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The invariance ratio in isoheptanes: a powerful tool for oil–oil correlation in the Tertiary Niger Delta, Nigeria
Organic Geochemistry Organic Geochemistry 35 (2004) 989–992 www.elsevier.com/locate/orggeochem
Note
The invariance ratio in isoheptanes: a powerful tool for oil–oil correlation in the Tertiary Niger Delta, Nigeria Chidi I. Eneogwe Mobil Producing Nigeria Unlimited, Geochemistry Laboratory, Qua Iboe Terminal, Eket, Akwa Ibom State, 3225 Gallows Road, Fairfax, VA 22037-0002, USA Received 1 March 2004; accepted 26 April 2004 (returned to author for revision 23 March 2004) Available online 15 June 2004
Abstract The invariance in isoheptanes, seen in all primary oils, proves a powerful tool for oil–oil correlation in Tertiary Niger Delta oils, Nigeria. Thirty-five oils from two fields in the offshore Eastern Niger Delta separate cleanly into two groups based on distinctions in their invariance ratios K1 and two ring preference ratios (P3 /N2 and CPs/CHs). One of the two Eastern Niger Delta oil sets compares well to 51 previously analyzed oils from the Western Niger Delta. The other set, however, remained distinct, implicating at least two very different sources for the Tertiary Niger Delta oil system. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction Light hydrocarbons (LHs) have proven very effective in oil–oil and oil-source rock correlation (Erdman and Morris, 1974; Koons et al., 1974; Williams, 1974; Deroo et al., 1977; Cardwell, 1977; Phillipi, 1981; Kornacki, 1993; Halpern, 1995). Mango (1987) has shown that a ratio of isoheptanes, the so-called invariance ratio K1 [(2MH + 2,3-DMP)/(3-MH + 2,4-DMP)], remains remarkably constant in all primary oils. Moreover, the variations in K1 that exist in all oils can be explained by the variations between what are called homologous oil sets, namely sets of oils from a common source. Thus, various source rocks appear to generate oils with constant and in some cases distinct K1 s. Differences in K1 have been used in oil–oil correlations generally and for oil-condensate and condensate–condensate in particular (ten Haven, 1996). Eneogwe (2003) separated 51 oils from 10 fields in the Western Niger Delta into four different sets based on
E-mail address: [email protected] (C.I. Eneogwe).
variations in their LHs. Before then, applying evidence from isotopes and biological markers (mainly triterpanes), Eneogwe and Ekundayo (2003) sorted the same oils into three families instead of the four sets achieved using LHs. This disconnect was assumed to have been related to the findings of ten Haven (1996), who argued that oils derived from Tertiary terrigenous organic matter are difficult to correlate using Mango’s invariance parameters because they usually have too narrow a range of variation. In support of this idea Mango (1997) pointed out that many homologous oil sets differ only slightly, if at all, in their invariance ratios K1 and K2 , limiting their use as correlation tools. Mango cited deltaic oils from the Gulf of Mexico as examples to support his argument. Mango (1994) divided C7 ratios into two categories: invariant ratios, which remain relatively constant in all oils, and ring preference ratios, which show substantial variance. According to Mango (1997) it is the ring preference ratios (ratios of daughter products sharing a common parent) that give the C7 s high resolution in distinguishing between genetically distinct oils.
0146-6380/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2004.04.005
990
C.I. Eneogwe / Organic Geochemistry 35 (2004) 989–992
Reported here are clear and unequivocal differences in K1 for the oils in two oil fields, offshore Eastern Niger Delta (Nigeria), implying that K1 is a definitive parameter in oil–oil correlations, defining two homogenous sets of Niger Delta oils. Mango’s invariance ratio K1 is therefore proposed as an effective tool in oil–oil correlations and for identifying homologous sets in Niger Delta oils. K1 could further elucidate the Niger Delta petroleum systems, which have been difficult to fully understand, partly because of unavailability of source rocks for detailed investigations. K1 compares well to the ring preference ratio P3 /N2 [{(sum dimethylpentanes + ethylpentane)/(1,1- + 1,3-DMCP (c þ t))}; Mango, 1997] in this regard and is markedly superior to cyclopentanes/cyclohexanes (CPs/CHs), a ratio often used in oil–oil correlations. Details of the analytical methods employed and a typical partial whole oil gas chromatogram showing how the C7 compounds were identified are as presented by Eneogwe (2003).
