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Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia
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Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia Geoff Clayton a,∗ , Robbie Goodhue b , Sami T. Abdelbagi c , Marco Vecoli c a
Department of Animal and Plant Sciences, University of Sheffield, Western Bank, S10 2TN Sheffield, United Kingdom b Department of Geology, Trinity College, University of Dublin, Dublin 2, Ireland c Saudi Aramco, Dhahran, Saudi Arabia
Abstract Palynomorph Darkness Index (PDI) determinations are presented from a sub-mature, cored, Carboniferous well section in northern Saudi Arabia and are correlated to Vitrinite Reflectance (Ro) and Vitrinite Reflectance (Calculated) results from the same section. PDI values from smooth, simple miospores range from 34–57%, whereas mean Ro and Vitrinite Reflectance (Calculated) both range from ca. 0.4–0.6%. The PDIs of the Recent spores Lycopodium clavatum and Cheilanthes viridis were also determined for comparative purposes. Considerable variation in PDI was recorded in all the samples investigated. A very tentative correlation of PDI to Ro is presented for samples with a range of thermal maturity from those unaffected by any heating, to those at the top of the oil window. © 2017 Elsevier Masson SAS. All rights reserved. Keywords: PDI; Spores; Vitrinite reflectance; Thermal maturity; Carboniferous; Saudi Arabia
1. Introduction Following publication of Staplin’s (1969) Thermal Alteration Index (TAI), numerous qualitative schemes relating palynomorph colour to thermal maturity have been established. Many of these are based on reference sets of palynomorph specimens or photomicrographs, and most employ numerical scales for colour categories. However, the numbers used in these schemes are simply labels with no quantitative significance. The few quantitative methods that have been proposed typically use expensive equipment not generally available to most workers, for example, the system based on microspectrophotometry described by Marshall (1991). The various methods for assessing maturity using palynomorph colour are comprehensively reviewed by Goodhue and Clayton (2010) and Hartkopf-Fröder et al. (2015). Palynomorph Darkness Index (PDI) was described by Goodhue and Clayton (2010) as a rapid and inexpensive method ∗
Corresponding author. E-mail address: [email protected] (G. Clayton).
of quantitatively describing palynomorph colour. This scheme differs from most earlier attempts in that it integrates colour intensities to produce greyscale values, rather than considering three variables (red, green and blue intensities). Although its methodology has been clearly described, the usefulness of PDI has been limited by the absence of correlation to vitrinite reflectance (VR), the industry standard for determination of maturity. Well 667-44 is a cored stratigraphic borehole drilled in 2009 by Saudi Aramco in northern Saudi Arabia. It includes one of the most complete Permo-Carboniferous successions in the region and is intended to serve as a litho- and biostratigraphic reference section for the Mississippian Berwath Formation. The location of the well and its generalised lithostratigraphy are shown in Figs. 1 and 2 respectively. In terms of thermal maturity, the section investigated ranges from submature at its top, to the ceiling of the oil window at its base, constituting a critical interval for the correlation of PDI to VR. In order to determine the PDI of palynomorphs that had not been subjected to any form of maturation process, spores of a Recent fern and lycopod were also studied.
http://dx.doi.org/10.1016/j.revmic.2017.07.002 0035-1598/© 2017 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Fig. 1. Location of Well 667-44.
