook of Solubility Parameters and Other Cohesion Parameters; Barton, A. F. M., Ed.; CRC Press: Boca Raton, FL, 1983. (41) Behar, F.; Vandenbrouke, M.; Tang, Y.; Marquis, F.; Espitalie, J. Org. Geochem. 1997, 26, 321. (42) VandenBroucke, M.; Bence, A.; Behar, F.; Kelemen, S.; Curry, D.; Xiao, Y.; Leblond, C. In Proceedings of the 21st IMOG Meeting, Krakow, Poland, Sept 2003; p 374.

been discussed in detail41 and is beyond the scope of this article. It is generally accepted that both open and closed system pyrolysis compositions fail to exactly match those of petroleum and bitumen in natural systems. Olefin products produced in open systems are not found in abundance in nature. Closed systems produce fewer olefin products but are influenced to some extent by secondary cracking of products. A direct correspondence of pyrolysis product with petroleum composition is not expected due to recognized fractionation effects that take place during expulsion. Indeed, a comparison of these primary product compositions to produced petroleum shows clearly that the produced oils are enriched in aliphatic components and deficient in NSOs relative to the modeled compositions (Figure 2). 2.2. Molecules and Molecular Lumps. Within a Regular Solution Theory framework, the amount and composition of hydrocarbons expelled from kerogen will depend not only on the chemical and physical structure of kerogen but also on the exact nature of the primary generated hydrocarbons. The density, molar volume, and solubility parameter of all primary products must be known. Rather than estimating these parameters for compositional lumps, specific molecules with well-defined properties are used as surrogates. Table 3 lists the molecules, their molecular properties, and their association to molecular lumps. Chemical structures of some of the more complex structures that are not obvious by their

Figure 1. Modeled composition of hydrocarbons produced by primary generation.

Figure 2. Observed produced oil composition.


Kelemen et al.

Table 3. Molecules and Molecular Properties Used in Model Calculationsa

a

ID#

molecular lump

name (structure #)

density (g/cm3)

molar volume (cm3)

solubility parameter (J/cm3)1/2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

C1 C2 C3 C4 C5 C6-C14 saturate C6-C14 saturate C6-C14 saturate C6-C14 saturate C6-C14 saturate C6-C14 aromatic C6-C14 aromatic C6-C14 aromatic C6-C14 aromatic C6-C14 aromatic C14+ saturate C14+ saturate C14+ saturate C14+ saturate C14+ saturate C14+ aromatic C14+ aromatic C14+ aromatic C14+ aromatic C14+ aromatic C14+ NSO C14+ NSO C14+ NSO C14+ NSO C14+ NSO C14+ NSO C14+ NSO C14+ NSO C14+ NSO C14+ NSO

methane ethane propane n-butane n-pentane n-heptane n-nonane n-undacane n-tridecane pentylcyclohexane toluene tertalin 1-methylnaphthylene ethylbenzene naphthylene2r n-pentadecane n-eicosane decylcyclohexane 2-pentyldecalin (#20) decylbenzene 1-pentylnaphthylene (#23) (#24) pentyldecylbenzene (#26) (#27) (#28) (#29) (#30) (#31) (#32) (#33) (#34) (#35)

0.33 0.47 0.56 0.61 0.65 0.69 0.74 0.76 0.78 0.8 0.87 0.97 1.02 0.88 1.02 0.79 0.81 0.82 0.84 0.87 0.89 0.98 1.04 1.11 0.89 0.95 0.96 0.99 1.11 1.07 1.00 0.99 1.11 1.07 1.09

47.8 64.0 79.3 95.4 111.0 141.4 173.7 205.1 236.5 211.3 106.1 136.1 139.2 120.4 124.9 267.8 346.2 274.0 248.8 301.9 245.4 201.2 240.1 269.4 323.8 269.6 332.8 275.8 318.9 344.1 215.7 261.7 328.9 324.3 330.5

12.4 12.4 13.6 14.3 14.8 15.4 15.8 16.1 16.3 17.3 18.2 19.4 20.2 18.0 20.3 16.4 16.7 17.3 17.3 17.7 18.1 20.0 21.1 22.0 17.9 20.1 20.4 20.3 21.7 21.8 23.7 21.2 22.3 22.2 22.2

See Figure 3 for structures of molecules designated by a number.

Figure 3. Chemical structures of selected molecules from Table 4.

chemical name are shown in Figure 3. Much work has been done in relating molecular structure to the solubility parameter of a given complex molecule. Different authors have compiled group contributions to the cohesive energy and molar volume.43-45 The present (43) Hoy, K. L. J. Paint Technol. 1970, 42, 76. (44) (a) Fedors, R. F. J. Polym. Sci. C 1974, 14, 147 and 472. A Method for Estimating Both the Solubility Parameters and Molar Volumes of Liquids. Also in CRC Handbook of Solubility Parameters and Other Cohesion Parameters; Barton, A. F. M., Ed.; CRC Press: Boca Raton, FL, 1983; p 64.

work calculates the cohesive energy using the group contribution data for the molar vaporization energy compiled by Fedors44 valid for 25 °C. The calculation method takes into account group contributions from structures such as aromatic carbon, primary, secondary, and tertiary aliphatic carbons, acid groups, sulfur groups, basic and nonbasic nitrogen, and other functionalities. A reliable estimate of the cohesive energy can be made for complex organic (45) van Krevelen, D. W. In Properties of Polymers, Their Estimation and Correlation with Chemical Structure; Elsevier Scientific Publishing: Amsterdam, 1976; Tables 7.1 and 7.2.