2. Results and discussion
3-MH + 2,4-DMP (wt % C 7)
Eighty-six oils are included in this study. Thirty-five are from offshore Eastern Niger Delta (OEND), while the other 51 oils, analyzed previously by Eneogwe (2003), are from the Western Niger Delta (WND). OEND oils are from two oil fields, OEND1 (12 oils) and OEND2 (23 oils). K1 averages 0.96 (SD ¼ 0.0474) for the 23 oils in OEND2. It is constant and statistically distinct from the OENDl (average K1 ¼ 1:05; SD ¼ 0:0172) and WND oils, where the average K1 is 1.05 (SD ¼ 0:0238) (Table 1). These data are consistent with two homologous sets. Fig. 1 illustrates the differences graphically.
11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
Table 1 Invariance data for WND, OEND1 and OEND2 oils Set
Number of oils
Average K1
Standard deviation
Western Niger Delta Offshore Niger Delta 1 Offshore Niger Delta 2
53 12 23
1.05 1.05 0.96
0.0238 0.0172 0.0474
Averages and standard deviations for the ratio K1 [(2-methylhexane) + (2,3-dimethylpentane)]/[(3-methylhexane) + (2,4-dimethylpentane)]. K1 ¼ [(2-MH + 2,3-DMP)/(3-MH + 2,4-DMP)].
Fig. 2, a ring preference plot, supports Fig. 1 in that OEND1 and OEND2 remain tightly constrained and distinct, again consistent with two homologous sets. Fig. 3 establishes definitively that OEND1 and OEND2 are two distinct homologous sets. WND oils, included in Figs. 1 and 3, are similar to OEND1 but different from OEND2. These results strongly suggest at least two distinctly different source rocks responsible for the oils in the Niger Delta. Therefore, the four oil sets sorted by Eneogwe (2003) based on the light hydrocarbon distribution in the oils belong to one super family of oils. The invariance ratio K1 , not expected to be useful in oil correlation studies (Mango, 1994, 1997; ten Haven, 1996), exhibits striking resolution in Niger Delta oils. The ring preference ratio P3 /N2 (Fig. 2) displays similar resolving power. On the other hand, CPs/CHs, used alone, would have only limited utility. Fig. 3 shows that differences in CHs/CPs are unable to explain why and how WND oils are related to OND1 and not OND2 oils. For this reason, the invariance plot (Fig. 1) and the other ring preference plot; (methylhexanes)/(P3/N2), Fig. 2, are considered to be superior to this plot in their use in oil–oil correlation studies. It is therefore advised that, in the Niger Delta, this plot must be used alongside
WND OEND1 OEND2
K1 = slope x/y
6.0
7.0
8.0
9.0
10.0
11.0
2-MH + 2,3-DMP (wt % C 7) Fig. 1. An invariance plot (crossplot of [2-methylhexane + 2,3-dimethylpentane] and [3-methylhexane + 2,4-dimethylpentane] shows that OEND2 oils constitute a distinct homologous set, which is different from OEND1 and WND oils. Also, it can be seen that OEND1 and WND oils constitute one homologous oilset.
C.I. Eneogwe / Organic Geochemistry 35 (2004) 989–992
991
Fig. 2. A ring preference plot of methylhexanes (2-methylhexane + 3-methylhexane) as % C7 and a ratio (as natural logarithm) of P3 / N2 , where P3 , sum of dimethylpentanes + ethylpentane; N2 , 1,1- + 1,3-dimethylcyclopentane (c þ t).
Fig. 3. A ring preference plot of n-heptane as % C7 and a ratio of cyclopentanes (CPs)/cyclohexanes(CHs), where CPs, ethylcyclopentane + 1,2-dimethylcyclopentane (c þ t); CHs, methylcyclohexane + toluene.
other plots/data in making interpretations in oil correlation studies. 3. Conclusions The invariance ratio K1 exhibits remarkable distinction of Niger Delta oils, differences not previously reported for deltaic oils. K1 and the ring preference ratio (P3 /N2 ) provide a powerful technique for correlating Niger Delta oils, suggesting two distinct homologous sets and thus two quite different oil sources.
Acknowledgements I am grateful to the management of ExxonMobil Co. for allowing the publication of these data, Sundararaman Padmanabhan (Sundar) for introducing me to light hydrocarbon research, and James Gormly for reviewing an initial version of the manuscript. I am
equally grateful to Lloyd Snowdon and Simon George for editing the manuscript, and to Frank Mango and Hans Lo ten Haven for their invaluable reviews and helpful suggestions. Associate Editor – Simon George
References Cardwell, A.L., 1977. Petroleum source-rock potential of Arbuckle and Ellenburger groups southern mid-continent United States. Quarterly of the Colorado School of Mines 72, 1–134. Deroo, G., Powell, T.G., Tissot, B., McCrossan, R.G., 1977. The origin and migration of petroleum in the western Canadian sedimentary basin, Alberta: a geochemical and thermal maturation study. Geological Survey of Canada Bulletin 262, Geological Survey of Canada, Ottawa, p. 136. Erdman, J.G., Morris, D.A., 1974. Geochemical correlation of petroleum. American Association of Petroleum Geologists Bulletin 58, 2326–2337.
