Composition of Crude Oil through Seven Carbons as Determined by Gas Chromatography RONALD 1. MARTIN and JOHN
C. WINTERS
Research and Development Department, Standard Oil Co. (Indiana), Whiting, Jnd.
b Methods for determining the lowerboiling components in crude oil have employed time-consuming distillation techniques. A rapid and accurate gas chromatographic method has been developed for components containing from two to seven carbon atoms. A gas chromatographic prefractionator column separates these components from the crude oil and transfers them to an analyzer column. Combining results from two analyzer columns determines the 2- to 7-carbon saturated hydrocarbons and benzene. Another column determines toluene. The key to the separations is the preparation of columns of the required selectivity by choosing the optimum percentage of liquid phase. One operator can make the analysis and calculate results in about 12 hours. Precision and accuracy are excellent. Analyses of 20 crude oils from major domestic and foreign producing areas show large variations in the total and relative amounts of the hydrocarbons and permit some generalizations concerning composition to be deduced.
C
6il has been extensively analyzed because its composition is of fundamental interest and is an important consideration in the development of improved refining processes. Determination of the individual hydrocarbons in the lower-boiling fraction has heretofore required n eeks or months of tedious separations by distillation and adsorption chromatography, followed by measurement of physical properties of the fractions (9, 10, 14-17). Infrared spectroscopy ( I ) , mass spectrometry ( 2 , S ) , and a combination of Raman spectroscopy Ivith dehydrogenation-hydrogenation techniques (11, 18) have helped in malyzing the fractions, but have not xpprcciably reduced the over-all time of analysis. Previous expcrienre has indicated that a t least the Ion-er-boiling ( ompounds in crude oil should be easier to dctcrinine by gas chromatography, A fast and accurate method has been tlevclopcd for determining the h j drocarbow in crude oil through seven carbon atoms. Gas chromatographic techniques arc usrd to st.parate this fraction from the crude oil and to detcrinine the RUDE
1954
ANALYTICAL CHEMISTRY
individual coniponents. The method has been applied to 20 crude oils from major domestic and foreign producing areas. Such extensive data, not previously available, have permitted some generalizations to be made about crudeoil composition. ANALYTICAL METHOD
The method consists essent'ia,lly of two parts: prefractionation of coniponents through seven carbons from the crude oil, and separation of this fraction into individual components. A gas chromatographic prefractionator column sepamtes the components through seven carbons from the crude oil and transfers them to one of three analyzer columns. where the desired sepsrations are made. A separate sample is prefractionatecl into each of the three analyzer columns; two determine the saturated hydrocarbons and benzcne, and the third determines toluene. Adding a knon-n amount of a reference compound to the crude oil eliminates the need for accurately measuring the sample.
1 gas density balance lvas chosen as the detector because it does not require calibration for individual components (12). .1 balancc of new and simplified design, developed in this laboratory by Serheim (13). was used with nitrogen eluting gas. Checking with synthetic blends shorved that detector errors did not exceed 1% of the amount present. Essential to the success of the method is handling of the crude-oil sample in a single closed system from sampling to detection. I n all of the columns the liquid phase is supported on Johnsllanville firebrick, prepared by screening to 35 to 48 mesh, treating with 10% sodium hydroxide, n-ashing with water until neutral, and drying a t 400" C. A weighed amount of the selected liquid phase as a solution in hexane or dry methanol is added to a known weight of firebrick; the solvent is evaporated sloivly on a hot plate, with stirring. -$mounts of liquid phase are expressed as percentages of the total packing. The detailed design and use of the prefractionator and analyzer columns involved diffrrent problems resulting from the use of chromatographic principles in two distinctly different operations. Design and Use of Prefractionator. T h e assembly used for prefractionation is shown in Figure 1. A 3-foot gas chromatographic column con-
'!
Figure
1.
Prefractionator assembly
taining lSyc Apiezon L grease on firebrick serves as the prefractionator. .\bout 50 pl. of crude oil. diluted with Tetralin to make it flox freely, is injected from a hypodermic needle and syringe through the silicone-rubber disk into the sample introduction zone. The compounds through seven carbons are quantitatively vaporized and eluted through the prefractionator into thcx connected analyzer column. The time needed to elute the coniponents through seven carbons from the prefractionator must be determined for the apparatus and conditions; 8 minutes were required with the apparatus a t a flow rate of 60 nil. per minute with 10p.s.i.g. pressure a t the head. An extra minute is allowed beyond thc. actual time needed to ensure complete removal of the desired compounds. The two valves are then switched so the prefractionator is bypassed and the nitrogen flows directly to the analyzer column. The same flow rate is maintained to the analyzer column a f t u bypassing the prefractionator by prvsetting the needle valve. After the valve positions arc changeti. the prefractionator is prepared for reuse by backflushing the remaining volntilc components with nitrogen. The column temperature may be increased slightly during the backflushing. The sample introduction zone and nitrogen preheater are maintained at 250" and 320' C., respectively. to ensure quantitative vaporization of all components to be determined. Tntroduction zone temperatures below 200" C. rmult in incomplete recovery of the
