Changes in Type II Kerogen Density as a Function of Maturity

Energy & Fuels 2005, 19, 2495-2499

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Changes in Type II Kerogen Density as a Function of Maturity: Evidence from the Kimmeridge Clay Formation Kenneth S. Okiongbo,† Andrew C. Aplin,*,† and Steve R. Larter‡ NRG, School of Civil Engineering and Geosciences, Devonshire Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, U.K., and PRG, Department of Geology and Geophysics, University of Calgary, 844 Campus Place, NW Calgary, Alberta, T2N 1N4, Canada Received July 1, 2005. Revised Manuscript Received September 13, 2005

Kerogens were purified from 26 samples of the Kimmeridge Clay Formation over the full range of maturities pertinent to petroleum generation. Most samples comprised >90% amorphous organic matter (AOM). Prior to and during the early phase of petroleum generation, kerogen densities range between 1.18 and 1.25 g cm-3. During peak and late stage petroleum generation, densities increase to ∼1.35 g cm-3 as hydrogen indices decrease from ∼350 to 50 mg HC/g C. The data are qualitatively consistent with the loss of alkyl carbon from kerogen to petroleum and the increased aromatization of remaining carbon. The density increase observed for AOM contrasts with the data for vitrinite, which exhibits a decrease in density at maturity levels relevant to petroleum generation. The contrasting behavior of AOM and vitrinite is thought to reflect the differing structural composition of the two kerogen types, most obviously the greater initial aromaticity of vitrinite.

1. Introduction In his classic text, van Krevelen1 published data showing how the density of vitrinite changed as a function of maturity. Densities decreased from ∼1.5 g cm-3 at a carbon content of 50% to a minimum of ∼1.27 g cm-3 at a carbon content of 87%, before increasing to 2.25 g cm-3 at 100%, the value of pure graphite. Van Krevelen1 explored these changes both qualitatively and quantitatively, explaining them in terms of a two-stage process involving an early loss of oxygen (density decrease) followed by a later loss of hydrogen. Of particular relevance to the maturity range of oil and gas generation, van Krevelen’s1 data suggest that the density of vitrinite decreases from ∼1.36 g cm-3 at 0.5% reflectance to a minimum of ∼1.28 g cm-3 at 1.1% reflectance. Fifty years later, there is no comparable dataset describing how the density of oil-prone, Type II amorphous kerogen changes as a function of maturity. This is surprising because the quantitative assessment of petroleum source rock volumetrics, especially changes in fluid pressure as a result of petroleum generation, requires this information.2,3 A key technical problem has been to isolate pure kerogen separates, especially the separation of kerogen from often intimately associated * Corresponding author. E-mail: [email protected]. † University of Newcastle. ‡ University of Calgary. (1) Van Krevelen, D. W. Coal; Elsevier: London, 1961. (2) Durand, B. Org. Geochem. 1988, 13, 445-459. (3) Durand, B.; Alpern, B.; Pittion, J. L.; Pradier, B. In Thermal history of sedimentary basins: Methods and case histories; Naeser, N. D., McCulloh, T. H., Eds.; 1989, pp 441-471.

pyrite with a density of 5.02 g cm-3.4 Density values between 1.15 and 1.65 g cm-3 have been previously reported for immature Type II kerogen.4-8 This range is as large as that for the full range of coal macerals at maturities below 1.1% Ro and probably reflects the difficulty of isolating pure kerogen. The key aim of this paper is thus to report density data for a suite of purified, largely amorphous, Type II kerogens isolated from the upper Jurassic Kimmeridge Clay Formation (KCF), the major North Sea petroleum source rock. The samples cover the full range of maturities pertinent to petroleum generation. 2. Samples and Methods 2.1. Sample Selection. This investigation is based on 26 KCF samples taken from the central and northern parts of the North Sea, the Norwegian Margin, and the area to the west of the Shetland Isles (Figure 1; Table 1). One was taken from the type locality of the KCF in Dorset, southern England. The samples are a mixture of archived ditch-cuttings and cores from petroleum wells. As a marine, clastic source rock, KCF kerogen often comprises a mixture of mainly Type II kerogen mixed with a minority of Type III kerogen. Aiming to isolate pure Type II, amorphous kerogen, we sampled uniquely from zones of the KCF with a gamma ray signal over 100 API because the reducing conditions responsible for the precipita(4) Hartgers, W. A.; Damste, J. S.; De Leeuw, J. W.; Yue, L.; Crelling, J. C. Org. Geochem. 1995, 23, 777-784. (5) Kinghorn, R. R. F.; Rahman, M. J. Pet. Geol. 1980, 6, 179-194. (6) Kinghorn, R. R. F.; Rahman, M. J. Pet. Geol. 1983, 6, 179-194. (7) Senftle, J. T.; Yordy, K. L.; Barron, L. S.; Crelling, J. C. Org. Geochem. 1991, 17, 275. (8) Stankiewicz, B. A.; Kruge, M. A.; Crelling, J. C. Energy Fuels 1994, 8, 1513-1521.