2. Materials and methods 2.1. Sampling and sample processing Grey mudrock samples were collected from the core for PDI, VR (Ro ) and VRcalculated determination. For PDI, standard palynological extraction and mounting techniques were employed, using hydrofluoric acid but no oxidation. For VR, whole-rock plugs were prepared using cold-setting epoxy resin. These were polished and detrital organic-matter fragments were identified microscopically in reflected and fluorescent light. Depending on fragment size, one or more random reflectance measurements (Ro) was made on each fragment using the standard technique coal petrography and reflectance investigation technique of Bustin et al. (1985). Two miospore genera were used for PDI determination in Well 667-44; Punctatisporites spp. and Waltzispora spp. Both produced consistent results but the latter only occurred commonly in two samples. The Recent spores investigated were obtained from two sources. Spores of the lycopod, Lycopodium clavatum were extracted from commercially available Lycopodium tablets which were dissolved in warm water. The spores were then concentrated by sieving at 15 m, and
®
®
mounted using Cellosize dispersal agent and Elvacite cold mounting medium. Spores of the fern, Cheilanthes viridis were obtained by shaking pinnules with mature sporangia over cold water, then concentrating and mounting the spores using the method described above. No chemical treatment was employed. Examples of the fossil and Recent palynomorphs used in this investigation are illustrated in Plate 1. 2.2. PDI determination RGB intensities were measured by the method described by Goodhue and Clayton (2010), using a Nikon Eclipse E600 microscope with a 12 V, 100 W halogen bulb, NCB filter, × 40 and × 60 plan fluor objective lenses, and a DXM1200 digital camera with ACT-1 software. The microscope was set up for Kohler illumination. Default settings were used for the camera with the NCB filter installed. With a slide in focus, the lamp voltage and condenser diaphragm were carefully adjusted to obtain RGB intensities less than, but as close as possible to 255, 255, 255. The automatic white balance control was not used. Images were captured, representative areas for measurement selected, and values for RGB intensity obtained.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Fig. 2. a. Generalised lithological succession of Well 667-44. b. Vitrinite reflectance (Ro) and Vitrinite Reflectance (calculated). c. Palynomorph Darkness Index (PDI) Punctatisporites spp. with 2 standard deviation bars and PDI Waltzispora spp. with 2 standard deviation bars.
Calculation of PDI is based on the National Television System Committee (NTSC) and Joint Photographic Experts Group (JPEG) RGB to greyscale conversion equation: Y = 0.299R + 0.587G + 0.114B where Y is the calculated greyscale value, R is red intensity, G is green intensity and B is blue intensity. However, as one or more of the measured white background intensities (Rb , Gb , Bb ) was invariable slightly less than 255, the correction published by Goodhue and Clayton (2010) was applied: Ya = (0.299R ∗ 255/Rb ) + (0.587G ∗ 255/Gb ) + (0.114B ∗ 255/Bb ) where Ya is the adjusted greyscale value. Palynomorph Darkness Index (PDI) is then defined as: PDI (%) = 100 − (100Ya /255) 2.3. Vitrinite reflectance (Ro) determination The system used consists of a Zeiss Axiotech incident and transmitted light microscope equipped with mechanical stage and tungsten, halogen and xenon gas discharge light sources. To measure VR, the microscope is equipped with a J&M Analytik’s TIDAS S MSP 200 photomultiplier that perform fast and highly
sensitive measurements, in conjunction with software that controls the photometer and the microscope to record data, build histograms, and calculate statistical parameters. KB standards; sapphire, YAG, and GGG were used for calibration. 2.4. Vitrinite reflectance calculated (VRcalculated ) determination To validate and confirm present day thermal maturity measured on vitrinite reflectance (RO ) for the sample set used for this study, vitrinite reflectance (calculated) (VRcalculated ) was determined from Rock-Eval Tmax using the conversion equation published by Jarvie et al. (2001): VRcalculated = 0.018.T max −7.16 The Jarvie Equation was developed for the Barnett Shale, and is extensively used to estimate thermal maturity in basins worldwide. The Vitrinite reflectance (calculated) (VRcalculated ) for the interval studied ranges from 0.36 to 0.63% with an average VRcalculated of 0.50% (Fig. 2b). 3. Results The PDI and VR results obtained are summarized in Table 1 and the VRcalculated results are shown in Table 2. The palynomorph assemblages studied were generally well preserved.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Plate 1. Figured specimens housed in the Palynology Collection of the Department of Animal and Plant Sciences, University of Sheffield. The 10 m scale bar in Fig. 9 applies to all figures. The images of the spores are optimised to best illustrate their morphology and may not accurately represent their colour. Figs. 1–4. Punctatisporites spp. Fig. 1. 667-44 (1), M40/4. Fig. 2. 667-44 (1), P49/2. Fig. 3. 667-44 (1), F36/1. Fig. 4. 667-44 (1), H28/0. Fig. 5. Waltzispora cf. W. polita (Hoffmeister, Staplin & Malloy) Smith & Butterworth). 667-44 (1), E42/0. Fig. 6. Waltzispora sagittata Playford. 667-44 (1), Q43/0. Figs. 7 & 8. Waltzispora cf. W. planiangulata Sullivan. Fig. 7. 667-44 (1), M54/4. Fig. 8. 667-44 (1), Q35/3. Figs. 9 & 10. Cheilanthes viridis Swartz. Fig. 9. F1 (1), M51/2. Fig. 10. F1 (1), F59/4. Fig. 11. Lycopodium clavatum Linnaeus (equatorial focus) L1 (2), M41/4. Fig. 12. Same specimen as Fig. 11: distal focus.
