Energy & Fuels 1990,4, 647-652
647
Role of Nickel and Vanadium in Petroleum Classification+ A. J. G. Barwise Exploration Technology Branch, BP Research Centre, Chertsey Road, Sunbury-on- Thames, Middlesex TW16 7LN, England Received June 22, 1990. Revised Manuscript Received October 1, 1990 A common problem in petroleum exploration is how to predict the source of a petroleum accumulation or a surface seepage when there are no source rock data available. There is a need to be able to determine source type from petroleum properties alone, and to this end, an oil classification scheme has been devised. This is based initially upon relatively simple petroleum properties such as API gravity, sulfur content, etc., and enables classification to be made where relatively few other data are available as is often the case in frontier exploration. One property of petroleum that is useful for classification purposes is the metal content. Nickel and vanadium are the two most abundant metals in petroleum and are thought to exist largely as porphyrin complexes which in turn are mainly derived from chlorophyll precursors. I t is shown that, in some oil classes, both the absolute concentration of metals and the nickel/vanadium ratio are useful oil classification parameters. Examples are given from a variety of oil classes from basins worldwide.
Introduction Classification of oils is an important topic for geochemists involved in exploration since it enables oils discovered in a basin to be grouped according either to a common source or to a common organic matter type. In well-explored basins, it allows the identification of the likely source for each oil group and can help to identify the possible presence of more than one family of oils. In frontier basins where there are often no source rock data to allow oil-source correlation, classification can lead to a rationalization of from what type of source rock the oil or oils may have been generated. Previous classification schemes such as the Tissot and Welte scheme1 classify oils using their chemical properties into class types called “paraffinic”, “napthenic”, etc., but make no attempt to relate oil types back to source rocks and source rock depositional environment. Therefore, while such schemes are useful for empirical correlation, a clearer relationship between oil properties and source rock properties is required. Such a classification scheme has been developed during this work, which describes five different generalized class types, A-E, and these are discussed below in more detail. The classification scheme use8 simple bulk and molecular properties of oils in order to classify oils, properties such as oil gravity, sulfur content, nitrogen content, and molecular parameters such as pristane/phytane ratios. Discussion of this scheme is beyond the scope of this paper and attention is focused on one set of parameters which is of use in the bulk classification scheme-the nickel and vanadium content of oils. This paper discusses the variation of these metals in oils derived from differing types of organic matter, what information can be gathered about the source of an oil from its metal content, and how parameters based on metals have been used for classification purposes. It must be stressed that the metal content of oils can only provide a limited amount of information, especially for oils of low metal content, and any classification must be backed up by inspection of other bulk properties and by detailed molecular and isotopic studies. However, exploration geochemisb are often faced
with limited geochemical information and must be able to infer as much as possible from such information about the likely source of an oil. This paper provides an overview of metal distribution in different oil classes and where information on metal concentration and type can aid classification.
Presented at the Symposium on Trace Elements in Petroleum Geochemistry, Divisions of Petroleum Chemistry and Geochemistry, 197th National Meeting of the American Chemical Society, Dallas, TX, April 9-14, 1989.
(1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer: Berlin, 1983. ( 2 ) Rohrbach, B. G. Advances in Organic Geochemistry, 1981; Wiley: London, 1983; pp 39-48.