10.1021/ef050194+ CCC: $30.25 © 2005 American Chemical Society Published on Web 10/05/2005

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Table 1. Sample and Analytical Dataa well location

well name

depth (m)

temp (°C)

TOC (wt %)

AOM (%)

phyto. (%)

palyno. (%)

Tmax (°C)

fluor. index

density (g/cm3)

HI (mg HC/g TOC)

Fe (%)

Al (%)

1 2 3 4 5 5 2 4 3 3 4 12 8 9 8 12 12 8 11 12 7 13 7 7 7 14

Dorset outcrop 204/27A-1 31/2-2 205/20-1 202/3-1 202/3a-3 204/28-2 205/26a-3 31/4-10 31/4-6 205/23-1 16/7b-20 211/12A-M1 25/2-6 211/12A-M16 16/7b-20 16/7b-20 211/12A-M16 34/8-6 16/7b-28 3/29-2 6507/2-3 3/29a-4 3/29a-4 3/29a-4 6205/3-1R

2043 1515 1986 1600 1773 2330 2414 2007 2132 3770 3932 3125 3161 3376 4134 4157 3401 3578 4132 4608 3848 4707 4742 4781 4450

n.d. 44 52 56 58 60 60 66 76 79 91 96 97 100 102 102 102 103 n.d. 106 130 135 141 142 144 157

8.22 6.5 3.83 2.29b 3.44 2.59b 9.98 7.64 4.87 7.1 4.23 7.71 7.52 7.7 8.71 8.47 8.8 8.32 9.04 9.63 6.07 2.42b 5.11 5.62 6.18 4

92 n.d. 80 93 n.d. n.d. 92 96 96 76 n.d. n.d. 76 92 96 n.d. 75 n.d. 97 92 95 99 n.d. n.d. 98 94



422 404 417 n.d. 417 n.d. 407 423 423 421 425 430 423 366 421 430 430 425 437 438 452 n.d 454 455 464 477

5 5 5 5 4 4 5 3 5 5 n.d. n.d. 4 4 4 n.d. 4 n.d. 3 3 1 1 n.d. n.d. 1 1

1.21 1.25 1.23 1.23 1.29 1.24 1.22 1.18 1.2 1.2 1.28 1.22 1.21 1.24 1.24 1.25 1.27 1.22 1.21 1.25 1.38 1.23 1.36 1.35 1.35 1.32

525 260 277 500b 260 308b 406 303 358 357 211 393 287 316 138 216 259 121 315 250 35 198 48 38 65 44

0.21 0.79 1.21 1.27 1.3 1.39 4.17 0.14 1.08 1.73 n.d. n.d. n.d. 1.59 0.43 n.d. 0.31 n.d. 2.11 0.48 0.41 1.45 n.d. n.d. 0.19 1.26

0.04 0.04 0.15 0.05 0.04 0.04 0.04 0.06 0.18 0.2 n.d. n.d. n.d. 0.2 0.07 n.d. 0.14 n.d. 0.27 0.12 0.15 0.23 n.d. n.d. 0.07 0.17

a AOM ) amorphous organic matter; phyto. ) phytoclasts; palyno. ) palynomorphs; n.d. ) not determined. b Isolated kerogen mixed with quartz sand. For well locations, see Figure 1.

