Energy Fuels 2009, 23, 6020–6025 Published on Web 11/11/2009
: DOI:10.1021/ef9007104
Application of Superheated Water Extraction in Geochemical Evaluation of Source Rocks Akinsehinwa Akinlua*,† and Roger M. Smith‡ †
Fossil Fuel and Environmental Research Group, Department of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria and ‡ Department of Chemistry, Loughborough University, Loughborough LE11 3TU, United Kingdom Received July 14, 2009. Revised Manuscript Received October 26, 2009
Application of superheated water was investigated for the extraction of organic compounds of petroleum exploration importance. Using Niger Delta samples as a case study, the geochemical ratios and parameters were calculated from n-alkane and isoprenoid hydrocarbon data. The pristane/phytane, pristane/nC17, Ph/ nC18 ratios and carbon preference index (CPI) ranged from 0.90 to 1.29, 0.61 to 2.30, 0.86 to 1.44, and 0.88 to 1.88 for the western Niger Delta samples, respectively, and from 0.35 to 3.49, 1.01 to 3.03, 0.87 to 1.98, and 1.00 to 2.15 for the eastern Niger Delta samples, respectively. The geochemical plots revealed that a preponderance of the samples from both western and eastern Niger Delta had mixed organic matter input and a good number of the samples also had contributions from terrestrial organic matter, while few samples had strong contributions from marine organic matter. The plots also indicated that the samples were sourced by organic matter deposited in more reducing environments than oxidizing environments. There was no significant difference in geochemical characteristics between western Niger Delta samples and eastern Niger Delta samples. Biomarker data also confirmed that the samples are mainly of terrestrial and mixed organic matter origin. The results of this study agreed with the results of previous studies based on Soxhlet extraction sample preparation. The results obtained from Soxhlet extraction of the same set of samples were comparable to those of superheated water extraction. The study showed that superheated water extraction provides a better alternative to Soxhlet extraction as the sample preparation procedure in geochemical evaluation of petroleum source rocks because of its environmentally friendly nature.
dioxide, which as been successfully applied to the extraction of organic compounds of petroleum exploration importance from source rocks.9-14 However, the SFE instrumentation required is relatively expensive. Hence, there is still a need for an alternative method that will be cheaper and equally environmentally friendly, and in recent years, superheated water extraction (SWE) has been shown to provide such an alternative. SWE has been applied to the extraction of organics from solid environmental samples, such as soil and sediments,15-18
1. Introduction In petroleum exploration, source rock studies are an important component of the evaluation of petroleum potential of any sedimentary basin. Organic compounds in the source rocks constitute invaluable tools to understand the quality of petroleum source rocks. Compounds of particular importance include n-alkanes, isoprenoid alicylic hydrocarbons, terpanes, steranes, etc. These compounds are useful to determine the origin, depositional environments, thermal maturity, and in some cases, the age of the organic matter.1-8 These compounds are traditionally obtained from the rock matrix by Soxhlet extraction with organic solvents prior to instrumental analysis. These organic solvents are toxic and environmentally unfriendly, which has led to the quest for benign green analytical extraction techniques. One of such techniques is supercritical fluid extraction (SFE) with carbon
(9) Monin, J. C.; Barth, D.; Perrut, M.; Espitalie, M.; Durand, B. Org. Geochem. 1998, 13, 1079–1086. (10) Hopfgartner, G.; Veuthey, J. L.; Gulac-ar, F. O; Buchs, A. Org. Geochem. 1990, 15, 397–402. (11) Greibrokk, T.; Kadke, M.; Skurdal, M.; Willsch, H. Org. Geochem. 1992, 18, 447–455. (12) Jaffe, R.; Diaz, D.; Hajje, N.; Chen, L.; Eckardt, C.; Furton, K. G. Org. Geochem. 1997, 26, 59–65. (13) Furton, K. G.; Chena, L.; Jaffe, R. J. J. High Resolut. Chromatogr. 1999, 22, 623–627. (14) Akinlua, A.; Torto, N.; Ajayi, T. R. J. Supercrit. Fluids 2008, 45, 57–63. (15) Di Corcia, A.; Caracciolo, A. B.; Crescenzi, C.; Guiliano, G.; Murtas, S.; Samperi, R. Environ. Sci. Technol. 1999, 33, 3271–3277. (16) Hawthorne, S. B.; Trembley, S.; Moniot, C. L.; Grabanski, C. B.; Miller, D. J. J. Chromatogr., A 2000, 886, 237–244. (17) Krieger, M. S.; Wynn, J. S.; Yoder, R. N. J. Chromatogr., A 2000, 897, 405–413. (18) Chienthavorn, O.; Su-in, P. Anal. Bioanal. Chem. 2006, 385, 83– 89. (19) Hawthorne, S. B.; Yang, Y.; Miller, D. J. Anal. Chem. 1994, 66, 2912–2920. (20) Kipp, S.; Peyrer, H.; Kleibohmer, W. Talanta 1998, 46, 385–393. (21) Young, T. E.; Ecker, S. T.; Synovec, R. E.; Hawley, N. T.; Lomber, J. P.; Wai, C. N. Talanta 1998, 45, 1189–1199.