Table 1 All PDI and Ro data. Borehole 667-44
Depth (Ft)
Punctatisporites spp. 4060.0 Punctatisporites spp. 4072.0 4190.0 Punctatisporites spp. Waltzispora spp. 4190.0 4356.0 Punctatisporites spp. Punctatisporites spp. 4275.8 4275.8 Waltzispora spp. 4412.5 Punctatisporites spp. Punctatisporites spp. 4505.0 4590.0 Punctatisporites spp. 4718.0 Punctatisporites spp. Punctatisporites spp. 4786.0 4827.0 Punctatisporites spp. Punctatisporites spp. 4950.0 4978.0 Punctatisporites spp. 5011.0 Punctatisporites spp. Punctatisporites spp. 5037.8 Cheilanthesviridis Lycopodiumsp.
Ro (%) 0.41 0.42 0.47 0.47 0.47 0.43 0.43 0.48 0.53 0.51 0.54 0.54 0.55 0.57 0.56 0.56 0.58 0.20 0.20
Standard deviation
No. particles
0.06 0.13 0.13
72 156 156
0.06 0.06
150 150
0.04
108
0.05
86
0.09 0.08
42 101
PDI (%)
Standard deviation
No. specimens
39 38 40 44 44 47 34 42 45 44 52 41 44 48 57 44 52 30 20
5 7
50 31
6 6 7 6 6
22 22 10 50 50
6 6 5 7 10 8 8 5 7 7
43 20 50 28 34 5 26 50 30 30
Measured PDI and VR (Ro ) values are shown in regular type and calculated values are shown in bold italic typeface.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Table 2 VRcalculated data. Depth (ft)
VR (calculated)
4072.50 4152.10 4191.10 4275.80 4278.90 4321.20 4322.90 4405.20 4479.00 4625.70 4687.50 4718.30 4718.30 4950.80 4978.70 4978.70 5011.40 5037.80
0.49 0.45 0.54 0.63 0.36 0.53 0.36 0.42 0.49 0.38 0.42 0.53 0.54 0.45 0.45 0.47 0.56 0.40
Best-fit trend lines were constructed for both PDI and VR (Ro ). Where possible, PDI and VR were determined from the same samples. Where this was not possible, derived VR values to match measured PDI values and derived PDI values to match measured VR values were calculated from the appropriate bestfit lines. In Table 1, measured PDI and VR (Ro ) values are shown in regular type and calculated values are shown in bold italic typeface. Measurements of PDI were made on representative areas of the scabrate to finely rugulate spore exine on C. viridis and within the lumens of the reticulate form, L. clavatum in order to establish the PDI of un-matured spores. Neither form is a perfect match in terms of morphology for the Punctatisporites spp. from well 667-44. The exine of C. viridis is typically slightly thicker than that of Punctatisporites spp. whereas that of L. clavatum is thinner. Waltzispora spp. are very similar in terms of exine thickness to the Punctatisporites spp. 4. Interpretation and discussion VR (Ro ) increases uniformly with depth through the interval studied from ca. 0.4% Ro to ca. 0.6%, the ceiling of the Oil Window (Fig. 2a). VRcalculated also increases but with greater scatter (Fig. 2a). PDI increases gradually with increasing depth but with a degree of scatter broadly comparable to that of VRcalculated (Fig. 2c). At the ceiling of the Oil Window, PDIPunctatisporites is ca. 50%. Even in samples with large numbers of PDI determinations, standard deviations are high, suggesting considerable variation in PDI within each population (Fig. 3). This corresponds to the observation made by Marshall (1991) of, “- - progressive colour change to a cusp at which a broad spread of colours from yellow to brown occur in a single sample (Fig. 4). This is the interval over which hydrocarbons are generated - - -.”