Oils Chosen for the Classification Study The basic bulk classification scheme which has been derived out of this work defines five fundamental classes of organic matter in oil called A, B, C, D,and E (Table I), and the definition of these class types is discussed below. Oil families from several basins around the world were chosen to represent each class type, and some bulk and molecular properties of the oils are shown in Table 11. A constant set of symbols has been used for each class type in the figures in this paper, and these are shown in Table 111. The oil data sets chosen are only representative of each broad class type, and it is acknowledged that many examples of each class type could have been included. However, such addition would not add to the general principles of behavior of metal concentration in each class type. Classes A, B, and C are defined as being derived from aquatic organic matter but vary according to the extent of sulfur incorporated into the kerogen. For class A, sulfur incorporation is the most extensive and is typical of organic matter in marine source rocks that have a low siliciclastic input such as siliceous or carbonate source rocks. Examples of class A oils were chosen from two areas in the Middle East-the Gulf of Suez and Abu Dhabi. Gulf of Suez oils have been examined by several geochemists2and are thought to represent one family of oils generated from carbonate source rocks. Class B organic matter is defined as containing a moderate quantity of organic sulfur and is typified by oils derived from marine shales. Oils from the North Sea have been chosen as type examples since these oils have been extensively studied and are thought to be derived from a single source unit-the Kimmeridge Clay Fm.3 Class C organic matter is defined as that which
0881-0624/90/ 2504-0647$02.50/0 0 1990 American Chemical Society
648 Energy & Fuels, Vol. 4, No. 6,1990
Barwise
SHORELINE. DELTA OR PARALIC
I
UPPER DELTA PLAIN
I
1
LAKE Frcshwalcr
LOWER DELTA PLAIN
SalinclAlkalinc
Playa Lake
A
1 MARINE
+
__
I . T Y P E iNPUT D E P E N D ON ENVIRONMENT 2. M A Y R E W O R K
I
I
M O D I F Y 0 M. ASSEMBLAGE WiTHOUT SIGNIFICANTLY CONTRIBUTING TO A S S E M B L A G E
I
I
I
ENCLOSED BASIN
+
-
Brackish
I
I
S8linc
Saline
- Hypcrralint
I
OPEN SHELF:
I
SLOPE
( c h a r i c l c r l r e d by aluminosilicale 8.dimOnlr) Non-dolrilal PREDOMINANTLY WOODY DETRlrUO OiVlmt RI6E TO REFRACTORY KEROOEN
PREDOMINANT PH~TOPLANKTON f
A
--
--
Friihwi1.r 8 Brichiih Alhalln. S Silln.
( c h r r a c l s r i s e d by carbonale redimcnlr)
BOTTOM-WATER
High biolopical productivily
-
(~ n o O~. M . c
o
N
o
i
T
1
o
N
s
( c h a r a e l c r i r c d by siliceous
sediments)
Figure 1. Relationship between organic matter type and depositional environment. Table I. Definition of the Organic Matter Class Types effects on oil properties of oil organic matter depositional organic depositional class input into kerogen environment of source matter input environment either/both of these A phytoplankton bacteria marine carbonates and other low wax high most other insuection urouerties - sulfur nonsiliciclastics are controlld by a combination B phytoplankton bacteria marine siliciclastics low wax moderate sulfur of factors high wax low sulfur C phytoplankton bacteria lacustrine D higher land plant (angiosperm) nonmarine high wax low sulfur debris, bacteria E higher land plant (gymnosperm) nonmarine high wax low sulfur debris, bacteria
has experienced relatively little organic sulfur incorporation and is typical of organic matter deposited in lacustrine environments. Oils were chosen from mainland Chinese basins to represent this class type. Many geochemical studies have shown that such oils are represent a distinctive organic matter type.*v5 Class D and E organic matter is defined as being largely derived from land plants. While class E represents organic matter derived from a "classical" land-plant input, class D differs in a greater contribution from resinous material. Class D oils are represented by oils from Indonesian basins which are derived from Tertiary deltaic source rocks! Class E oils were chosen from the Gippsland Basin, Australia, where the Cretaceous sources are known to be land-plant dominated.' The relationships between the type of organic matter expected in various depositional environments and the likely classes of oil that are expected to be generated from source rocks from each environment are shown in Figures (3)Cooper, B.S.;Barnard, P. C Petroleum Geochemistry and Basin Evaluation; AAPG Mem. 1984,s. (4)Yang Wanli; Li Yongkang; Gao Ruiqi. AAPG Bull. 1986,69(7), 1112-1122. (5)Yang Zhiqiong; Gu Xinzhang; Zhang Ling. Energy Sources 1987, 9, 211-227. (6)Schyll, M.; Teechner, M.;Wehner, H.;Durand, B.; Oudin, F.L. Advances zn Organic Geochemistry, 1981; Wiley: London, 1983; pp 39-48. (7)Philp, R. P.; Simoneit, B. R. T.; Gilbert, T. D. Advances in Organic Geochemistry, 1981; Wiley: London, 1983;pp 698-704.