*To whom correspondence should be addressed. E-mail: aakinlua@ oauife.edu.ng. (1) Bray, E. E.; Evans, E. D. Geochim. Cosmochim. Acta 1961, 22, 2–15. (2) Meyers, P. A. Org. Geochem. 1997, 27, 213–250. (3) Obaje, N. G. NAPE Bull. 2000, 15, 29–45. (4) Peters, K. E.; Snedden, J. W.; Sulaeman, A.; Sarg, J. F.; Enrico, R. J. AAPG Bull. 2000, 84, 12–44. (5) Nytoft, H. P.; Bojesen-Koefoed, J. A. Org. Geochem. 2001, 32, 841–856. (6) Banerjee, A.; Pahari, S.; Jha, M.; Sinha, A. K.; Jain, A. K.; Kumar, N.; Thomas, N. J.; Misra, K. N.; Chandra, K. AAPG Bull. 2002, 86, 433– 456. (7) Hanson, A. D.; Ritts, B. D.; Moldowan, J. M. AAPG Bull. 2007, 91, 1273–1293. (8) Alsharhan, A. S.; Abd El-Gawad, E. A. J. Pet. Geol. 2008, 3, 191– 212. r 2009 American Chemical Society
6020
pubs.acs.org/EF
Energy Fuels 2009, 23, 6020–6025
: DOI:10.1021/ef9007104
Akinlua and Smith
Figure 1. Schematic diagram of SWE.
reported in our earlier work.24 The SWE method was found to be over 100% more efficient than the Soxhlet extraction. A direct comparison of data and geochemical ratios from SWE and Soxhlet extraction had also been undertaken in the earlier study24 and were found to be in a good agreement. The hydrocarbons in the extractant water were extracted using poly(dimethylsiloxane) solid-phase microextraction (SPME) (7 μm) fibers (Supelco, Dorset, U.K.). The SPME fiber was exposed to the water extract for 15 min, which was stirred vigorously during the sorption step using a magnetic stir bar. The fiber was withdrawn and analyzed directly by gas chromatography-mass spectrometry (GC-MS) with a Fisons Instruments GC 8000 series and MS MD800 system (Fisons, Manchester, U.K.). The fiber was inserted into the injection port, which was held at 300 °C for 3 min in the splitless mode. The separations were performed on a DB-1 capillary column (50 m 0.2 mm 0.5 μm film thickness), using helium as the carrier gas. The oven temperature program was 50 °C for 5 min, then 50-300 °C at 5 °C/min, and finally, 300 °C for 20 min. The mass spectrometer was operated at an electron energy of 70 eV, an ion source temperature of 250 °C, and transfer line temperature of 300 °C. Soxhlet extraction of the samples was performed in a standard Soxhlet extractor using a dichloromethane/methanol (3:1) solvent mixture for 48 h. Samples of 20 g were used. Extracts were then concentrated with a rotary evaporator and further concentrated by evaporating the solvent in a stream of nitrogen gas (i.e., by gentle nitrogen gas blown down to prevent the high volatile components from being carried off by the nitrogen gas). Gas chromatographic analysis of the rock extracts was performed using the same GC system as above.