Fig. 3. Cumulative mean of PDIPunctatisporites in sample 5037.8 ft with 2 standard deviation bars.
Fig. 4. Correlation of PDIPunctatisporites and PDIWaltzispora from Well 667-44 (solid circles and crosses respectively), PDICheilanthes viridis (open circle) and PDI Lycopodium clavatum (open square) v. Ro .
Ro for vitrinite precursors (i.e. woody plant tissues at normal Earth surface temperature and pressure) is assumed to be 0.2% (Taylor et al., 1998). This value correlates with the PDI determined for Cheilanthes viridis of 30% and Lycopodium clavatum of 20%. Assuming a linear relationship, tentative correlation of PDIPunctatisporites and PDIWaltzispora to Ro in Well 667-44, combined with the PDI values determined from the Recent spores can be expressed as: PDI = 65.01 R0 + 11.857. 2 R = 0.74 However, this result is based on a very small data set and much more work would be needed to establish this relationship with a high degree of confidence.
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5. Conclusions In the submature Mississippian section in Well 66744, PDIPunctatisporites gradually increases down-section from approximately 40 to 50% at the ceiling of the Oil Window. PDIPunctatisporites is very variable within each sample throughout the section investigated. Recent fern and lycopod spores with similar exine thickness to the Punctatisporites spp. from Well 667-44 have a PDI of ca. 30 and 20% respectively. Combining the PDI results from Well 667-44 with the PDI from spores of the Recent taxa, Cheilanthes viridis and Lycopodium clavatum, the relationship between PDI and Ro can be very tentatively established to be: PDI = 65.01 Ro + 11.857. Disclosure of interest The author declares that he has no competing interest. Acknowledgements We are grateful to Saudi Aramco for permission to publish. We also thank Steve Waldron for providing the Recent fern and
lycopod sporangia for study and Pete Coxon for useful discussion of Recent spore taxonomy and preservation. Thanks also to the reviewers, Cortland Eble and John Marshall for their very helpful comments. References Goodhue, R., Clayton, G., 2010. Palynomorph Darkness Index (PDI)–a new technique for assessing thermal maturity. Palynology 34, 147–156. Bustin, R.M., Cameron, A., Grieve, D., Kalkreuth, W., 1985. Coal petrology, its principles, methods and applications, Short Course Notes, 2nd ed. Geological Association of Canada, Victoria, 273 pp. Hartkopf-Fröder, C., Königshof, P., Littke, R., Schwarzbauer, J., 2015. Optical thermal maturity parameters and organic geochemical alteration at low grade diagenesis to anchimetamorphism: a review. International Journal of Coal Geology 150151, 74–119. Jarvie, D.M., Claxton, B.L., Henk, F., Breyer, J.T., 2001. Oil and shale gas from the Barnett Shale, Fort Worth Basin. AAPG Annual Meeting Program, v.10, Texas, p. A100. Marshall, J.E.A., 1991. Quantitative spore colour. Journal of the Geological Society 148, 223–233. Staplin, F.L., 1969. Sedimentary organic matter, organic metamorphism, and oil and gas occurrence. Bulletin of Canadian Petroleum Geology 17, 47–66. Taylor, G.H., Teichmuller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic petrology. Gebrüder Borntraeger, Berlin, Stuttgart, 704 pp.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia Geoff Clayton a,∗ , Robbie Goodhue b , Sami T. Abdelbagi c , Marco Vecoli c a