Figure 4. Plot of solubility parameter versus molar volume for the representative molecules used in calculations of chemical fractionation.

structures for which measured data are unavailable. Molecular density from correlation and the molecular weight are used to calculate the molar volume. The solubility parameter is defined as the square root of the ratio of the cohesive energy to the molar volume. A plot of solubility parameter versus molar volume shows that the representative molecules span the range projected for most abundant petroleum species (Figure 4). The relative amount of each molecule within a molecular lump was equal in all calculations. However, the relative proportion of the molecular lumps was scaled to match the primary product composition specified in Table 3. All calculations are done at 25 °C and assume that the primary generated composition is constant and does not change as a function of maturity. It is recognized that temperature affects the parameters used in the calculation of solubility parameter (e.g., density and cohesive energy) and therefore the accuracy of the conclusions drawn at 25 °C relative to an elevated temperature will depend on the magnitude of the temperature difference. All molecules are assumed to be completely dissolved in either the oil or bitumen (e.g., no gas phase). With this assumption, the major effect of elevated pressure is the indirect effect of a decrease in molar volume that can be determined but is relatively unimportant44. Therefore, the conclusions reached in the present study are expected to be relevant to conditions of elevated pressure. 2.3. Average Thermodynamic Parameters for Kerogen. Table 1 shows the thermodynamic parameters used in model calculations for representative Type II and Type IIIC kerogens. The parameters associated with the kerogen designated A1 correspond closely with values determined experimentally for the average Type II kerogen.37 The parameters associated with the kerogens designated Structure B1 correspond closely to the values determined experimentally for the average Type IIIC kerogen.37 It can be seen that the solubility parameter varies little among the models while the cross-link density varies significantly. The thermodynamic parameters of kerogen are expected to change with increasing thermal maturation. Only a small increase (1.0 (J/cm3)1/2) in solubility parameter is expected from the lowest maturity (A1, B1) to the highest maturity (A4, B4) models for respective Type II and IIIC kerogens. This does not imply that the chemical composition of the kerogen is not changing during maturation. It is well-known that the H/C and O/C ratios decrease with increasing maturation for both Type II and IIIC kerogens. Basic chemical transformations can be understood with a van Krevelen diagram, and their impact on solubility parameter can be understood using a group additive approach for calculating the solubility parameter of kerogen.40 The loss of oxygen functional groups during maturation results in a decrease in oxygen content, and this contributes to a decrease in the solubility parameter of kerogen. In contrast, aromatization and selective production of low aromatic content thermal products result in increasing kerogen aromaticity with increasing maturity, and this contributes to an increase in the solubility parameter of kerogen. The effects of these chemistries on the solubility parameter of kerogen offset one another. The idea

Energy & Fuels, Vol. 20, No. 1, 2006 313 that the primary product composition is (to first order) constant with increasing kerogen maturity is consistent with this concept. The progressive loss of oxygen-rich and aromatic poor products serves to offset one another at all stages of maturity within the oil window. The apparent relative constancy of the solubility parameter with increasing maturity is a consequence of offsetting chemical factors. A general decrease in mean swelling ratio with increasing kerogen maturity is observed for Type I,33 Type II,35,37 and Type IIIC37 kerogens. The models reflect an increase in cross-link density with increasing kerogen maturity based on these observations. The drop in mean swelling ratio is a good indication that the cross-link density of kerogen increases during the later stages of maturation. The increase in cross-link density associated with Type II structures A3 and A4 reflects an increase that would be needed to model the swelling behavior of higher maturity kerogen samples. Likewise, structures B3 and B4 reflect increases in cross-link density needed to explain the magnitude of decreased swelling by solvents for higher maturity Type IIIC kerogen. There also is a slight increase in the solubility parameter associated with higher maturity Type II and IIIC kerogen structures to account for the well-established increase in aromaticity with increased kerogen maturity. Supporting this, the solubility parameters of the representative chemical structural models of kerogen were calculated with a group additivity approach40,41 to obtain the average solubility parameter for immature and mature kerogens. These results show that the average solubility parameter of kerogen increases slightly, ∼1.0 (J/cm3)1/2, on transitioning from immature to the most mature Type II and IIIC kerogens. The maturity trends discussed above for solubility parameter and cross-link density are incorporated into modeled average thermodynamic parameters in Table 1. Low maturity kerogens with crosslink densities higher than those used in our experiments may exist as covalent sulfide linkages or noncovalent hydrogen bonds associated with oxygen functionalities. These cross-linkages are lost as maturation proceeds. To explore this potential situation, additional variant Structures A2 and B2 are considered for low maturity (25% conversion) Type II and IIIC kerogens, respectively. These structures have a solubility parameter set equal to their respective immature A1 and B1 counterparts but with a cross-link density and native swelling fraction set equal to the most mature respective kerogen structure. These theoretical structures likely span the potential for high cross-link densities that may exist within immature kerogens, but they have not yet been rigorously determined experimentally. For example, we determined that a Monterey Type IIS kerogen with a Rock-Eval Tmax ) 411 °C and HI ) 621 mg/g has a mean swelling ratio of Qv ) 1.19 in a range of solvents used. This compares favorably with the mean predicted Qv ) 1.17 that is calculated using the thermodynamic parameters of structure A2 and is significantly lower than that observed with immature Type II kerogens in the same solvents.37 Immature Type III kerogens contain oxygen functionalities with the potential for hydrogen bonding that serve as noncovalent cross-links.31-33 A mean experimental Qv of 1.13 has been measured for Beulah Zap lignite coal. A high apparent cross-link density such as that in hypothesized structure B2 could explain these results. 2.4. Closed System Model Calculations. The amount of primary generated products relative to kerogen will vary as kerogen matures and more products are produced. In a closed system, generated bitumen will equilibrate between the kerogen and the expelled oil and conditions will vary considerably as the kerogen matures (Figure 5). Initially, the amount of kerogen is far in excess of the amount of generated products. At the latest maturity stage, the amount of products will be comparable to the amount of kerogen. The relative amounts of kerogen and its decomposition products are calculated at specific stages of maturity, defined at 25, 50, 75, and 100% conversion, based on hydrogen index evolution (section 2.2). The capacity for kerogen to retain bitumen is limited by its cross-link density and native swelling, which varies with increasing maturation and kerogen type. With the thermodynamic parameters of kerogen (section 2.1) and the molecular composition of generated