including the extraction of polycyclic aromatic hydrocarbons (PAHs).19-23 These works suggested that it would be a potentially feasible method for the extraction of organics from source rocks, and in an earlier study, the optimum conditions and yields compared to a Soxhlet extraction were investigated.24 In this study, the applicability of SWE of organic compounds for the geochemical evaluation of petroleum source rocks in the Niger Delta basin was used as a case study. The main aim of this study is to evaluate whether the geochemical characterization obtained from the superheated water extracts of source rocks would be comparable to that obtained previously, mainly from Soxhlet extracts of source rocks. 2. Experimental Section Rock samples from western and eastern offshore fields in the Niger Delta were obtained for this study. The rock samples were pulverized, homogenized, and passed through a 120 mesh screen. The SWE of the rock samples was performed using the experimental setup, as shown in the schematic diagram (Figure 1). The water was delivered at a constant flow rate of 0.5 mL min-1 by a Jasco PU-980 Intelligent HPLC pump (JASCO, Tokyo, Japan) through a stainless-steel preheating coil (1 m length and 0.17 mm inner diameter) to a stainless-steel extraction cell (50 10 mm inner diameter) fitted with 2 μm frits. The extraction cell and the preheating coil were placed in a Philips PU 4500 gas chromatograph (GC) oven (Cambridge, U.K.), and maintained at a constant temperature. The water was passed vertically through the extraction cell for downflow extractions. The extraction cell outlet was connected to a 2 m length of stainless tubing (0.17 mm inner diameter), which provided back-pressure control, with cooling fins attached outside the oven to conduct heat away and cool the extractant to ambient temperature. The water extract was collected in a vial immersed in ice. Each SWE experiment was carried out in two stages: the static stage, which was undertaken for 10 min after the cell had been filled, followed by a dynamic stage, during which the water was pumped at 0.5 mL min-1. A temperature of 250 °C and extraction time of 50 min had been determined in previous work to give the maximum yields of the hydrocarbons,24 at a pressure of more than 5.0 MPa, which was sufficient to prevent the water from boiling. Efficiency and reproducibility of the method have been
3. Results and Discussion 3.1. n-Alkane and Isoprenoid Hydrocarbons. A total of 20 sedimentary organic rock samples from western and eastern offshore fields in the Niger Delta were collected for this study. A total of 10 samples were obtained from the western Niger Delta (with sample code WND), and a total of 10 samples were obtained from eastern Niger Delta (with sample code END). The total organic carbon (TOC) levels, which are a measure of the amount of organic matter present in a source rock, were found to range from 0.64 to 1.63 wt %, with an average of 1.18 wt % for the western Niger Delta samples, and from 0.19 to 3.35, with an average of 1.21 wt % for the eastern Niger Delta samples (Table 1). All of the samples, except END-B, had total organic carbon (TOC) values above the 0.50 wt % threshold value for potential source rocks to generate oils and gas.
(22) Fernandez-Perez, V.; Luque de Castro, M. D. J. Chromatogr., A 2002, 902, 357–367. (23) Hyotylainen, T.; Andersson, T.; Hartonen, K.; Kuosmanen, K.; Riekkola, M.-L. Anal. Chem. 2000, 72, 3070–3076. (24) Akinlua, A.; Smith, R. M. Chromatographia 2009, 69, 1333–1339.