Figure 1. Location of samples. Refer to Table 1 for names of well locations. tion of uranium are also conducive to the preservation of amorphous organic matter.9 Temperatures, which we believe are maxima in all but the Dorset outcrop, were generally assessed from bottomhole temperatures measured during logging runs, corrected by Horner type methods. These methods account for borehole environmental parameters such as diameter, drilling disturbance time, and the time since circulation stopped. In some cases, temperatures were estimated from Drill Stem Test data (9) Bordenave, M. L.; Espitalie, J.; Leplat, P.; Oudin, J. L.; Vandenbroucke, M. In Applied Petroleum Geochemistry; Bordenave, M. L., Ed.; Editions Technip: Paris, 1993, pp 218-278.

from nearby reservoirs, corrected for shut-in times and extrapolated according to an average thermal gradient for the section. Corrected temperatures for the samples range from 44 to 157 °C (Table 1). 2.2. Chemical and Optical Analysis. Samples were ground, and organic carbon contents (TOC) were determined using a LECO R LS-100 Carbon-Sulfur Analyzer. Bulk pyrolysis parameters were obtained by Rock-Eval pyrolysis.10 Carbonate and silicate minerals were removed from the samples with hydrochloric and hydrofluoric acids. 0.5 g of the enriched kerogens was then depyritized using chromous chloride in hydrochloric acid, following the procedure of Zhabina and Volkov11 and Canfield et al.12 In some cases, this step was repeated. Apart from its efficacy at dissolving pyrite, one of the benefits of the chromous chloride technique is that it is a reductive process and, unlike oxidative methods for removing pyrite, is unlikely to alter the structure or composition of kerogen. Petrographic analysis was performed using standard preparation techniques, similar to those described by Barss and Williams.13 Quantitative palynological analysis of the overall kerogen composition was carried out on sieved, unoxidized kerogen material mounted on glass slides to determine the general character of the organic matter. Over 300 counts of the various particle species were made during several traverses over different parts of the slide, distinguishing between amorphous organic matter (AOM), phytoclasts (high plant debris), and palynomorphs (organic-walled microfossils). The count data were recorded on an automatic point counter device and used to calculate the percentage particle abundance. Following the counting, the kerogen slides were examined under a UV fluorescence microscope to provide a qualitative assessment of the character of the amorphous (10) Espitalie, J.; Mader, M.; Tissot, B. Offshore Technol. Conf. 1977, 439-443. (11) Zhabina, N. N.; Volkov, I. I. In Environmental Biogeochemistry and Geomicrobiology; Krumbein, W. E., Ed.; Ann Arbor Sci.: Ann Arbor, MI, 1978; Vol. 3, pp 735-745. (12) Canfield, D. E.; Raiswell, R.; Westrich, J. T.; Reaves, C. M.; Barker, R. A. Chem. Geol. 1986, 54, 149-155. (13) Barss, M. S.; Williams, G. L. Geol. Surv. Can., Pap. 1973, 7326, 1-25.

Density of Type II Kerogen

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organic matter. Estimation of the fluorescence intensity was based on Tyson’s14 fluorescence preservation scale. To assess the purity of the depyritized kerogen isolates, quantitative analysis of the iron, aluminum, and silicon contents of the depyritized kerogens was carried out using conventional aqua regia digestion15 as a way of inferring the presence of aluminosilicates, residual quartz, and pyrite. As a final purity check, some samples, including the one with the highest Fe content (204/28-2), were analyzed by X-ray diffraction. 2.3. Density Gradient Centrifugation. A detailed description of the experimental setup and principles of the density gradient technique is given by Dyrkacz and Horwitz16 and Dyrkacz and Bloomquist.17,18 A cesium chloride density gradient ranging from 1 to 1.5 g cm-3 was made in a 30 cm3 plastic centrifuge tube using a BDH gradient maker. About 40 mg of kerogen was suspended in 1.0 cm3 of water, and 0.4 g of nonionic surfactant Brij-35 (polyoxyethylene-23-lauryl ether) was added as a wetting and dispersing agent. To disperse the particles completely, the suspension was also subjected to intermittent treatment with an ultraturax type blender for several seconds. The kerogen slurry was then carefully placed as a layer on the gradient liquid. This suspension was centrifuged in a Sorval RC-5B Plus centrifuge using a Sorval SS-34 zonal rotor, which rotated at 17 500 rpm (gmax ) 30 500) for 1 h. After centrifugation, the gradient was fractionated and the densities of the kerogen particles were determined by measuring the mass of 1 cm3 of that part of the gradient containing the kerogen particles.