Kelemen et al.

the amount and composition of expelled oil and retained bitumen for a Type II kerogen with a hydrogen index of 400 mg/g Corg (Tables 6 and 7) and for Type IIIC kerogen with a hydrogen index of 350 mg/g Corg (Tables 8 and 9) or 200 mg/g Corg (Tables 10 and 11). In several cases, a two-phase solution to the equations for phase equilibrium could not be determined (nd). The lack of convergence is most likely due to an insufficient amount of thermal decomposition primary products to form a second phase; that is, it reflects a condition where most of the products generated are retained as bitumen. This interpretation, however, was not rigorously verified by supplemental calculations.

Figure 5. Cartoon of the closed chemical system for kerogen thermal maturation and partitioning into kerogen, bitumen, and expelled oil.

products in well-defined quantities (section 2.3), the multicomponent equilibrium between the specific molecules sorbed within kerogen and those existing in a separate hydrocarbon phase may be calculated by solving the equations for phase equilibrium in a Regular Solution Theory modeling framework.36 Compositions of the retained bitumen and expelled oil are calculated to reflect the behavior of the representative molecules.

4. Discussion The equilibrium composition of bitumen and expelled fluid was calculated for well-defined mixtures of model compounds and Type II and IIIC kerogen structures (defined by experimentally derived thermodynamic parameters of cross-link density, native swelling, and solubility parameter). The composition of the mixtures was chosen to reflect the expected range of kerogen thermal decomposition products. The compositions were held constant as a function of thermal maturity, and no consideration was given to changes in the composition of products due to secondary thermal cracking. The thermodynamic parameters of kerogen change with maturity. The parameters associated with immature Type II and IIC kerogens correspond closely with values determined experimentally. Changes in the solubility parameter for kerogen reflect changes in its chemical composition due to decreases in oxygen and hydrogen content

3. Results The amount and composition of expelled oil (Table 4) and retained bitumen (Table 5) are calculated for a Type II kerogen with a hydrogen index of 650 mg/g organic carbon for different kerogen structures (Table 1) and primary generated product compositions (Table 2). Phase equilibrium was actually determined for mixtures of molecules with well-defined thermodynamic parameters (Table 3) that are used to represent petroleum fractions. Similar calculations were made to determine

Table 4. Predicted Amount and Composition of Expelled Petroleum for Type II Kerogen with HI ) 650 mg/g Organic Carbon kerogen structure A1 A1 A2 A2 A1 A1 A1 A1 A3 A3 A3 A3 A4 A4 A4 A4

maturity

primary product

amount expelled mg/g Corg

C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100

P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4

nd nd 87 85 163 142 131 116 368 366 366 366 596 596 597 598

nd nd 11.9 12.2 12.8 15.0 16.3 18.4 9.6 9.6 9.6 9.6 8.1 8.1 8.1 8.0

nd nd 3.1 3.2 3.2 3.8 4.1 4.7 2.4 2.4 2.4 2.4 2.0 2.0 2.0 2.0

nd nd 3.2 3.3 3.3 3.9 4.2 4.8 2.4 2.5 2.4 2.4 2.0 2.0 2.0 2.0

nd nd 3.3 3.4 3.4 3.9 4.3 4.8 2.5 2.5 2.5 2.5 2.0 2.0 2.0 2.0

nd nd 3.3 3.4 3.3 3.9 4.2 4.8 2.5 2.5 2.5 2.5 2.0 2.0 2.0 2.0

weight percent C6 -14 Sat C6-14 Aro nd nd 30.5 31.3 29.1 34.5 37.6 42.5 22.1 22.2 22.2 22.2 18.2 18.2 18.2 18.2

