Methyl diamantane index (MDI) as a maturity parameter
for Lower Palaeozoic carbonate rocks at high maturity and
overmaturity
Li Jinggui
a, Paul Philp
b,*, Cui Mingzhong
aaLanzhou Institute of Geology, Academia Sinica, Lanzhou, 730000, China bSchool of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, USA
Received 23 June 1998; accepted 10 January 2000 (returned to author for revision 24 August 1999)
Abstract
Diamantanes were identi®ed in extracts from the Lower Ordovician Majiagou Formations (O1m5) of the central gas
®eld, Shanganning Basin, China. At 3100±3800 m, corresponding to Ro 1.9±3.9%, the MDI [methyl diamantane
index=4-MD/(1-MD+3-MD+4-MD)] ranges from 40 to 65% for the source rock extracts. Changes in the MDI index in the very mature sections (Ro>2.0%) are relatively small and no linear correlation between MDI andRo, or
MDI and depth, is noted, as previously reported, suggesting the possibility that the MDI may have limited applic-ability in maturity determinations.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:China; Shanganning Basin; Carbonate source rocks; MDI (methyl diamantane index); Overmaturity
1. Introduction
Diamondoids are a class of petroleum hydrocarbons. They are rigid, fused-ring alkanes with diamond-like structures and unique thermal stabilities. Their formation from polycyclic hydrocarbon precursors, probably cat-alyzed by a strong Lewis acid catalyst, is driven by accompanying increases in their thermodynamic stabi-lity (Wingert, 1992). Diamondoids are more stable than most hydrocarbons and, once formed, are resistant to thermal and biological destruction (Wingert, 1992). Several papers on the occurrence of these compounds and their stability have been published (Fort and Schleyer 1964; Petrov et al., 1974; Petrov, 1984; Lin and Wilk, 1995), and it is not the intent of this paper to review this literature in detail. A recent paper on dia-mondoids (Dahl et al., 1999) introduces the possibility of their use for evaluating the occurrence and extent of oil destruction and oil deadline in a speci®c basin. Although the concentrations of these compounds are
often very low (selective ion monitoring is typically used to determine their distribution), they are widely dis-tributed in crude oils and source rocks. Variation in the thermal stability of methyl-substituted diamondoids has lead to the use of certain isomer ratios as maturity parameters for crude oils and source rocks, especially at high and overmature stages of hydrocarbon generation (Chen et al., 1995, 1996, 1997). For example, 1-methyl-adamantane (1-MA) is more stable than 2-methyl-adamantane (2-MA), 4-methyldiamantane (4-MD) is more stable than 1-methyldiamantane (1-MD) and 3-methyl-diamantane (3-MD). Hence, the ratios 1-MA/(1-MA +2-MA) and 4-MD/(1-MD+3-MD+4-MD) should increase with increasing thermal stress (or depth). In other words the greater the ratio, the higher the maturity of the oils and source rocks.
Although these diamondoid hydrocarbon ratios are aected by organic input to a limited extent (Chen et al., 1996), it has been proposed that diamondoid hydrocarbon ratios can be used as maturity indices for overmature crude oils and source rocks (Ro0.9±2.0%;
Chen et al., 1996). In this paper, results from a study of the proposed diamondoid maturity parameters applied
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 0 1 6 - 4
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* Corresponding author.
to source rocks from the Shanganning Basin, China show that changes in the MDI in the very mature sec-tions (Ro>2.0%) were relatively small. There is no
lin-ear correlation between MDI and Ro, or MDI and
depth, as reported previously. This suggests that the MDI may have limited applicability in maturity deter-minations.
2. Experimental
Twenty carbonate source rock samples from three wells (Shancan 1, Shan 42 and Shan 17) were collected from the central gas ®eld of the Shanganning Basin (Fig. 1). These are assumed to be the source rocks for these gases, because no oils have been found in this basin in the vicinity of these source rocks. The whole-rock sam-ples were powdered to 100 mesh after surface cleaning, and subsequently extracted with chloroform. This extract is referred to as chloroform bitumen A. The rock residues were treated with 6% hydrochloric acid to remove the carbonate minerals, and the residue re-extracted with chloroform to give chloroform bitumen C. The saturate hydrocarbon fractions were isolated from the chloroform bitumen A and C samples by silica/ alumina column chromatography and elution with hex-ane.1They were then concentrated by solvent removal,
and analyzed by GC±MS in MID and full scan modes. An HP 5989A mass spectrometer coupled to an HP 5890 GC and equipped with a 30 m (0.25 mm) HP-5 column was used for GC±MS analysis. The oven tem-perature was programmed from 80 to 300C at a rate of 4C minÿ1, and held for 25 min at 300C. The distribu-tions of adamantanes and diamantanes were determined by selective ion monitoring usingm/z135 and 149, and m/z187 and 201, respectively. The MAI was not calcu-lated due to evaporative losses of these components in their elution region of the chromatogram. Methyldia-mantanes were quanti®ed using them/z187 fragmento-grams (Fig. 2). Structural assignments for the various substituted diamantanes are given in Table 1. The diamantanes in the samples eluted between the n-C15
andn-C16alkanes, and their characteristic mass spectra
were virtually identical to those published by Wingert (1992).