3. Results and Discussion 3.1. Kerogen Type and Purity. The kerogens are dominated by amorphous organic matter, which comprises greater than 75% and generally greater than 90% of the total assemblage (Table 1). Phytoclasts (wood debris), generally dominated by black material, comprise less than 25% and usually less than 10% of the organic matter. Palynomorphs are rare, comprising less than 6% of the assemblage. The fluorescence intensity of immature samples is high, consistent with the wellpreserved, oil-prone nature of the organic matter.19 At higher temperatures, the fluorescence intensity decreases as a function of petroleum generation. Chemical and mineralogical data indicate that the kerogen isolates lack significant inorganic contaminants. The abundance of aluminum, silicon, and iron was measured as potential indicators of the presence of aluminosilicates, quartz, and pyrite, respectively. Silicon concentrations are below detection limit, while aluminum concentrations are always below 0.3% and generally below 0.1%. In contrast, iron concentrations average 1.1% and range between 0.14% and 4.17%. In the form of pyrite (FeS2), 1.1% iron corresponds to 2.4% pyrite, which would increase the apparent kerogen density by around 0.09 g cm-3, an unacceptably large number. However, there is no XRD evidence for any pyrite. Figure 2 shows the X-ray diffractogram of the (14) Tyson, R. V. An introduction to Palynological Fluorescence Microscopy; http://nrg.ncl.ac.uk/people/staff/Tyson/microscopy3.html, 1995; pp 1-3. (15) Chen, M.; Ma, L. Q. Soil Sci. Soc. Am. J. 2001, 65, 491-499. (16) Dyrkacz, G. R.; Horwitz, E. P. Fuels 1982, 61, 3-12. (17) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 1992a, 6, 357-374. (18) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 1992b, 6, 374-386. (19) Tyson, R. V. Sedimentary organic matter: Organic Facies and Palynofacies; Chapman and Hall: London, 1995.

Figure 2. X-ray diffractogram of sample 204/28-2 2330 m (a) before and (b) after treatment with CrCl2 to remove pyrite. P ) pyrite; Q ) quartz.

sample with the highest amount of iron (sample 204/ 28-2; 2330 m; 4.17% Fe), both before and after CrCl2 treatment to remove pyrite. While pyrite is the major mineral phase prior to treatment, there is no evidence for pyrite in the treated sample. Indeed, there is almost no evidence of mineral matter in the treated samples, except for minor peaks that we were unable to assign to specific minerals. As a final check, we repeated the CrCl2 treatment on some samples, but did not measure changes in either the sulfur contents or the densities of the retreated samples. We conclude that our samples are essentially pure kerogens, comprising more than 75% and generally more than 90% AOM. The iron is most likely organically bound. 3.2. Geochemistry. Samples were taken at temperatures ranging between 44 and 157 °C (Table 1). At heating rates typical of the North Sea, petroleum generation occurs between ∼100 °C (Ro ≈ 0.5%) and 150 °C (Ro ≈ 1.2%). We can therefore compare variations in the organic matter prior to oil generation as well as changes that occur as a function of generation. Total organic carbon ranges from 3.44% to 9.8%, mainly between 4% and 9% (Table 1). Because petroleum generation and expulsion reduces the TOC content of oil-prone source rocks by around one-half, 20 the original TOC contents of the most mature samples (20) Larter, S. R. Mar. Pet. Geol. 1988, 5, 194-204.