1 and 2 (Fleet, personal communication, 1989). While there are subtle variations in the type of organic matter that contributes to source rocks in each of these environments, these variations cannot be distinguished by using a bulk classification scheme alone-only broad classification can be made. More detailed classification schemes such as the application of detailed molecular schemess need to be applied in order to provide further subdivisions. Nevertheless, such broad classification is useful in frontier exploration.
Nickel and Vanadium Contents of Crude Oils Nickel and vanadium are the two most abundant metals in petroleum with nickel reaching concentrations up to 340 ppm and vanadium up to 1580 ppm of total crude? The metals are known to be bound into crude oil partly as organometallic complexes (porphyrins) and partly as high molecular weight complexes (non-porphyrins?) associated with asphaltenic components of crude oil.('' Many authors have used nickel/vanadium ratios for empirical oil-oil and (8) Mello, M. R.; Talnaes, N.; Gaglianone, P. C.; Chicarelli, M. I.; Brassell, S.C.; Maxwell, J. R. Advances in Organic Geochemistry 1987; Pergamon Press: Oxford, 1988; pp 31-45. (9)Barwise, A. J. G.; Whitehead E. V. Trace Elements in Petrogenesis; Theophrastus Publications: Athens, pp 599-644. (10)Fish, R. H.;Reynolds, J. G.;Gallegos, E. J. MetGl Complexes in Fossil Fuels, ACS Symposium Series 344;American Chemical Society: Washington, DC, 1987;pp 332-349.
~
Role of Ni and V in Petroleum Classification
class A: Abu Dhabi oils
class A: Suez oils
class B: North Sea oils
class c: Chinese oils
class D: Indonesian oils
class E Gippsland Basin oils
oil 1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10
API gravity 24.3 26.3 27.2 29.9 39.6 1.7 13.7 17.8 21.6 25.2 28.6 29.1 30.5 31.1 32.7 27.3 29.3 31.0 31.5 32.7 33.2 34.2 35.5 35.5 35.8 36.0 36.0 36.3 36.8 37.3 37.3 37.4 37.6 39.2 39.4 40.5 43.9 48.2 48.2 nd nd 26.8 33.6 58.7 20.2 43.4 26.8 39.6 40.4 52.7 44.2 46.9 22.8 42 37.8 22.7 39.6 36 37 29 33 31 38 26.1 29.5 29.3 35.2 38.8 39.3 40.3 41.1 42.3 42.7 42.6 44.6 53.3
Energy & Fuels, Vol. 4, No. 6, 1990 649
Table 11. Prowrties of Oils Chosen for This StudP % nickel, vanadium, nitrogen, % sulfur ppm ppm Pr/Ph ppm saturates 2.93 27 92 0.73 2330 32 34 2.67 20 68 0.78 1990 27 1.04 41 1.83 16 1660 1440 40 1.9 16 48 0.69 4 39 0.76 0.49 794 8 10 28 nd nd nd nd 5.2 148 190 0.76 14 3630 3.79 145 5020 0.70 29 162 3.32 101 4460 0.60 37 100 2.19 3200 0.72 37 54 50 1.85 2770 0.80 40 26 23 1.57 42 2070 1.00 57 28 1.65 1950 0.82 49 32 23 1.21 1930 0.90 53 29 17 1.03 1980 0.90 58 10 4 1.46 21 22 1615 33 nd 0.93 3400 nd 3 11 nd 0.85 nd 16 nd 49 3 5
0 2.c
A Marine Carbonates
B Marine Clastics C Lacustrine
U
1.c
Figure 7. Nickel/vanadium ratios for class A, B, and C oils.