Department of Animal and Plant Sciences, University of Sheffield, Western Bank, S10 2TN Sheffield, United Kingdom b Department of Geology, Trinity College, University of Dublin, Dublin 2, Ireland c Saudi Aramco, Dhahran, Saudi Arabia
Abstract Palynomorph Darkness Index (PDI) determinations are presented from a sub-mature, cored, Carboniferous well section in northern Saudi Arabia and are correlated to Vitrinite Reflectance (Ro) and Vitrinite Reflectance (Calculated) results from the same section. PDI values from smooth, simple miospores range from 34–57%, whereas mean Ro and Vitrinite Reflectance (Calculated) both range from ca. 0.4–0.6%. The PDIs of the Recent spores Lycopodium clavatum and Cheilanthes viridis were also determined for comparative purposes. Considerable variation in PDI was recorded in all the samples investigated. A very tentative correlation of PDI to Ro is presented for samples with a range of thermal maturity from those unaffected by any heating, to those at the top of the oil window. © 2017 Elsevier Masson SAS. All rights reserved. Keywords: PDI; Spores; Vitrinite reflectance; Thermal maturity; Carboniferous; Saudi Arabia
1. Introduction Following publication of Staplin’s (1969) Thermal Alteration Index (TAI), numerous qualitative schemes relating palynomorph colour to thermal maturity have been established. Many of these are based on reference sets of palynomorph specimens or photomicrographs, and most employ numerical scales for colour categories. However, the numbers used in these schemes are simply labels with no quantitative significance. The few quantitative methods that have been proposed typically use expensive equipment not generally available to most workers, for example, the system based on microspectrophotometry described by Marshall (1991). The various methods for assessing maturity using palynomorph colour are comprehensively reviewed by Goodhue and Clayton (2010) and Hartkopf-Fröder et al. (2015). Palynomorph Darkness Index (PDI) was described by Goodhue and Clayton (2010) as a rapid and inexpensive method ∗
Corresponding author. E-mail address: [email protected] (G. Clayton).
of quantitatively describing palynomorph colour. This scheme differs from most earlier attempts in that it integrates colour intensities to produce greyscale values, rather than considering three variables (red, green and blue intensities). Although its methodology has been clearly described, the usefulness of PDI has been limited by the absence of correlation to vitrinite reflectance (VR), the industry standard for determination of maturity. Well 667-44 is a cored stratigraphic borehole drilled in 2009 by Saudi Aramco in northern Saudi Arabia. It includes one of the most complete Permo-Carboniferous successions in the region and is intended to serve as a litho- and biostratigraphic reference section for the Mississippian Berwath Formation. The location of the well and its generalised lithostratigraphy are shown in Figs. 1 and 2 respectively. In terms of thermal maturity, the section investigated ranges from submature at its top, to the ceiling of the oil window at its base, constituting a critical interval for the correlation of PDI to VR. In order to determine the PDI of palynomorphs that had not been subjected to any form of maturation process, spores of a Recent fern and lycopod were also studied.
http://dx.doi.org/10.1016/j.revmic.2017.07.002 0035-1598/© 2017 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Fig. 1. Location of Well 667-44.