992
C.I. Eneogwe / Organic Geochemistry 35 (2004) 989–992
Eneogwe, C., 2003. The distribution of light hydrocarbons in Western Niger Delta oils. Journal of Petroleum Geology 26, 479–488. Eneogwe, C., Ekundayo, O., 2003. Geochemical correlation of crude oils in the NW Niger Delta, Nigeria. Journal of Petroleum Geology 26, 95–103. Halpern, H.I., 1995. Development and application of lighthydrocarbon-based star diagrams. American Association of Petroleum Geologists Bulletin 79, 801–815. Koons, C.B., Bond, J.G., Pierce, F.L., 1974. Effects of depositional environments and post-depositional history on chemical composition of Lower Tuscaloosa oils. American Association of Petroleum Geologists Bulletin 58, 1272–1280. Kornacki, A.S., 1993. C7 chemistry and origin of Monterey oils and source rocks from the Santa Maria Basin, California. Abstract in American Association of Petroleum Geologists Annual Convention Program, New Orleans, April 25–28, p. 131.
Mango, F.D., 1987. Invariance in the isoheptanes of petroleum. Science 327, 514–517. Mango, F.D., 1994. The origin of light hydrocarbons in petroleums: ring preference in the closure of carbocyclic rings. Geochimica et Cosmochimica Acta 54, 1315– 1323. Mango, F.D., 1997. The light hydrocarbon in petroleum: a critical review. Organic Geochemistry 26, 417–440. Phillipi, G.T., 1981. Correlation of crude oils with their source formation, using high resolution GLC C6 –C7 component analysis. Geochimica et Cosmochimica Acta 45, 1495– 1513. ten Haven, H.L., 1996. Application and limitation of Mango’s light hydrocarbon parameters in petroleum correlation studies. Organic Geochemistry 24, 957–976. Williams, J.A., 1974. Characterization of oil types in the Williston Basin. American Association of Petroleum Geologists Bulletin 58, 1243–1252.
Note
The invariance ratio in isoheptanes: a powerful tool for oil–oil correlation in the Tertiary Niger Delta, Nigeria Chidi I. Eneogwe Mobil Producing Nigeria Unlimited, Geochemistry Laboratory, Qua Iboe Terminal, Eket, Akwa Ibom State, 3225 Gallows Road, Fairfax, VA 22037-0002, USA Received 1 March 2004; accepted 26 April 2004 (returned to author for revision 23 March 2004) Available online 15 June 2004
Abstract The invariance in isoheptanes, seen in all primary oils, proves a powerful tool for oil–oil correlation in Tertiary Niger Delta oils, Nigeria. Thirty-five oils from two fields in the offshore Eastern Niger Delta separate cleanly into two groups based on distinctions in their invariance ratios K1 and two ring preference ratios (P3 /N2 and CPs/CHs). One of the two Eastern Niger Delta oil sets compares well to 51 previously analyzed oils from the Western Niger Delta. The other set, however, remained distinct, implicating at least two very different sources for the Tertiary Niger Delta oil system. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction Light hydrocarbons (LHs) have proven very effective in oil–oil and oil-source rock correlation (Erdman and Morris, 1974; Koons et al., 1974; Williams, 1974; Deroo et al., 1977; Cardwell, 1977; Phillipi, 1981; Kornacki, 1993; Halpern, 1995). Mango (1987) has shown that a ratio of isoheptanes, the so-called invariance ratio K1 [(2MH + 2,3-DMP)/(3-MH + 2,4-DMP)], remains remarkably constant in all primary oils. Moreover, the variations in K1 that exist in all oils can be explained by the variations between what are called homologous oil sets, namely sets of oils from a common source. Thus, various source rocks appear to generate oils with constant and in some cases distinct K1 s. Differences in K1 have been used in oil–oil correlations generally and for oil-condensate and condensate–condensate in particular (ten Haven, 1996). Eneogwe (2003) separated 51 oils from 10 fields in the Western Niger Delta into four different sets based on
E-mail address: [email protected] (C.I. Eneogwe).