nd nd 6.5 7.2 4.0 4.6 4.9 5.2 8.4 8.8 8.9 9.0 8.4 8.5 8.4 8.5

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 11.1 5.7 36.5 24.9 13.6 7.7 27.8 16.0 8.0 4.0 22.9 13.1 6.5 3.3

nd nd 11.8 6.4 3.4 4.2 9.3 5.2 5.6 6.1 12.5 6.5 6.1 6.1 12.3 6.2

nd nd 15.3 23.9 0.9 1.4 1.5 1.8 16.8 27.4 29.1 39.0 28.2 37.9 38.3 47.7

Table 5. Predicted Amount and Composition of Retained Bitumen for Type II Kerogen with HI ) 650 mg/g Organic Carbon kerogen structure A1 A1 A2 A2 A1 A1 A1 A1 A3 A3 A3 A3 A4 A4 A4 A4

maturity

primary product

amount retained mg/g Corg

C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 76 77 162 184 194 209 119 121 121 121 54 54 53 52

nd nd 2.4 2.3 2.1 1.7 1.6 1.4 1.0 1.0 1.1 1.1 1.0 1.2 1.3 1.3

nd nd 0.5 0.4 0.5 0.4 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

nd nd 0.3 0.3 0.4 0.3 0.3 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2

nd nd 0.2 0.2 0.4 0.3 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

nd nd 0.2 0.2 0.4 0.3 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

weight percent C6-14 Sat C6-14 Aro nd nd 1.0 0.8 4.2 3.1 2.7 2.4 0.4 0.3 0.3 0.3 0.2 0.2 0.2 0.3

nd nd 10.2 9.4 12.6 11.1 10.5 9.9 7.8 6.5 6.4 6.0 6.3 6.0 6.0 5.8

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 0.1 0.0 5.3 2.1 0.9 0.4 0.2 0.1 0.0 0.0 0.1 0.0 0.0 0.0

nd nd 12.3 5.6 8.6 7.4 13.8 6.4 7.2 5.6 10.5 4.6 5.4 4.4 8.4 3.7

nd nd 72.7 80.8 65.5 73.3 69.2 78.4 82.9 85.9 81.1 87.4 86.5 87.8 83.5 88.3



Table 6. Predicted Amount and Composition of Expelled Petroleum for Type II Kerogen with HI ) 400 mg/g Organic Carbon kerogen structure A1 A1 A2 A2 A1 A1 A1 A1 A3 A3 A3 A3 A4 A4 A4 A4

maturity

primary product


C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 62 60 62 64 48 44 164 150 146 139 310 307 309 309

nd nd 14.9 17.0 16.2 19.2 21.6 23.7 12.3 13.7 14.2 14.9 9.4 9.4 9.4 9.4

nd nd 3.9 4.5 4.1 4.8 5.4 5.9 3.1 3.5 3.6 3.8 2.4 2.4 2.4 2.4

nd nd 4.2 4.8 4.2 4.9 5.5 6.0 3.2 3.5 3.7 3.9 2.4 2.4 2.4 2.4

nd nd 4.4 5.0 4.2 5.0 5.6 6.2 3.2 3.6 3.7 3.9 2.4 2.4 2.4 2.4

nd nd 4.4 5.0 4.1 4.8 5.4 5.9 3.2 3.6 3.7 3.9 2.4 2.4 2.4 2.4

weight percent C6-14Sat C6-14 Aro nd nd 40.5 45.9 30.6 35.9 39.7 42.7 28.6 32.3 33.5 35.4 21.6 21.7 21.6 21.6

nd nd 2.5 2.7 1.7 1.8 1.8 1.8 4.5 6.2 7.0 8.1 8.3 8.7 8.7 8.8

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 15.4 8.7 33.4 22.1 12.0 6.4 36.3 23.5 12.2 6.4 27.1 15.6 7.8 3.9

nd nd 8.9 5.2 1.3 1.4 2.8 1.4 4.0 5.2 11.3 6.3 5.7 6.1 12.5 8.4

nd nd 1.0 1.3 0.1 0.1 0.1 0.1 1.6 4.0 7.2 13.4 18.4 29.0 30.5 40.4

Table 7. Predicted Amount and Composition of Retained Bitumen for Type II Kerogen with HI ) 400 mg/g Organic Carbon kerogen structure A1 A1 A2 A2 A1 A1 A1 A1 A3 A3 A3 A3 A4 A4 A4 A4

maturity

primary product


C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 38 40 138 137 152 156 136 142 154 161 90 93 91 91

nd nd 3.8 3.4 3.6 3.2 3.0 2.9 1.7 1.3 1.2 1.1 1.0 1.1 1.2 1.2

nd nd 0.9 0.8 0.9 0.8 0.8 0.7 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.2

nd nd 0.7 0.6 0.8 0.8 0.7 0.7 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0.1