2. Materials and methods 2.1. Sampling and sample processing Grey mudrock samples were collected from the core for PDI, VR (Ro ) and VRcalculated determination. For PDI, standard palynological extraction and mounting techniques were employed, using hydrofluoric acid but no oxidation. For VR, whole-rock plugs were prepared using cold-setting epoxy resin. These were polished and detrital organic-matter fragments were identified microscopically in reflected and fluorescent light. Depending on fragment size, one or more random reflectance measurements (Ro) was made on each fragment using the standard technique coal petrography and reflectance investigation technique of Bustin et al. (1985). Two miospore genera were used for PDI determination in Well 667-44; Punctatisporites spp. and Waltzispora spp. Both produced consistent results but the latter only occurred commonly in two samples. The Recent spores investigated were obtained from two sources. Spores of the lycopod, Lycopodium clavatum were extracted from commercially available Lycopodium tablets which were dissolved in warm water. The spores were then concentrated by sieving at 15 m, and
®
®
mounted using Cellosize dispersal agent and Elvacite cold mounting medium. Spores of the fern, Cheilanthes viridis were obtained by shaking pinnules with mature sporangia over cold water, then concentrating and mounting the spores using the method described above. No chemical treatment was employed. Examples of the fossil and Recent palynomorphs used in this investigation are illustrated in Plate 1. 2.2. PDI determination RGB intensities were measured by the method described by Goodhue and Clayton (2010), using a Nikon Eclipse E600 microscope with a 12 V, 100 W halogen bulb, NCB filter, × 40 and × 60 plan fluor objective lenses, and a DXM1200 digital camera with ACT-1 software. The microscope was set up for Kohler illumination. Default settings were used for the camera with the NCB filter installed. With a slide in focus, the lamp voltage and condenser diaphragm were carefully adjusted to obtain RGB intensities less than, but as close as possible to 255, 255, 255. The automatic white balance control was not used. Images were captured, representative areas for measurement selected, and values for RGB intensity obtained.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Fig. 2. a. Generalised lithological succession of Well 667-44. b. Vitrinite reflectance (Ro) and Vitrinite Reflectance (calculated). c. Palynomorph Darkness Index (PDI) Punctatisporites spp. with 2 standard deviation bars and PDI Waltzispora spp. with 2 standard deviation bars.
Calculation of PDI is based on the National Television System Committee (NTSC) and Joint Photographic Experts Group (JPEG) RGB to greyscale conversion equation: Y = 0.299R + 0.587G + 0.114B where Y is the calculated greyscale value, R is red intensity, G is green intensity and B is blue intensity. However, as one or more of the measured white background intensities (Rb , Gb , Bb ) was invariable slightly less than 255, the correction published by Goodhue and Clayton (2010) was applied: Ya = (0.299R ∗ 255/Rb ) + (0.587G ∗ 255/Gb ) + (0.114B ∗ 255/Bb ) where Ya is the adjusted greyscale value. Palynomorph Darkness Index (PDI) is then defined as: PDI (%) = 100 − (100Ya /255) 2.3. Vitrinite reflectance (Ro) determination The system used consists of a Zeiss Axiotech incident and transmitted light microscope equipped with mechanical stage and tungsten, halogen and xenon gas discharge light sources. To measure VR, the microscope is equipped with a J&M Analytik’s TIDAS S MSP 200 photomultiplier that perform fast and highly
sensitive measurements, in conjunction with software that controls the photometer and the microscope to record data, build histograms, and calculate statistical parameters. KB standards; sapphire, YAG, and GGG were used for calibration. 2.4. Vitrinite reflectance calculated (VRcalculated ) determination To validate and confirm present day thermal maturity measured on vitrinite reflectance (RO ) for the sample set used for this study, vitrinite reflectance (calculated) (VRcalculated ) was determined from Rock-Eval Tmax using the conversion equation published by Jarvie et al. (2001): VRcalculated = 0.018.T max −7.16 The Jarvie Equation was developed for the Barnett Shale, and is extensively used to estimate thermal maturity in basins worldwide. The Vitrinite reflectance (calculated) (VRcalculated ) for the interval studied ranges from 0.36 to 0.63% with an average VRcalculated of 0.50% (Fig. 2b). 3. Results The PDI and VR results obtained are summarized in Table 1 and the VRcalculated results are shown in Table 2. The palynomorph assemblages studied were generally well preserved.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Plate 1. Figured specimens housed in the Palynology Collection of the Department of Animal and Plant Sciences, University of Sheffield. The 10 m scale bar in Fig. 9 applies to all figures. The images of the spores are optimised to best illustrate their morphology and may not accurately represent their colour. Figs. 1–4. Punctatisporites spp. Fig. 1. 667-44 (1), M40/4. Fig. 2. 667-44 (1), P49/2. Fig. 3. 667-44 (1), F36/1. Fig. 4. 667-44 (1), H28/0. Fig. 5. Waltzispora cf. W. polita (Hoffmeister, Staplin & Malloy) Smith & Butterworth). 667-44 (1), E42/0. Fig. 6. Waltzispora sagittata Playford. 667-44 (1), Q43/0. Figs. 7 & 8. Waltzispora cf. W. planiangulata Sullivan. Fig. 7. 667-44 (1), M54/4. Fig. 8. 667-44 (1), Q35/3. Figs. 9 & 10. Cheilanthes viridis Swartz. Fig. 9. F1 (1), M51/2. Fig. 10. F1 (1), F59/4. Fig. 11. Lycopodium clavatum Linnaeus (equatorial focus) L1 (2), M41/4. Fig. 12. Same specimen as Fig. 11: distal focus.