3. Results and discussion
The Shanganning Basin includes the east part of Gansu Province, a part of Inner Mongolia Autonomous
Region and Ningxia Moslem Autonomous Region, and a large part of Shanxi Province. The central gas ®eld of Shanganning Basin is the biggest gas ®eld in China, and the gas from this ®eld is utilized for domestic supplies in China. The gases are reservoired in the upper portions of the Lower Ordovician Majiagou Formation, a car-bonate unit.
On the basis of carbon isotopic data it has been pro-posed that gases of this ®eld originate mainly from the carbonate rocks of the Lower Ordovician Majiagou Formation (Chen, 1994; Huang et al., 1996). The Fig. 1. Location of the three wells in the Shanganning Basin from which core samples were collected for this study.
1 These samples were prepared in Lanzhou. It is standard
Majiagou Formation belongs to a set of platform-lagoon facies carbonates interlayered with gypsum. There are sapropelic and overmature gas source rocks, with vitrinite re¯ectance values from 2.0 to 5.0% (when converted from solid bitumen re¯ectance values using the formulaRo=0.967Rb+0.586; Xu Zhengqui, 1995).
The high level of maturity in this basin has lead to the proposal that the majority of gases in the central gas ®eld are formed from thermal cracking of crude oils (Huang et al., 1996).
Based on TOC, genetic potential (S1+S2), and
stra-tum thickness (Table 2), the hydrocarbon potential of the ®fth section of the Majiagou Formation (O1m5) is
superior to that of the ®rst to fourth sections of the Majiagou Formation (O1m1-O1m4; Wang, 1993).
Fur-thermore, gas reservoirs are only present in the ®fth sections (O1m51-2, O1m54), and it is concluded that these
are also the most important for hydrocarbon genera-tion. The 20 carbonate samples from the three wells (Shancan 1, Shan 42 and Shan 17) studied in this paper were collected mainly from the O1m5sections.
Lower Palaeozoic marine strata do not contain vitri-nite, so vitrinite re¯ectance values (Ro) could not be
determined. Furthermore the biomarker concentrations are relatively low for these samples, making it dicult to estimate the molecular maturity of oils and source rocks. Many other conventional parameters are not applicable to Chinese Lower Palaeozoic basins, due to high levels of maturity. Two diamondoid hydrocarbon ratios (MAI-methyl adamantane index (1-MA/(1-MA +2-MA)), and MDI-methyl diamantane index (4-MD/ (1-MD+3-MD+4-MD)), ®rst proposed by Chen et al. (1996) as new maturity parameters for highly mature and overmature crude oils and source rocks (Ro 0.9±
2.0%) of Tarim, Yinggehai and Qiongdongnan Basins of China, do not appear to be applicable to the Lower Ordovician Formations of Shanganning Basin. In pre-vious studies, Chen et al. (1996) noted that for sixteen rock samples (mainly ranging in age from Silurian to Cambrian) in the Tarim and Erdos Basins, there was a good correlation between MDI and Ro. MDI values
measured in source rocks increased with increasing vitrinite re¯ectance, as determined from the solid bitu-men re¯ectance (Rb) of the rocks.
The diamantane hydrocarbon ratios (MDI) of the 20 rock samples from the three wells in O1m5sections from
the Shanganning Basin samples examined in this study (Table 2) show a very dierent relationship between MDI andRofrom that observed by Chen et al. (1996) in
the Tarim and Erdos Basins.
MDI values for the chloroform bitumen extracts A and C do not increase with increasing vitrinite re¯ec-tance (Ro) values, and there is no linear correlation
between MDI andRo. In the vitrinite re¯ectance range
Ro1.9±3.9%, the MDI values for 80% of the
chloro-form bitumen A samples are in the 55±65% range. The Fig. 2. Partial mass chromatograms for them/z187 and 201
fragment ions of diamantanes in an extract from the O1m5unit
of the Shanganning Basin. The peak numbers are the same as in the original paper by Chen et al. (1996) and the identities are: (18) diamantane(D); (19) 4-methyldiamantane(4MD); (20) 4,9-dimethyldiamantane; (21) 1-methyldiamantane(1MD); (22) 1,4 and 2,4-dimethyldiamantane; (23) 4,8-dimethyldiamantane; (25) 3-methyldiamantane(3MD); (26) 3,4-dimethyldiamantane.