nd nd 0.6 0.5 0.8 0.7 0.7 0.7 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1

nd nd 0.6 0.5 0.9 0.8 0.8 0.7 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1

weight percent C6-14 Sat C6-14 Aro nd nd 5.0 4.2 10.5 9.8 9.5 9.4 2.5 1.2 0.9 0.6 0.3 0.3 0.3 0.3

nd nd 11.1 10.6 11.2 10.6 10.3 10.1 12.7 10.3 9.4 8.4 8.0 6.9 6.7 6.3

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 1.3 0.5 15.4 8.3 4.1 2.0 2.6 0.5 0.2 0.0 0.1 0.0 0.0 0.0

nd nd 13.5 6.4 8.1 7.7 14.9 7.3 8.4 6.8 12.7 5.7 7.1 5.5 10.4 4.6

nd nd 62.4 72.5 47.8 57.3 55.2 65.5 70.7 79.1 74.9 83.6 82.9 86.8 80.9 87.1

Table 8. Predicted Amount and Composition of Expelled Petroleum for Type IIIC Kerogen with HI ) 350 mg/g Organic Carbon kerogen structure B1 B1 B2 B2 B1 B1 B1 B1 B3 B3 B3 B3 B4 B4 B4 B4

maturity

primary product


C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 46 43 78 75 70 66 171 167 164 159 313 313 313 313

nd nd 20.8 22.5 23.7 24.9 26.8 28.6 18.5 19.0 19.4 20.0 13.8 13.8 13.8 13.8

nd nd 5.4 5.8 6.0 6.2 6.7 7.2 4.7 4.8 4.9 5.0 3.5 3.5 3.5 3.5

nd nd 5.5 5.9 6.0 6.3 6.8 7.3 4.7 4.8 4.9 5.1 3.5 3.5 3.5 3.5

nd nd 5.6 6.1 6.1 6.4 6.9 7.3 4.7 4.9 4.9 5.1 3.5 3.5 3.5 3.5

nd nd 5.7 6.1 6.0 6.3 6.8 7.2 4.7 4.9 5.0 5.1 3.5 3.5 3.5 3.5

during maturation. Only a small increase (1.0 (J/cm3)1/2) in the solubility parameter with maturity was considered for each kerogen type due to offsetting factors. The progressive loss oxygen decreases the solubility parameter for kerogen while loss of hydrogen reflects increases in aromaticity and increases the solubility parameter for kerogen. These factors serve to offset one another at all stages of maturity. Decreases in the crosslink density with sample maturity were modeled based on the experimental observation that the mean swelling ratio of kerogen decreases with increasing sample maturity.33,35,37 Provisions were


nd nd 4.5 4.9 1.8 1.8 1.8 1.9 6.7 7.2 7.7 8.3 8.4 8.4 8.4 8.5

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 9.4 5.0 19.4 16.2 8.7 4.6 19.1 15.7 8.0 4.1 14.0 11.2 5.6 2.8

nd nd 8.2 4.4 2.0 1.4 2.9 1.5 5.8 4.1 8.7 4.7 7.5 5.1 10.2 5.2

nd nd 4.2 6.1 0.1 0.1 0.1 0.1 5.7 8.3 9.9 15.0 23.8 28.9 29.4 37.2

made to include low maturity kerogens with higher cross-link densities. Examination of the amount and composition of expelled oil and retained bitumen under the different modeled situations provides insight on which properties are most significant in influencing chemical fractionation during expulsion. Figure 6 shows the amount of retained bitumen and expelled fluid plotted as a function of maturity. These include all of the cases listed in Tables 4-11. The amount of bitumen reaches a maximum between 50 and 75% maturity in all cases. The greatest amount


Kelemen et al.

Table 9. Predicted Amount and Composition of Retained Bitumen for Type IIIC Kerogen with HI ) 350 mg/g Organic Carbon kerogen structure B1 B1 B2 B2 B1 B1 B1 B1 B3 B3 B3 B3 B4 B4 B4 B4

maturity

primary product


C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 41 44 97 100 105 109 91 96 99 104 37 37 37 37

nd nd 3.2 2.9 3.4 3.2 3.0 2.8 1.3 1.2 1.1 1.0 1.4 1.4 1.5 1.5

nd nd 0.6 0.5 0.8 0.8 0.7 0.7 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

nd nd 0.4 0.4 0.8 0.7 0.7 0.6 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1

nd nd 0.3 0.3 0.7 0.7 0.6 0.6 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

nd nd 0.3 0.2 0.8 0.8 0.7 0.7 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1


nd nd 12.5 11.5 13.5 13.1 12.5 12.1 11.2 10.0 9.2 8.1 7.2 6.7 6.7 6.3

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 0.1 0.0 6.9 5.3 2.6 1.2 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0

nd nd 12.1 5.6 11.9 7.7 14.7 7.1 10.7 6.5 12.2 5.5 7.6 4.5 8.5 3.6

nd nd 69.6 78.1 54.2 61.1 58.2 68.1 75.6 81.4 76.6 84.7 83.2 86.8 82.8 88.1

Table 10. Predicted Amount and Composition of Expelled Petroleum for Type IIIC Kerogen with HI ) 200 mg/g Organic Carbon kerogen structure B1 B1 B2 B2 B1 B1 B1 B1 B3 B3 B3 B3 B4 B4 B4 B4