Table 1 All PDI and Ro data. Borehole 667-44
Depth (Ft)
Punctatisporites spp. 4060.0 Punctatisporites spp. 4072.0 4190.0 Punctatisporites spp. Waltzispora spp. 4190.0 4356.0 Punctatisporites spp. Punctatisporites spp. 4275.8 4275.8 Waltzispora spp. 4412.5 Punctatisporites spp. Punctatisporites spp. 4505.0 4590.0 Punctatisporites spp. 4718.0 Punctatisporites spp. Punctatisporites spp. 4786.0 4827.0 Punctatisporites spp. Punctatisporites spp. 4950.0 4978.0 Punctatisporites spp. 5011.0 Punctatisporites spp. Punctatisporites spp. 5037.8 Cheilanthesviridis Lycopodiumsp.
Ro (%) 0.41 0.42 0.47 0.47 0.47 0.43 0.43 0.48 0.53 0.51 0.54 0.54 0.55 0.57 0.56 0.56 0.58 0.20 0.20
Standard deviation
No. particles
0.06 0.13 0.13
72 156 156
0.06 0.06
150 150
0.04
108
0.05
86
0.09 0.08
42 101
PDI (%)
Standard deviation
No. specimens
39 38 40 44 44 47 34 42 45 44 52 41 44 48 57 44 52 30 20
5 7
50 31
6 6 7 6 6
22 22 10 50 50
6 6 5 7 10 8 8 5 7 7
43 20 50 28 34 5 26 50 30 30
Measured PDI and VR (Ro ) values are shown in regular type and calculated values are shown in bold italic typeface.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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Table 2 VRcalculated data. Depth (ft)
VR (calculated)
4072.50 4152.10 4191.10 4275.80 4278.90 4321.20 4322.90 4405.20 4479.00 4625.70 4687.50 4718.30 4718.30 4950.80 4978.70 4978.70 5011.40 5037.80
0.49 0.45 0.54 0.63 0.36 0.53 0.36 0.42 0.49 0.38 0.42 0.53 0.54 0.45 0.45 0.47 0.56 0.40
Best-fit trend lines were constructed for both PDI and VR (Ro ). Where possible, PDI and VR were determined from the same samples. Where this was not possible, derived VR values to match measured PDI values and derived PDI values to match measured VR values were calculated from the appropriate bestfit lines. In Table 1, measured PDI and VR (Ro ) values are shown in regular type and calculated values are shown in bold italic typeface. Measurements of PDI were made on representative areas of the scabrate to finely rugulate spore exine on C. viridis and within the lumens of the reticulate form, L. clavatum in order to establish the PDI of un-matured spores. Neither form is a perfect match in terms of morphology for the Punctatisporites spp. from well 667-44. The exine of C. viridis is typically slightly thicker than that of Punctatisporites spp. whereas that of L. clavatum is thinner. Waltzispora spp. are very similar in terms of exine thickness to the Punctatisporites spp. 4. Interpretation and discussion VR (Ro ) increases uniformly with depth through the interval studied from ca. 0.4% Ro to ca. 0.6%, the ceiling of the Oil Window (Fig. 2a). VRcalculated also increases but with greater scatter (Fig. 2a). PDI increases gradually with increasing depth but with a degree of scatter broadly comparable to that of VRcalculated (Fig. 2c). At the ceiling of the Oil Window, PDIPunctatisporites is ca. 50%. Even in samples with large numbers of PDI determinations, standard deviations are high, suggesting considerable variation in PDI within each population (Fig. 3). This corresponds to the observation made by Marshall (1991) of, “- - progressive colour change to a cusp at which a broad spread of colours from yellow to brown occur in a single sample (Fig. 4). This is the interval over which hydrocarbons are generated - - -.”