variations in their LHs. Before then, applying evidence from isotopes and biological markers (mainly triterpanes), Eneogwe and Ekundayo (2003) sorted the same oils into three families instead of the four sets achieved using LHs. This disconnect was assumed to have been related to the findings of ten Haven (1996), who argued that oils derived from Tertiary terrigenous organic matter are difficult to correlate using Mango’s invariance parameters because they usually have too narrow a range of variation. In support of this idea Mango (1997) pointed out that many homologous oil sets differ only slightly, if at all, in their invariance ratios K1 and K2 , limiting their use as correlation tools. Mango cited deltaic oils from the Gulf of Mexico as examples to support his argument. Mango (1994) divided C7 ratios into two categories: invariant ratios, which remain relatively constant in all oils, and ring preference ratios, which show substantial variance. According to Mango (1997) it is the ring preference ratios (ratios of daughter products sharing a common parent) that give the C7 s high resolution in distinguishing between genetically distinct oils.
0146-6380/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2004.04.005
990
C.I. Eneogwe / Organic Geochemistry 35 (2004) 989–992
Reported here are clear and unequivocal differences in K1 for the oils in two oil fields, offshore Eastern Niger Delta (Nigeria), implying that K1 is a definitive parameter in oil–oil correlations, defining two homogenous sets of Niger Delta oils. Mango’s invariance ratio K1 is therefore proposed as an effective tool in oil–oil correlations and for identifying homologous sets in Niger Delta oils. K1 could further elucidate the Niger Delta petroleum systems, which have been difficult to fully understand, partly because of unavailability of source rocks for detailed investigations. K1 compares well to the ring preference ratio P3 /N2 [{(sum dimethylpentanes + ethylpentane)/(1,1- + 1,3-DMCP (c þ t))}; Mango, 1997] in this regard and is markedly superior to cyclopentanes/cyclohexanes (CPs/CHs), a ratio often used in oil–oil correlations. Details of the analytical methods employed and a typical partial whole oil gas chromatogram showing how the C7 compounds were identified are as presented by Eneogwe (2003).
2. Results and discussion
3-MH + 2,4-DMP (wt % C 7)
Eighty-six oils are included in this study. Thirty-five are from offshore Eastern Niger Delta (OEND), while the other 51 oils, analyzed previously by Eneogwe (2003), are from the Western Niger Delta (WND). OEND oils are from two oil fields, OEND1 (12 oils) and OEND2 (23 oils). K1 averages 0.96 (SD ¼ 0.0474) for the 23 oils in OEND2. It is constant and statistically distinct from the OENDl (average K1 ¼ 1:05; SD ¼ 0:0172) and WND oils, where the average K1 is 1.05 (SD ¼ 0:0238) (Table 1). These data are consistent with two homologous sets. Fig. 1 illustrates the differences graphically.
11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
Table 1 Invariance data for WND, OEND1 and OEND2 oils Set
Number of oils
Average K1
Standard deviation
Western Niger Delta Offshore Niger Delta 1 Offshore Niger Delta 2
53 12 23
1.05 1.05 0.96
0.0238 0.0172 0.0474
Averages and standard deviations for the ratio K1 [(2-methylhexane) + (2,3-dimethylpentane)]/[(3-methylhexane) + (2,4-dimethylpentane)]. K1 ¼ [(2-MH + 2,3-DMP)/(3-MH + 2,4-DMP)].
Fig. 2, a ring preference plot, supports Fig. 1 in that OEND1 and OEND2 remain tightly constrained and distinct, again consistent with two homologous sets. Fig. 3 establishes definitively that OEND1 and OEND2 are two distinct homologous sets. WND oils, included in Figs. 1 and 3, are similar to OEND1 but different from OEND2. These results strongly suggest at least two distinctly different source rocks responsible for the oils in the Niger Delta. Therefore, the four oil sets sorted by Eneogwe (2003) based on the light hydrocarbon distribution in the oils belong to one super family of oils. The invariance ratio K1 , not expected to be useful in oil correlation studies (Mango, 1994, 1997; ten Haven, 1996), exhibits striking resolution in Niger Delta oils. The ring preference ratio P3 /N2 (Fig. 2) displays similar resolving power. On the other hand, CPs/CHs, used alone, would have only limited utility. Fig. 3 shows that differences in CHs/CPs are unable to explain why and how WND oils are related to OND1 and not OND2 oils. For this reason, the invariance plot (Fig. 1) and the other ring preference plot; (methylhexanes)/(P3/N2), Fig. 2, are considered to be superior to this plot in their use in oil–oil correlation studies. It is therefore advised that, in the Niger Delta, this plot must be used alongside
WND OEND1 OEND2
K1 = slope x/y
6.0
7.0
8.0
9.0
10.0
11.0
2-MH + 2,3-DMP (wt % C 7) Fig. 1. An invariance plot (crossplot of [2-methylhexane + 2,3-dimethylpentane] and [3-methylhexane + 2,4-dimethylpentane] shows that OEND2 oils constitute a distinct homologous set, which is different from OEND1 and WND oils. Also, it can be seen that OEND1 and WND oils constitute one homologous oilset.