6021
Energy Fuels 2009, 23, 6020–6025
: DOI:10.1021/ef9007104
Akinlua and Smith
END-B, representing both western and eastern Niger Delta origins, have strong terrestrial organic matter input. While only three samples, WND-ML3, WND-MF3, and END-C2, have the strong marine organic matter input.28,29 The crossplot of CPI versus pristane/phytane (Figure 4) indicated that most of the samples had organic matter input deposited in reducing paleoenvironments, while only three samples, WND-ML2, WND-MJ3, and END-C1, were derived from organic matter deposited in oxidizing paleoenvironments. The geochemical characterization obtained above is similar to the results of previous studies based on Soxhlet extraction.27,28,30,31 The V/(V þ Ni) ratios for all of the samples determined are high (Table 2), which indicate that the samples were deposited under reducing conditions.32 The results of the V/(V þ Ni) ratio are correlative to those of the Pr/Ph ratio, except for samples WND-MJ3, WND-ML2, and END-C1, thus suggesting that Pr/Ph ratios of these samples are also influenced by organic matter input.34 Direct comparison of SWE and Soxhlet extraction of these samples was performed by carrying out Soxhlet extraction of some of the samples, and the extracts were examined by gas chromatography. The results (Table 3) show that the pristane/phytane (Pr/Ph) ratios ranged from 0.88 to 3.10, which suggest more anoxic than oxic conditions.6,25,26 Pr/nC17 ratios for the samples ranged from 0.97 to 2.74, while Ph/ nC18 ratios ranged from 0.93 to 1.46. The values of the ratios suggest contributions from both terrestrial organic matter and marine organic matter. The CPI values for the samples ranged from 0.96 to 1.48. Most of the samples have values that are about 1, suggesting mixed organic matter input. The above results are comparable to those obtained from the SWE. The geochemical characteristics of the source rock extracts from the Soxhlet extraction were further evaluated from the geochemical plots made from n-alkane and isoprenoid ratios. The cross-plot of Pr/nC17 versus Ph/C18 (Figure 3) revealed that the distribution of the samples among terrestrial, mixed, and marine organic matter fields is similar to that of SWE. Besides discriminating the samples into different organic matter sources, cross-plot of Pr/nC17 versus Ph/C18 also indicates that most of the samples are thermally mature, with few immature ones, because of their relatively high levels of these ratios (Figure 3). The cross-plot of CPI versus pristane/phytane (Figure 4) showed that a preponderance of the samples had organic matter input deposited in reducing paleoenvironments, which is comparable to what was obtained from SWE. Each of these parameters from SWE and Soxhlet extraction were cross-plotted to determine their correlation coefficients. The cross-plots of values of the Pr/Ph ratio from SWE and Soxhlet extraction (Figure 5) show a correlation coefficient of 0.967, which indicates that SWE and Soxhlet extraction are strongly correlated for the Pr/Ph ratio. While the correlation coefficient for CPI is 0.004, which suggests a very low correlation of SWE and Soxhlet extraction for CPI, the
Table 1. Geochemical Data of the Source Rock from Superheated Water Extracts from Offshore Fields, Niger Deltaa sample
depth (ft)
WND-MJ1 WND-MJ2 WND-MJ3 WND-MF1 WND-MF2 WND-MF3 WND-ML1 WND-ML2 WND-ML3 WND-ML4 END-S1 END-S2 END-S3 END-S4 END-S5 END-S6 END-B END-D END-C1 END-C2
7030-7060 8170-8200 9610-9640 7120-7150 7870-7890 8530-8560 6170-6200 7130-7160 7820-7850 8180-8210 5460-5490 6870-6900 8005-8030 8990-9020 10670-10700 11060-11090 10199 10410-10430 10280-10310 11580-11610
a
Pr/Ph Pr/nC17 Ph/nC18 CPI TOC (wt %) 0.90 0.96 1.04 0.91 0.96 0.90 0.94 1.29 0.92 0.94 0.91 0.93 0.91 0.94 0.92 0.95 0.92 0.83 3.49 0.35
1.00 1.86 1.85 1.00 1.09 1.00 1.11 2.30 0.61 1.07 1.01 1.07 3.03 1.12 1.05 ND 1.89 1.15 1.30 1.26
0.99 1.01 1.01 0.99 0.99 1.45 1.02 1.00 0.86 1.01 1.00 0.87 0.95 1.07 1.03 0.97 1.01 1.07 1.02 1.98
1.07 1.88 0.88 0.96 0.99 1.20 1.21 1.31 1.02 1.06 2.15 1.05 1.00 1.28 1.11 1.08 1.03 1.07 1.14 1.01
0.64 1.16 1.08 1.04 1.48 1.42 1.13 1.17 1.63 1.01 0.62 0.93 0.98 0.77 0.66 0.56 0.19 0.83 3.35 3.23
NA, not available; ND, not detected; CPI, carbon preference index.