Table 1
Identi®cation of diamantanes labeled in Fig. 2
Peak
22 1,4 and 2,4-Diamethyldiamantane 216 201
23 4,8-Dimethyldiamantane 216 201
25 3-Methyldiamantane 202 187
MDI values of 78% of the chloroform bitumen C extracts are in the 40±55% range (Fig. 3).
Analogously, the MDI values of these samples do not increase with burial depth and there is no linear corre-lation between MDI and depth. At 3100±3800 m, MDI values for 75% of the chloroform bitumen A samples are in the 55±65% range, and for 86% of the chloro-form bitumen C samples they are in 40±55% range (Fig. 4). In the chloroform bitumen A and chloroform bitumen C extracts examined outside of this depth interval, other MDI values show decreasing trends with increasing burial depth andRovalue (Figs. 3 and 4).
The reason for the discrepancies between the MDI-Ro
results presented in this study and those presented pre-viously by Chen et al. (1996) may be that the MDI maturity parameter is only applicable in the range of 0.9±2.0% Ro values. From the data of Chen et al.
(1996), Rovalues of 13 samples from among 16 rock
samples in Tarim and Erdos Basins are <2.0% (accounting for 81% of the total samples). In other words, the correlation between MDI andRoreported by
Chen et al. (1996) is based mainly on data from rock samples whoseRovalues are <2.0%. TheRovalues of
samples from the O1m5 section in the Shanganning
Basin are all >2.0% (with one exception: sample Shan 42-2, depth 3320 m Ð 1.95%), with a maximum re¯ec-tance of 3.8% at 3785 m (sample Shancan 1-8). The samples from the O1m5 section of the Shanganning
Basin are clearly at higher thermal maturity and evolu-tion than those samples from the Tarim and Erdos Basins examined by Chen et al. (1996).
In these overmature O1m5sections (Ro>2.0),
methyl-diamantane isomerization dierences are not obvious, and the relative abundances of the three methyl-substituted isomers do not show any signi®cant change with increasing depth. The present results suggest that Table 2
Lithology, MDI values and re¯ectance values for samples examined in this study
MDI%a
1 Shancanl-1 3400 O1m51 0.60 464 0.02 Grey limestone 38.10 ± 1.90 2.42
2 1-2 3465 O1m52 0.31 466 0.03 Grey black limestone 56.41 ± 1.86 2.39
3 1-3 3490 O1m53 0.25 449 0.04 Grey black dolomite 66.17 46.81 ± ±
4 1-4 3550 O1m55 0.14 445 0.03 Grey black limestone 60.00 52.73 2.17 2.68
5 1-5 3670 O1m59 0.16 409 0.05 Grey black dolomite 62.50 36.59 3.08 3.56
6 1-6 3750 O1m4 0.12 587 0.04 Grey black dolomite 64.52 50.00 3.03 3.52
7 1-7 3770 O1m4 0.07 444 0.02 Dark grey dolomite 60.00 ± 3.19 3.67
14 42-6 3350 O1m53 0.12 466 0.02 Gray black dolomite 44.44 38.18 3.06 3.55
15 42-7 3400 O1m54 0.15 550 0.03 Gray dolomite 55.00 42.27 2.94 3.43
16 42-8 3430 O1m55 0.13 490 0.03 Gray black limestone 60.47 55.00 2.61 3.11
17 Shan17-1 3125 O1m51 0.09 464 0.03 Dark gray dolomite 46.67 52.73 1.54 2.08
18 17-2 3135 O1m52 0.13 478 0.03 Dark gray dolomite 64.84 42.31 1.58 2.11
19 17-3 3150 O1m53 0.16 494 0.02 Dark gray dolomite 63.86 51.52 ± ±
20 17-4 3180 O1m54 0.04 470 0.02 White grey dolomite
containing gypsum
60.00 51.43 3.17 3.65
a MDI: Methyl diamantane index (4-MD/1-MD+3-MD+4-MD%). b R
b(%): re¯ectance of solid bitumen in whole rock thin section. Whole rock thin section is used because particles of solid bitumen
in Lower Paleozoic carbonate rocks of Shanganning Basin are very few in number and very thin.
c Values forR
oof the three wells are converted from the aboveRbwith the following equationRo=0.967Rb+0.586 established in
1995 (unpublished) by the Research Institute of the Exploration and Development, Changqing Petroleum Exploration Bureau, China National Petroleum Corporation.
d Note Ð the convention used to describe the Age and Formation of the Source rocks can be explained using O
1m51-(4)as an
example. O refers to Ordovician; m5refers to the ®fth section of the Majiagou Formation; m51-this is the ®rst subsection of the ®fth
either MDI is not useful in the Shanganning Basin, or an MDI value of 65% is the equilibrium value for the MDI. In rocks, oils, and condensates from Tarim, Yinggehai and Qiongdongnan Basins of China, all of the MDI values were also <65%, with one exception; namely a sample from well KN1 in the Tarim Basin, a dark black mudstone, had an MDI value of 75.79% (Chen et al., 1996). From Fig. 5, it is clear that the MDI is not a good indicator of maturity for the overmature O1m5 sections (Ro>2.0%) in the Shanganning Basin,
and is probably of limited use as a maturity parameter for overmature carbonate gas source rocks in general.