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Figure 3. Hydrogen index as a function of temperature (left) and depth (right).

would have been around 9-12%. Hydrogen index (HI) values of immature samples vary from 260 to 530, with most samples showing values between 300 and 400. Given the fact that most of our samples comprise relatively pure AOM, we can only speculate as to the reason for the variation in HI; because partial oxidation of organic matter close to the sediment-water interface would preferentially remineralize more hydrogen-rich, low-density organic matter, one possibility is that variations in density reflect a variable degree of organic matter preservation, in turn reflecting variations in the oxicity of the bottom water at the time of deposition.21 At temperatures between 100 and 150 °C, petroleum generation and expulsion is reflected in a reduction in HI from 250 to 450 mg HC/g TOC to less than 50 mg HC/g TOC (Figure 3). The reduction in HI occurs mainly over a depth range between 3500 and 4500 m (Figure 3). 3.3. Kerogen Density. Densities of immature samples (below 100 °C) range from 1.18 and 1.29 g cm-3 and are mainly between 1.2 and 1.25 g cm-3 (Table 1; Figure 4). For these samples, there is no significant correlation between density and (a) iron, (b) temperature, (c) HI, and (d) fraction of nonamorphous kerogen, suggesting that this range reflects the true density range of immature, AOM dominated Type II kerogen. These values are similar to those previously reported by Stankiewicz et al. (1994)8 (1.18-1.22 g cm-3) and Senftle et al. (1991)7 (1.12-1.32 g cm-3). The effects of maturation on kerogen density are shown in Figure 4. Densities are essentially constant prior to petroleum generation, with little or no change at HI values above ∼300. Densities then increase from ∼1.25 to ∼1.35 g cm-3 as HI is reduced by petroleum generation from ∼300 to ∼50-100 (Figure 4). In these samples, this occurs over a temperature range of 110140 °C and a depth range of 3.5-4.5 km. In terms of thermal maturity, a clear increase in density occurs only at Tmax values greater than 440 °C, corresponding to an approximate vitrinite reflectance of 0.8%. Type II kerogen densities thus increase at maturity levels related to peak and, most obviously, late oil generation. Without detailed structural data for these kerogens, we can only speculate as to the cause of the increasing density. However, Patience et al.22 published 13C NMR

Figure 4. Kerogen density as a function of hydrogen index (upper left), depth (upper right), temperature (lower left), and Tmax (lower right).

data for a similar set of KCF kerogens. Critically, these data showed (a) a loss of heteroatom-bonded carbon (to O or S) with increasing maturity, both prior to and during petroleum generation, (b) that the aromaticity of the kerogen increased only modestly prior to petroleum generation but substantially thereafter, and (c) that the increased aromaticity is not simply a concentration of existing aromatic carbon as a result of the loss of alkyl carbon, but that aromatization reactions occur during petroleum generation. The major changes in aromaticity occur as HI decreases from ∼350 to ∼70 mg HC/g C. Qualitatively, these results explain the density trends reported here. They also suggest that for AOM dominated Type II kerogen, the major factor driving increasing density is not the loss of oxygen but rather the loss of alkyl carbon and the related aromatization of remaining kerogen. This contrasts with van Krevelen’s1 explanation for the reduction of vitrinite density at oil window maturities, which centered on the loss of oxygen moieties. The contrasting behavior of AOM and vitrinite reflects the differing structural composition of the two kerogen types, most obviously the greater initial aromaticity of vitrinite. (21) Ebukason, E. J.; Kinghorn, R. R. F. J. Pet. Geol. 1985, 8, 435462. (22) Patience, R. L.; Mann, A. L.; Poplett, I. J. F. Geochim. Cosmochim. Acta 1992, 56, 2725-2742.

Density of Type II Kerogen

4. Summary and Conclusions The density of immature, AOM dominated Type II kerogen in the Kimmeridge Clay Formation is generally between 1.18 and 1.25 g cm-3 and is approximately constant prior to, and during the early stage of, petroleum generation. Through the peak and late phase of petroleum generation, densities increase to ∼1.35 g cm-3, as HI decreases from ∼350 to 50 mg HC/g C. The increasing density is qualitatively consistent with the loss of alkyl carbon to petroleum and the increased aromatization of remaining carbon. The density increase observed for AOM contrasts with the data for vitrinite,1

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which exhibits a decrease in density at maturity levels relevant to petroleum generation. Acknowledgment. We thank Niger Delta University for the financial support of K.S.O.’s postgraduate research. BP and Norsk Hydro, via Steve Cawley and Balazs Badics, kindly supplied samples and ancillary data, and Richard Tyson and Lisa Buckley guided our microscopy. The two reviewers’ constructive comments were much appreciated. EF050194+

Changes in Type II Kerogen Density as a Function of Maturity

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