The source rock samples were extracted with superheated water, and the extracts were examined by gas chromatography (see example of the gas chromatogram in Figure 2). From these results (Table 1), the pristane/phytane (Pr/Ph) ratios for the western Niger Delta samples ranged from 0.90 to 1.29, while they ranged from 0.35 to 3.49 for the eastern Niger Delta samples, which suggest more anoxic than oxic conditions.6,25,26 Pr/nC17 ratios for the western Niger Delta samples ranged from 0.61 to 2.30, while the eastern Niger Delta samples had values from 1.01 to 3.03. The western Niger Delta samples had Ph/nC18 ratios from 0.86 to 1.44, while Ph/nC18 ratios ranged from 0.87 to 1.98 for the eastern Niger Delta samples. There were no clear-cut differences in the values of Pr/nC17 and Ph/nC18 for samples from both regions of the delta, which suggested a predominance of mixed organic matter.27-29 The carbon preference index (CPI) values for the western Niger Delta samples ranged from 0.88 to 1.88, while they ranged from 1.00 to 2.15 for the eastern Niger Delta samples. These CPI values did not indicate any significant difference in terms of the geochemical signature between the eastern and western Niger Delta samples. Most of the samples represent a predominantly mixed organic matter input, with few samples having either marine or terrestrial organic matter input. They also suggest that most of the samples were thermally mature, with few samples of thermal immaturity, such as END-C1 and ENDC2. Because CPI values could be influenced by both source and maturity, confirmation of the two factors will be obtained by biomarkers. Standard geochemical plots were generated from these parameters. The cross-plots of Pr/nC17 versus Ph/C18 (Figure 3) revealed that a preponderance of the samples from both western and eastern Niger Delta have mixed organic matter input. Many of the samples, WND-ML2, WND-MJ2, WND-MJ3, END-S2, END-S3, END-C1, and
(30) Ekweozor, C. M.; Okogun, J. I.; Ekong, D. U. E.; Maxwell, J. R. Chem. Geol. 1979, 27, 29–37. (31) Udo, O. T. Some aspects of the petroleum geochemistry of the opuama clay channel complex of the Niger Delta. Ph.D. Thesis, University of Ibadan, Ibadan, Oyo, Nigeria, 1985; p 385. (32) Lewan, M. D. Geochim. Cosmochim. Acta 1984, 48, 2231–2238. (33) Huang, W. Y.; Meinschein, W. G. Geochim. Cosmochim. Acta 1979, 43, 739–745. (34) Peters, K. E.; Walters, C. C.; Moldowan, J. M. Biomarkers and Isotopes in Petroleum Exploration and Earth History; Cambridge University Press: Cambridge, U.K., 2005; pp 475-1155.
(25) Hughes, W. B.; Holba, A. G.; Dzou, L. I. P. Geochim. Cosmochim. Acta 1995, 59, 3581–3598. (26) Pancost, R. D.; Freeman, K. H.; Patzowsky, M. A.; Wavrek, D. A.; Collister, J. W. Org. Geochem. 1998, 29, 1649–1662. (27) Akaegbobi, I. M. J. Min. Geol. 2000, 36, 175–189. (28) Okoh, A. F.; Nwachukwu, J. I. J. Min. Geol. 1997, 33, 103–114. (29) Ashraf-Khorassani, M.; Memarianm, M.; Angaji, M. Fresenius’ J. Anal. Chem. 1992, 344, 492–496.
6022
Energy Fuels 2009, 23, 6020–6025
: DOI:10.1021/ef9007104
Akinlua and Smith
Figure 2. Typical gas chromatogram of the (a) superheated water and (b) Soxhlet extracts of the source rock sample.