generated oils (classes D and E) is low, nickel/vanadium ratios are only measurable and therefore of use for distinguishing class A, B, and C oils. A cross-plot of the metal content of class A, B, and C oils used in this study (Figure 6) reveals that nickel/vanadium ratios do show systematic variation. Nickel/vanadium ratios for the oils are also shown in histogram form in Figure 7. It must be stressed that often low metal concentrations are encountered, especially for thermally mature oils and or many class D and E oils regardless of maturity. In these cases, metal ratios are subject to large errors and cannot be used with confidence. Class A and B oils have Ni/V ratios of 1or less and tend to contain more vanadium than nickel as predicted by Lewan’s work. The dominance of vanadium over nickel may be a consequence of sulfate reduction in marine environments leading to Eh-pH conditions favourable to vanadium incorporation into kerogen. If this link is correct, then it might be expected that there should be a correspondence between the vanadium and sulfur contents of oils. A plot of the sulfur content versus vanadium content of the crude oils in this study (Figure 8) shows that this is the case. High vanadium content class A oils have the highest sulfur contents whereas the low sulfur content class C, D, and E oils have low vanadium contents. Oils from lacustrine environments (class C) have higher Ni/V ratios (Ni/V > 2) compared to oils generated from marine source rocks (classes A and B) since nickel incorporation into kerogen is a relatively more dominant process for this type of organic matter. The relative abundance of nickel over vanadium has been noted by several authors for Chinese oils. For example, Yang Zhiqiong et al.5 examined crude oils from several basins from continental China and reported the relative abundance of nickel compared to vanadium in all the oils they examined (Ni/V > 10 for all oils). Thus nickel/vanadium ratios greater than 2 appear to be consistently observed for oils generated from lacustrine source rocks. Besides being able to help in separating class types, nickel/vanadium ratios can help to distinguish oils from the same class type. For example, the Abu Dhabi and Gulf of Suez oils examined in this study have both been gen-
C l a s s D.E Oils, Low V.S C l a s s C Oils Low V,S 0
40
60
I
I
I
120
160
200
Vanadium (ppm)
Figure 8. Vanadium versus sulfur content for class types.
erated from carbonate source rocks and yet they have distinctly different nickel/vanadium ratios (Figure 6). This presumably arises from subtle variations in Eh-pH conditions at the time of deposition of the source rocks resulting in different relative rates of incorporation of nickel or vanadium into porphyrins during diagenesis. Once this ratio is fixed into the source rock, it appears that it becomes a characteristic ratio of the oils, regardless of variations in thermal maturity as is evident for the Suez and Abu Dhabi oils. This constancy of ratio explains why application of nickel/vanadium ratios to correlation have been successfully applied by many different geochemists.
Conclusions From this work it has been shown that there is a systematic variation in the nickel and vanadium content of crude oils which can be related to source rock type and depositional environment. The highest concentrations of metal are to be found in low maturity crude oils derived from source rocks which have a low clay content and a high organic sulfur content such as carbonate source rocks. Moderate quantities of metals are found in oils derived from marine shales or lacustrine source rocks whereas little nickel or vanadium is found in land-plant-derived oils. Nickel/vanadium ratios are useful correlation parameters for oils in which measureable quantities of metals are encountered and can easily distinguish marine from nonmarine (lacustrine) sourced oils. Overall the metal content of crude oils can provide a useful insight into the source of a petroleum accumulation, especially when combined with classification parameters derived from molecular, isotopic, and bulk parameters. Acknowledgment. I thank BP management for permission to publish this paper and in particular thank Drs. A. L. Mann and A. J. Fleet for useful contributions to this work.