Fig. 3. Cumulative mean of PDIPunctatisporites in sample 5037.8 ft with 2 standard deviation bars.
Fig. 4. Correlation of PDIPunctatisporites and PDIWaltzispora from Well 667-44 (solid circles and crosses respectively), PDICheilanthes viridis (open circle) and PDI Lycopodium clavatum (open square) v. Ro .
Ro for vitrinite precursors (i.e. woody plant tissues at normal Earth surface temperature and pressure) is assumed to be 0.2% (Taylor et al., 1998). This value correlates with the PDI determined for Cheilanthes viridis of 30% and Lycopodium clavatum of 20%. Assuming a linear relationship, tentative correlation of PDIPunctatisporites and PDIWaltzispora to Ro in Well 667-44, combined with the PDI values determined from the Recent spores can be expressed as: PDI = 65.01 R0 + 11.857. 2 R = 0.74 However, this result is based on a very small data set and much more work would be needed to establish this relationship with a high degree of confidence.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002
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5. Conclusions In the submature Mississippian section in Well 66744, PDIPunctatisporites gradually increases down-section from approximately 40 to 50% at the ceiling of the Oil Window. PDIPunctatisporites is very variable within each sample throughout the section investigated. Recent fern and lycopod spores with similar exine thickness to the Punctatisporites spp. from Well 667-44 have a PDI of ca. 30 and 20% respectively. Combining the PDI results from Well 667-44 with the PDI from spores of the Recent taxa, Cheilanthes viridis and Lycopodium clavatum, the relationship between PDI and Ro can be very tentatively established to be: PDI = 65.01 Ro + 11.857. Disclosure of interest The author declares that he has no competing interest. Acknowledgements We are grateful to Saudi Aramco for permission to publish. We also thank Steve Waldron for providing the Recent fern and
lycopod sporangia for study and Pete Coxon for useful discussion of Recent spore taxonomy and preservation. Thanks also to the reviewers, Cortland Eble and John Marshall for their very helpful comments. References Goodhue, R., Clayton, G., 2010. Palynomorph Darkness Index (PDI)–a new technique for assessing thermal maturity. Palynology 34, 147–156. Bustin, R.M., Cameron, A., Grieve, D., Kalkreuth, W., 1985. Coal petrology, its principles, methods and applications, Short Course Notes, 2nd ed. Geological Association of Canada, Victoria, 273 pp. Hartkopf-Fröder, C., Königshof, P., Littke, R., Schwarzbauer, J., 2015. Optical thermal maturity parameters and organic geochemical alteration at low grade diagenesis to anchimetamorphism: a review. International Journal of Coal Geology 150151, 74–119. Jarvie, D.M., Claxton, B.L., Henk, F., Breyer, J.T., 2001. Oil and shale gas from the Barnett Shale, Fort Worth Basin. AAPG Annual Meeting Program, v.10, Texas, p. A100. Marshall, J.E.A., 1991. Quantitative spore colour. Journal of the Geological Society 148, 223–233. Staplin, F.L., 1969. Sedimentary organic matter, organic metamorphism, and oil and gas occurrence. Bulletin of Canadian Petroleum Geology 17, 47–66. Taylor, G.H., Teichmuller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic petrology. Gebrüder Borntraeger, Berlin, Stuttgart, 704 pp.
Please cite this article in press as: Clayton, G., et al., Correlation of Palynomorph Darkness Index and vitrinite reflectance in a submature Carboniferous well section in northern Saudi Arabia. Revue de micropaléontologie (2017), http://dx.doi.org/10.1016/j.revmic.2017.07.002