4. Conclusions
1. The MDI (methyl diamantane index) can be used to measure the maturity of source rocks at highly mature stages in the vitrinite re¯ectance range (Ro) of 0.9±2.0%.
2. The MDI cannot be used as a maturity parameter for overmature carbonate gas source rocks at vitrinite re¯ectance of >2.0%.
Fig. 3. Diagram displaying the relationship of 4-MD/(1-MD+3-MD+4-MD) (or MDI, methyl diamantane index) and
R0of the O1m5sections. The open circles represent the values
obtained from the chloroform extract C and the shaded circles represent values from chloroform extract A.
Fig. 4. Diagram displaying the relationship of 4-MD/(1-MD +3-MD+4-MD) and depth of O1m5sections. The open circles
represent the values obtained from the chloroform extract C and the shaded circles represent values from chloroform extract A.
Fig. 5. Comparative diagram displaying the relationship between 4-MD/(1-MD+3-MD+4-MD) vs.Ro, for chloroform
3. It is proposed that an MDI value of 65% repre-sents an apparent equilibrium value for this ratio.
Acknowledgements
This paper has greatly bene®ted from the original reviews by Eric Michael and Jeremy Dahl plus addi-tional comments from both Ken Peters and Joe Curiale.
Associate EditorÐK. Peters
References
Chen, A.D., 1994. Origin and migration of natural gas in the Ordovician reservoir in Shanganning Basin central gas ®eld. Acta Petrolei Sinica 15 (2), 1±10, (in Chinese).
Chen, J.H., Fu, J.M., Sheng, G.Y., Liu, D.H. and Zhang, J.J, 1995. The diamondoid hydrocarbon ratios: novel maturity indices for over-mature crude oils. In: Grimalt J, Dorron-sorro C, (Eds.), Organic Geochemistry: Development and Applications to Energy, Climate, Environment and Human History. pp. 407±409.
Chen, J.H., Fu, J.M., Sheng, G.Y., Liu, D.H., Zhang, J.J., 1996. Diamondoid hydrocarbon ratios: novel maturity indices for highly mature crude oils. Org. Geochem 25, 179± 190.
Chen, J.H., Fu, J.M., Sheng, G.Y., Liu, D.H., 1997. The study of distribution characteristics of diamondoid compounds in
crude oils. Advances in Natural Sciences 7 (3), 365±367, (in Chinese).
Dahl, J.E., Moldowan, J.M., Peters, K.E., Claypool, G.E., Rooney, M.A., Michael, G.E. et al., 1999. Diamondoids as a quantitative indicator of natural oil cracking. Nature 399, 54±57.
Huang, D.F., Xiong, C.W., Yang, J.J., Xu, Z.Q., Wang, K.R., 1996. Gas source discrimination and natural gas genetic types of central gas ®eld in Erdos Basin. Natural Gas Indus-try 16 (6), 1±6.
Fort, R.C., Schleyer, P.V.R., 1964. Adamantane: consequences of the diamondoid structure. Chemical Reviews 64, 277±300. Lin, R., Wilk, Z.A., 1995. Natural occurrence of tetramantane (C22H28), pentamantane (C26H32) and hexamantane (C30H36)
in a deep petroleum reservoir. Fuel 74, 1512±1521.
Petrov, A.A., 1984. Petroleum Hydrocarbons. Springer-Verlag, Berlin (p. 84).
Petrov, A.A., Arefjef, O.A., Yakubson, Z.V., 1974. Hydro-carbons of adamantane series as indices of petroleum cata-genesis process. In: Tissot, B., Bienner, F. (Eds.), Advances in Organic Geochemistry 1973. Editions Technip, Paris, France, pp. 517±522.
Wang, K., 1993. Hydrocarbon Generation Evaluations of the Ordovician Carbonates of Shangannin Basin. Unpublished Research Report (in Chinese) of the Research Institute of the Exploration and Development, Changqing Exploration Bureau, China Natiional Petroleum Corporation, p. 1±57. Wingert, W.S., 1992. GC±MS analysis of diamondoid
hydro-carbons in Smackover Petroleum. Fuel 71, 37±43.