C.I. Eneogwe / Organic Geochemistry 35 (2004) 989–992
991
Fig. 2. A ring preference plot of methylhexanes (2-methylhexane + 3-methylhexane) as % C7 and a ratio (as natural logarithm) of P3 / N2 , where P3 , sum of dimethylpentanes + ethylpentane; N2 , 1,1- + 1,3-dimethylcyclopentane (c þ t).
Fig. 3. A ring preference plot of n-heptane as % C7 and a ratio of cyclopentanes (CPs)/cyclohexanes(CHs), where CPs, ethylcyclopentane + 1,2-dimethylcyclopentane (c þ t); CHs, methylcyclohexane + toluene.
other plots/data in making interpretations in oil correlation studies. 3. Conclusions The invariance ratio K1 exhibits remarkable distinction of Niger Delta oils, differences not previously reported for deltaic oils. K1 and the ring preference ratio (P3 /N2 ) provide a powerful technique for correlating Niger Delta oils, suggesting two distinct homologous sets and thus two quite different oil sources.
Acknowledgements I am grateful to the management of ExxonMobil Co. for allowing the publication of these data, Sundararaman Padmanabhan (Sundar) for introducing me to light hydrocarbon research, and James Gormly for reviewing an initial version of the manuscript. I am
equally grateful to Lloyd Snowdon and Simon George for editing the manuscript, and to Frank Mango and Hans Lo ten Haven for their invaluable reviews and helpful suggestions. Associate Editor – Simon George
References Cardwell, A.L., 1977. Petroleum source-rock potential of Arbuckle and Ellenburger groups southern mid-continent United States. Quarterly of the Colorado School of Mines 72, 1–134. Deroo, G., Powell, T.G., Tissot, B., McCrossan, R.G., 1977. The origin and migration of petroleum in the western Canadian sedimentary basin, Alberta: a geochemical and thermal maturation study. Geological Survey of Canada Bulletin 262, Geological Survey of Canada, Ottawa, p. 136. Erdman, J.G., Morris, D.A., 1974. Geochemical correlation of petroleum. American Association of Petroleum Geologists Bulletin 58, 2326–2337.
992
C.I. Eneogwe / Organic Geochemistry 35 (2004) 989–992
Eneogwe, C., 2003. The distribution of light hydrocarbons in Western Niger Delta oils. Journal of Petroleum Geology 26, 479–488. Eneogwe, C., Ekundayo, O., 2003. Geochemical correlation of crude oils in the NW Niger Delta, Nigeria. Journal of Petroleum Geology 26, 95–103. Halpern, H.I., 1995. Development and application of lighthydrocarbon-based star diagrams. American Association of Petroleum Geologists Bulletin 79, 801–815. Koons, C.B., Bond, J.G., Pierce, F.L., 1974. Effects of depositional environments and post-depositional history on chemical composition of Lower Tuscaloosa oils. American Association of Petroleum Geologists Bulletin 58, 1272–1280. Kornacki, A.S., 1993. C7 chemistry and origin of Monterey oils and source rocks from the Santa Maria Basin, California. Abstract in American Association of Petroleum Geologists Annual Convention Program, New Orleans, April 25–28, p. 131.
Mango, F.D., 1987. Invariance in the isoheptanes of petroleum. Science 327, 514–517. Mango, F.D., 1994. The origin of light hydrocarbons in petroleums: ring preference in the closure of carbocyclic rings. Geochimica et Cosmochimica Acta 54, 1315– 1323. Mango, F.D., 1997. The light hydrocarbon in petroleum: a critical review. Organic Geochemistry 26, 417–440. Phillipi, G.T., 1981. Correlation of crude oils with their source formation, using high resolution GLC C6 –C7 component analysis. Geochimica et Cosmochimica Acta 45, 1495– 1513. ten Haven, H.L., 1996. Application and limitation of Mango’s light hydrocarbon parameters in petroleum correlation studies. Organic Geochemistry 24, 957–976. Williams, J.A., 1974. Characterization of oil types in the Williston Basin. American Association of Petroleum Geologists Bulletin 58, 1243–1252.