maturity

primary product


C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 34 33 nd nd nd nd 78 75 71 66 157 156 155 155

nd nd 21.7 23.9 nd nd nd nd 21.3 22.5 23.8 25.8 15.6 15.7 15.7 15.8

nd nd 5.7 6.3 nd nd nd nd 5.4 5.7 6.0 6.5 3.9 4.0 4.0 4.0

nd nd 6.0 6.6 nd nd nd nd 5.5 5.8 6.1 6.6 3.9 4.0 4.0 4.0

nd nd 6.3 7.0 nd nd nd nd 5.6 5.9 6.2 6.7 4.0 4.0 4.0 4.0

nd nd 6.3 6.9 nd nd nd nd 5.5 5.8 6.2 6.7 4.0 4.0 4.0 4.0

weight percent C6-14Sat C6-14 Aro nd nd 34.6 37.8 nd nd nd nd 28.8 30.5 32.6 35.4 21.3 21.5 21.5 21.6

nd nd 1.7 1.7 nd nd nd nd 2.4 2.6 2.8 3.1 8.0 8.3 8.4 8.6

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 10.8 5.9 nd nd nd nd 21.4 18.2 9.8 5.3 15.9 12.9 6.4 3.2

nd nd 6.4 3.4 nd nd nd nd 3.8 2.7 6.1 3.4 7.0 4.8 9.9 5.1

nd nd 0.4 0.4 nd nd nd nd 0.2 0.3 0.3 0.4 16.3 20.9 22.2 29.6

Table 11. Predicted Amount and Composition of Retained Bitumen for Type IIIC Kerogen with HI ) 200 mg/g Organic Carbon kerogen structure B1 B1 B2 B2 B1 B1 B1 B1 B3 B3 B3 B3 B4 B4 B4 B4

maturity

primary product


C1

C2

C3

C4

C5

25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100


nd nd 16 17 nd nd nd nd 72 75 79 84 43 44 45 45

nd nd 6.0 5.5 nd nd nd nd 2.8 2.6 2.3 2.0 1.4 1.4 1.4 1.4

nd nd 1.3 1.2 nd nd nd nd 0.6 0.6 0.5 0.5 0.2 0.2 0.2 0.2

nd nd 1.1 1.0 nd nd nd nd 0.5 0.5 0.4 0.4 0.1 0.1 0.1 0.1

nd nd 0.9 0.8 nd nd nd nd 0.5 0.4 0.4 0.3 0.1 0.1 0.1 0.1

nd nd 0.9 0.8 nd nd nd nd 0.5 0.4 0.4 0.3 0.1 0.1 0.1 0.1

of bitumen (209 mg/g) is found in the case of high HI Type II kerogen. The highest amount of bitumen for Type IIIC is 109 mg/g. These amounts of bitumen compare favorably with those of natural extracted bitumen. In all cases, the amount of bitumen declines at the highest maturity. Strong chemical fractionation between saturate and NSO fractions is observed with all kerogens, source richness, and product compositions modeled. Figure 7 shows the C4+ saturates composition in the retained bitumen and the expelled fluid. The C4+ saturate level is low in all bitumen but very high in the expelled fluids. Even though the expelled C4+

weight percent C6-14Sat C6-14 Aro nd nd 4.1 3.8 nd nd nd nd 3.5 3.0 2.6 2.1 0.1 0.1 0.1 0.1

nd nd 12.9 12.3 nd nd nd nd 14.6 13.9 13.1 12.3 9.1 8.2 7.9 7.1

C14+ Sat

C14+ Aro

C14+ NSO

nd nd 0.9 0.4 nd nd nd nd 2.7 1.8 0.7 0.3 0.0 0.0 0.0 0.0

nd nd 12.5 6.0 nd nd nd nd 11.6 7.2 13.5 6.3 9.5 5.6 10.5 4.6

nd nd 59.5 68.4 nd nd nd nd 62.6 69.6 66.1 75.6 79.3 84.2 79.7 86.4

saturate level is high in all cases, there is significant variation in the amounts depending on sample maturity and source richness. Figure 8 shows that the fractionation of C4+ aromatics between bitumen and expelled fluids is not that great. In contrast, Figure 9 shows strong fractionation for the C4+ NSO fraction, and it is the dominant fraction in all bitumens. There is considerable variation in the amount of C4+ NSOs in the expelled fluid. For all kerogens and source richness the level of NSOs rise at highest maturity. NSOs appear early in the case of high cross-link density immature kerogen model systems.


Figure 6. Retained and expelled yields for different source richness Type II and IIIC kerogens as a function of maturity.


Figure 8. C4+ aromatics composition of retained and expelled for different source richness Type II and IIIC kerogens as a function of maturity.

Figure 7. C4+ saturates composition of retained and expelled for different source richness Type II and IIIC kerogens as a function of maturity.