Figure 3. Cross-plot of Pr/nC17 versus Ph/nC18 of rock extracts of SWE and Soxhlet extraction of Niger Delta source rocks. Figure 4. Cross-plot of pristane/phytane versus CPI showing the depositional environment of Niger Delta source rocks based on rock extracts of SWE and Soxhlet extraction.
very low correlation is as a result of discrepancies in the CPI values of some of the samples, WND-MJ2, WND-MF1, 6023
Energy Fuels 2009, 23, 6020–6025
: DOI:10.1021/ef9007104
Akinlua and Smith
Table 2. Nickel and Vanadium Contents and Ratios of the Samplesa Sample
depth (ft)
Ni (ppm)
V (ppm)
V/(V þ Ni)
WND-MJ1 WND-MJ2 WND-MJ3 WND-MF1 WND-MF2 WND-MF3 WND-ML1 WND-ML2 WND-ML3 WND-ML4 END-S1 END-S2 END-S3 END-S4 END-S5 END-S6 END-B END-D END-C1 END-C2
7030-7060 8170-8200 9610-9640 7120-7150 7870-7890 8530-8560 6170-6200 7130-7160 7820-7850 8180-8210 5460-5490 6870-6900 8005-8030 8990-9020 10670-10700 11060-11090 10199 10410-10430 10280-10310 11580-11610
10.77 8.87 10.39 12.59 8.72 7.39 13.48 18.56 9.11 9.46 8.38 9.52 14.86 10.55 -
356.35 396.39 649.36 407.99 293.64 584.11 500.44 552.64 457.36 311.29 327.31 280.52 193.65 290.92 -
0.97 0.98 0.98 0.97 0.97 0.99 0.97 0.97 0.98 0.97 0.98 0.97 0.93 0.97 -
a
Figure 5. Comparison of Pr/Ph ratios and CPI values of rock extracts of SWE and Soxhlet extraction.
-, not determined.
Table 3. Geochemical Data of Source Rock Extracts from Soxhlet Extraction from Offshore Fields, Niger Delta sample
depth (ft)
Pr/Ph
Pr/nC17
Ph/nC18
CPI
WND-MJ1 WND-MJ2 WND-MJ3 WND-MF1 WND-MF3 WND-ML1 WND-ML2 END-S3 END-B END-C1
7030-7060 8170-8200 9610-9640 7120-7150 8530-8560 6170-6200 7130-7160 8005-8030 10199 10280-10310
1.00 0.88 1.25 0.92 0.86 0.95 1.48 0.97 0.76 3.10
1.05 1.20 1.46 0.95 1.00 0.97 1.70 2.74 1.13 1.21
0.95 1.46 1.12 0.93 1.11 1.13 0.94 0.93 1.21 1.12
0.96 1.17 1.16 1.23 1.07 1.28 1.08 1.06 1.48 1.12
WND-ML2, and END-B. The discrepancies might be due to differential extraction yields because CPI was calculated from a range of components. One or two of such components might not have exhaustive extraction especially in the Soxhlet extraction because SWE had better yields than Soxhlet extraction at optimum conditions.24 The cross-plots of values of Pr/nC17 and Ph/C18 ratios from SWE and Soxhlet extraction (Figure 6) show correlation coefficients of 0.82 and 0.53, respectively. These correlation coefficients indicate that SWE and Soxhlet extraction are strongly correlated for the Pr/nC17 ratio and not strongly correlated for Ph/C18. The relatively lower correlation coefficient of Ph/C18 was a result of the relatively large range between the values of WND-MJ2 and WND-MF3; however, the geochemical significance of these values are similar. Statistical evaluation of the data was carried out using dendrogram cluster analysis. Dendrogram cluster analysis of the rock extract data from SWE of the samples using Pr/Ph, Pr/nC17, Ph/nC18, and CPI as variables revealed two main classes, END-S3 and END-C1, which fall into one class, and the remaining samples fall into the second class, although there are subclasses in the second class (Figure 7). Dendrogram cluster analysis of the rock extract data from Soxhlet extraction of the samples (Figure 8) showed classification very similar to that obtained from SWE. This indicates that data obtained from the two extractions are highly comparable. 3.2. Biomarker Chemistry. The geochemistry of the rock extracts was also evaluated on the basis of biomarker
Figure 6. Comparison of Pr/nC17 ratios and Ph/nC18 ratios of rock extracts of SWE and Soxhlet extraction.