Figure 9. C4+ NSO composition of retained and expelled for different source richness Type II and IIIC kerogens as a function of maturity.

The model developed36 from basic Regular Solution Theory appears to predict correctly not only the nature but also the extent of chemical fractionation that is observed between expelled oil and retained bitumen. A compilation of all modeled results shows that the expelled fluids are enriched in saturated hydrocarbons and depleted in NSO compounds relative to the primary products generated by kerogen thermal decomposition (Figure 10). They closely correspond to the observed range for produced petroleum (Figure 2). Conversely, the retained fluids are dominated by polar NSO compounds and are typical of natural bitumens found in source rocks of low to moderate thermal maturity that have not undergone secondary cracking.

The amount of primary generated product relative to the capacity of the kerogen to retain bitumen is an overriding factor that determines the composition of the expelled and retained fluid. Lesser effects are imposed by variations in chemical composition of the primary generated products. A closer examination of a maturation series of any kerogen structure shows that the selective retention of polar NSO compounds and expulsion of saturated hydrocarbons decreases with increasing cross-link density. Consider, for example, the maturation series results for the Type II kerogen with HI ) 650 mg/g Corg (Tables 4 and 5). No two-phase solutions were found at 25% conversion for kerogen structures A1, implying


Figure 10. Predicted composition of expelled and retained hydrocarbons.

that oil was not expelled under these conditions. Kerogen structures at 50% conversion with A1 structures expel ∼1728% of the total generated product (depending on composition), and this expelled oil is highly depleted in NSO compounds. The differences between these cases are exclusively due to the amount of primary product produced as the kerogen thermodynamic parameters are identical (structure A1). At 75% conversion, the capacity to fully fractionate the generated primary products is exceeded in the A3 kerogen structure due to the higher cross-link density and amount of product. Expelled oils account for ∼53-58% of the total product yield and contain ∼17-39% polar compounds depending on product composition. The trend continues at 100% conversion with the A4 structures having a high cross-link density and a lower capacity to retain bitumen and chemically fractionate the expelled oil. Similar trends are observed for the maturation series of the other kerogens. The influence of kerogen structure on chemical fractionation also can be seen by comparing Type II and IIIC kerogen structures at 25% conversion that differ in their thermodynamic properties. In all cases, calculations based on Type II A1 and IIIC B1 kerogens with low cross-link density (0.12 and 0.16, respectively) do not converge to a two-phase solution, suggesting that oil is not expelled. In contrast, Type II A2 and IIIC B2 kerogens with high cross-link density (0.93 and 0.70, respectively) expelled ∼52-68% of the generated primary products because of their lower absorption capacity. The nature and extent of chemical fractionation upon expulsion also is strongly dependent on the amount of generated primary products. For example, at 25% conversion Type II kerogen with A2 structures expel ∼53 and ∼61% of the generated primary products for HI values of 650 and 400 mg/g Corg, respectively. The composition of the expelled oil varies considerably. With only 100 mg/g total generated product,

Kelemen et al.

kerogen capacity for chemical fractionation is optimal and the expelled oil is composed of only ∼1% polar NSO compounds. With 162 mg/g total generated product, the amount exceeds the kerogen capacity and the expelled oil is composed of ∼1524% polar NSO compounds. Similar effects are seen in the Type IIIC B2 kerogens where at 25% conversion the relative concentration of NSO compounds differ from ∼5 to 0.4% for Type IIIC kerogen with initial HI values of 350 and 200 mg/g Corg, respectively. Low maturity kerogens with high cross-link density likely exist and are mostly probably associated with Type IIS and IIIV kerogen structures. However, their thermodynamic parameters have not been rigorously established. This is partly due to their weak swelling response to solvents that translates into large experimental uncertainty in determining their thermodynamic parameters. This weak swelling response is a clear indication that the cross-link and native swelling parameters are significantly higher for these kerogens relative to most other kerogen structures. Immature sulfur-rich Type IIS kerogens contain weak sulfur and oxygen linkages that are modeled to impart high cross-link densities. During early catagenesis, large quantities of low maturity, polar-rich oil are produced and expelled. With advancing thermal stress, atoms associated with these crosslinking bonds break and the kerogen structure becomes similar to typical Type II kerogens and the generated products are more typical as well. The simultaneous occurrence during early catagenesis of kerogen with high cross-link density and large amounts of polar-rich primary products would promote the expulsion of a large proportion of the generated oil with apparently little chemical fractionation. The retention of bitumen in low potential source rocks through catagenesis is another consequence of the interaction between kerogen structure and the amount of primary generated product. At 50% conversion, Type IIIC kerogen with an initial HI value of 350 mg/g Corg expels ∼37-44% of the total generated products (175 mg/g). The expelled oil is highly enriched in saturated hydrocarbons with most of the aromatic and nearly all of the polar species retained in the bitumen. In contrast, no solution to the equilibrium equations is found when a Type IIIC source has a lower initial HI 200 mg/g Corg. We interpret the lack of convergence as indicating no oil is expelled when only 75 mg/g of product is generated. At 75% conversion, both the low and high potential Type IIIC kerogens expel oil. Even with the increase in cross-link density due to changes in the kerogen structure, the amount of product (150 mg/g) generated from low potential source does not exceed conditions needed to overcome chemical fractionation, and the expelled oil is depleted in polar compounds. The greater product yield (263 mg/g) from the higher potential Type IIIC source coupled with the higher crosslink density is sufficient that the calculated chemical fractionation is less than the conditions described above. This decrease in fractionation continues into the 100% conversion level when both the high and low potential Type IIIC kerogens are modeled to expel oil with ∼16-37% polar compounds. Although enhanced expulsion of polar compounds under late catagenicmetagenic thermal conditions seems paradoxical, polar C14+ compounds will undergo secondary cracking in natural systems and will not actually be present in the fluid products. When secondary cracking is considered, Type IIIC kerogen with high potential is modeled to expel a highly paraffinic, polar-depleted fluid during peak oil generation and hydrocarbon-rich condensate and gas at higher levels of thermal stress. Under identical conditions with the same kerogen structures and primary product compositions, a lower potential source will not expel oil within