parameters. The relative abundance of regular C27, C28, and C29 steranes is a good means of evaluating the origin of organic matter. C29 steranes predominated in most of the samples from western Niger Delta, indicating a greater input of terrestrial organic matter than from the marine source (Table 4). However, sample WND-MJ1 had greater input from marine organic matter, and samples WND-MJ3 and WND-ML1 had almost equal contributions from marine and terrestrial organic matters, indicating mixed organic matter input. The relative abundance of C27, C28, and C29 (Table 4) streanes indicated that more than a half of the samples from eastern Niger Delta had mixed organic matter input, while the remaining samples had a strong contribution from terrestrial organic matter.3,29,33 Thermal maturity of the samples was assessed using Ts/(Ts þ Tm).34 The Ts/(Ts þ Tm) values for the western Niger Delta samples were generally above 0.5, which indicate that the samples are thermally mature. However, two samples, WND-MF3 and WND-ML1, had Ts/(Ts þ Tm) values below 0.5, indicating low thermal maturity. The CPI values of these two samples were relatively high, indicating low thermal maturity rather than being terrestrially sourced. Ts/(Ts þ Tm) values of the eastern Niger Delta samples 6024
Energy Fuels 2009, 23, 6020–6025
: DOI:10.1021/ef9007104
Akinlua and Smith
Figure 7. Dendrogram cluster analysis of rock extracts from SWE of rock samples using Pr/Ph, Pr/nC17, Ph/nC18, and CPI as variables.
Figure 8. Dendrogram cluster analysis of rock extracts from Soxhlet extraction of rock samples using Pr/Ph, Pr/nC17, Ph/nC18, and CPI as variables.
the samples were mainly of mixed and terrestrial origin, although few samples were of marine origin. The results of this study strongly agreed with the results of previous studies on Niger Delta source rocks based on the Soxhlet extraction sample preparation technique and other geochemical methods that Niger Delta source rocks are mainly of terrestrial and mixed organic matter origin.28,30,31,35,36
Table 4. Biomarker Data of Source Rock Samples from Offshore Fields, Niger Deltaa sample
C27 (%)
C28 (%)
C29 (%)
Ts/(Ts þ Tm)
WND-MJ1 WND-MJ2 WND-MJ3 WND-MF1 WND-MF2 WND-MF3 WND-ML1 WND-ML2 WND-ML3 WND-ML4 END-S1 END-S2 END-S3 END-S4 END-S5 END-S6 END-B END-D END-C1 END-C2
46.87 5.03 44.06 16.19 11.52 37.55 31.22 14.10 23.91 43.42 42.92 30.27 40.83 34.85 34.94 28.31 36.71
31.83 15.69 18.17 30.16 34.15 13.98 33.12 22.76 16.17 16.56 25.63 20.82 31.80 18.09 15.40 30.00
21.30 79.28 55.94 65.65 58.32 28.30 54.80 52.79 53.32 40.41 40.52 44.11 38.35 33.36 46.97 56.30 33.29
0.73 0.53 0.96 0.89 0.54 0.34 0.32 0.57 0.6 0.6 0.8 0.29 0.67 0.89 0.79 0.7 0.52 0.62 0.55 0.63
4. Conclusions A geochemical evaluation of source rock samples based on a SWE sample preparation procedure was undertaken. The study clearly showed that there were no significant geochemical differences in the samples from offshore western and eastern Niger Delta. The results from this study showed that the Niger Delta source rocks are mainly of terrestrial and mixed organic matter origin, which agreed with the results of pervious studies based on a Soxhlet extraction sample preparation step and other geochemical techniques. The results of this study demonstrated that SWE is a viable analytical sample preparation procedure for geochemical evaluation of petroleum source rocks, as a preferred alternative to Soxhlet extraction.
a Ts, 22,29,30-trisnor-18R(H)hopane; Tm, 22,29,30-trisnor-17R(H)hopane.
showed that they are thermally mature, with only one sample having low thermal maturity. Generally, the biomarker data support the conclusion from the results of the n-alkanes and isoprenoid hydrocarbons that
Acknowledgment. We are very grateful to the Royal Society for the International Incoming Short Visit Research Grant offered to A. Akinlua to visit Loughborough University.
(35) Ekweozor, C. M.; Daukoru, E. M. AAPG Mem. 1994, 599–614.
(36) Akinlua, A.; Ajayi, T. R.; Jarvie, D. M.; Adeleke, B. B. J. Pet. Geol. 2005, 28, 39–48.
6025