the oil window. The retained bitumen will crack, and hydrocarbon-rich condensate and gas will be expelled at higher levels of thermal stress. The nature and extent of chemical fractionation during expulsion is influenced by the composition of the primary generated products. For the Type II kerogens, the rank order for the percentage of C14+ NSOs in the expelled fluid (P4 > P3 ≈ P2 > P1) is related to the rank order of C14+ NSOs in the primary products (P4 > P3 ) P2 > P1) at a given level of conversion. Similarly, the rank order for the percentage of C14+ NSOs in the fluids expelled from Type IIIC kerogens (P8 > P7 ≈ P6 > P5) is related to the rank order of C14+ NSOs in the primary products (P8 > P7 ) P6 > P5). Chemical fractionation of the C14+ saturated and aromatic hydrocarbons behaves in an identical manner. Ranking of the C15+ saturate/aromatic ratio for the expelled fluids (P1 > P2 > P3 ≈ P4) from Type II kerogens mirrors the ranking of this ratio in the primary products (P1 > P2 > P3 ) P4). Ranking of the C14+ saturate/aromatic ratio for the expelled fluids from Type IIIC kerogens (P5 > P6 . P7τ P8) mostly reflects the ranking of this ratio in the primary products (P5 > P6 . P7 > P8). Results from the various multicomponent equilibrium calculations also explain the apparently anomalous range in the extent of oil saturation that is needed for expulsion to occur. In many simulators, expulsion threshold values of oil saturation are derived empirically such that the model results agree with field observations. These saturation values range from as low as 246 to 40%,47 or higher.3 Our calculations show that the amount of primary generated product is only one factor in determining the extent of expulsion and that the kerogen structure and to a lesser extent product composition are other key parameters that must be taken into account.


lated using an extended Flory-Rehner Regular Solution Theory framework. Thermodynamic parameters for kerogen (solubility parameter, cross-link density, and native swelling) were derived experimentally and were modeled for primary generative products based on representative molecules. Equilibrium calculations were then made for a closed system as a function of maturity. From these calculations we conclude: (1) The amount of generated product relative to the capacity of the kerogen to retain bitumen exerts a controlling influence on expelled fluid composition. Lower source potential and crosslink density promote bitumen retention and enrich expelled oil in saturated hydrocarbons. Conversely, higher source potential and cross-link density promote expulsion during early catagenesis and enrich the expelled fluid in polar compounds. The crosslink density of kerogens can vary between organic matter type and level of thermal maturity. (2) Differences in the measured solubility parameter between Type II and IIIC kerogens and variations in the composition of primary generated products appear to exert less influence on the expelled fluid composition. (3) The range in composition of calculated C4+ expelled products closely matches that observed in unaltered produced petroleum. The predicted bitumen (kerogen-retained, soluble organic compounds) compositions are dominated by C14+ NSOs (>50%) at all levels of maturity for all modeled kerogens. (4) The most significant mechanisms for the chemical fractionation that occurs during expulsion have been identified, and a theoretical model that describes this process has been constructed. Coupled with models of thermal generation and secondary cracking, it is possible to accurately predict the quantity and quality of expelled petroleum and retained bitumen.

5. Conclusions Multicomponent equilibrium between model Type II and IIIC kerogens and their thermal decomposition products was calcu(46) Burrus, J.; Wolf, S.; Osadetz, K.; Visser, K. Bull. Can. Pet. Geol. 1996, 44, 429-445. (47) Braun, R. L.; Burnham, A. K. User’s Manual for PMOD, a Pyrolysis and Primary Migration Model; LLNL Report UCRL-MA-107789; Lawrence Livermore National Laboratory: Livermore, CA, 1991.

Acknowledgment. We thank Linda M. Kwiatek for conducting all of the swelling studies and P. J. Mankiewicz, A. E. Bence, W. A. Symington, Y. Xiao, J. W. Larsen, M. L. Gorbaty, B. G. Silbernagel, T. C. Halsey, and M. Siskin for valuable discussions during the course of the work. EF058023S

Petroleum Expulsion Part 3. A Model of Chemically ... - ACS Publications

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