heptanes. d glass-wool plug at the top of the sample introduction zone provides surface area to facilitate vaporization. The plug is changed after about five runs, and the entire column is cleaned nnd repacked after about 80 runs, because sniall amounts of coke accumulate a t the top. The heated parts of the prefraction:itor assembly (nitrogen preheater, sample introduction zone, and prefractionntor column) are made of stainless steel tubing and are brazed together; they m e maintained at different teniperatures by a new electrical heating techiiique (19). K h e n a high current a t lo\\ voltage is passed through the n-alls of stainless steel tubing, the tubing msunies a temperature dependent on its rlectrical resistance; the resistance of the tubing :ind the temperature imp r t e d by a given current increase as the cross-sectional area of t'he walls decreases. Conditions employed for lieating the three sections are:
ical plates ( 4 ) , as measured for nhexane. Of t h e 35 compounds boiling below 108" C . (hydrocarbons of two through seven carbons and three with either carbons) 31 give individual peaks on one column or the other. The four reniaining compounds-2,2,3-triniethylbutane, &methylhexane, 1-cis-2-diniethylcyclopentane, and ethylcyclopentane-are determined by the difference from peaks containing two components, one of which is determined individually on the other column. Cycloheptane, a trace constituent boiling a t 119" C.. is eluted in the eight-carbon region and was not determined on these columns. The oqdipropionitrilt, column used to determine toluene also gives an additional determination of benzene. This column retains benzene and toluene iiiuch longer than any of the t.nturated
Stainless Steel Tubing, Inches Outer Thickness Length diameter Sitrogcii preheat er Sample introduction zone
Prefrurtionator column
0.25 0 31 0.31
The leads for the electrical heating are connected to the top of the nitrogen preheater and to the bottom of the prefractionator column. Design and Use of Analyzer Columns. Gas chroniatographic determination of saturated hydrocarbons containing two t o five carbons is common, b u t complete separation of t h e many six- and seven-carbon isomers is difficult. Several methods are available for t h e hexane isomers and eycloperitane (5, 7 , H),but the separations obtained are incomplet'e. The five-to-seven-carbon saturates in reformer charge stock have been determined by combining the results from single runs made on three different columns ( 5 ) , but analysis was not obtained for 3-ethylpentane, benzene, or toluene, and each run required about 3 hours. Better columns were needed to determine all of the hydrocarbons through seven carbons. The three :inalyzer colunins developed :ire made of coiled copper tubing 0.25 inch in outer tlianieter and are operated nt room temperature. -\26-foot column containing 13.4% isoquinoline and a 40foot column containing 3.17, l-chloronaphthalene :ire used to determine the saturated hydrocarbons and benzene. A 12-foot column containing 5 7 , ~,~'-oxydipropioriitrile is used to determine toluene. Chroniatogrnms from the isoquinoline :ind chloronaphthalene columns arc shown in Figure 2 . Attenuation factors : r e shown along the base. The abbrevi:ttions used for components are identified in Table I. Gas flow rates in the columns were 55 ml: per niinute for isoquinoline, and 65 ml. per minute for chloronaphthalene. They possessed, respectively, 11,000 and 13,000 theoret-
n 012
24 6 36
0 oi2
0 040
Temp., O C. with 37 .\nip. and 5.4 I-olts
320 "0 125 ~~
11)-drocarbons passed into it from the prefractionittor. About 30 minutes are required for this anal) sis. -4bout 2% of a reference compound neighed into the crude oil inakes the results quantitative. cis-2-Pentene is eluted in a vicant spot on the clironiatograms from the isoquiiioline and the chloronnphthalene columns as indicated in Figure 2, and is used as the reference compound. A 95% commercial product (Phillips Petroleum Co.) is purified to 99y0 by distillation. .I nii.;ture of
s:
Y
the cis and trans isomers (available in 99% purity) also can be used, but it yields a n unsynimetrical peak that must be integrated for area. Ethyl acetate is used as the reference compound with the oxydipropionitrile column and is eluted between benzene and toluene. Determining each of the seven-carbon saturates and benzene with only two columns depended on choosing columns with optimum selectivity for cyclop a r a f h s and btnzene. For both the isoqninoline and the chloronaphthalene columns, the retention times of benzene and the cycloparaffins, relative to those of the paraffins, increase markedly :IS the percentage of the liquid phase is increased. For example, benzene is eluted with n-heptane from a 40-foot column of 3.8% 1-chloronaphthalene, but the two components are conipletelJ, separated on columns with 3.1y0 and 4.47, liquid phase-the benzene being eluted before n-heptane from the 3.1% column and after i t from the 4.4% column. A similar but somewhat smaller variation in relative retention times exists for the cycloparaffins. Variation in selectivity for cycloparaffins with changes in the percentage of liquid phase has been discussed (6) but not widely applied in preparing columns to separate complex niixtures of paraffins and cycloparaffins. I n this work, the optimum perctwtagc,s of liquid phase avoidcd superimposition of benzene and cycloparaffins. This approach should be useful in performing other difficult separations of mixtures with many components. Calculation. T h e areas of the peaks from the clironiatograms are measured
ISOOUINOLINE COLUMN
VOL. 31, NO. 12, DECEMBER 1959
1955
by multiplying peak height b y band width at half t h e height, or by using a planimeter. With detection by a gas density per cent by weight of component x is as follows:
x=- A z D z R ARDR where A , is the area of peak z D, is the density correction for
(molecular weight of z) 2) - (mol. wt. of eluting gas) R is the concentration of reference (weight of R added) (100) conipound = (weight of crude oil) A Bis the area of reference peak D Ris the density correction for reference compound
Table 1.
x = (mol. wt. of
In routine operation the analysis requires about 12 hours, including calculations. RESULTS A N D DISCUSSION
D a t a are given in Table I from duplicate runs on a mid-continent composite crude oil with the isoquinoline and the chloronaphthalene columns. The small
Analysis of a Mid-Continent Crude Oil
(Per cent by weight) Compound Ethane Propane Isobutane n-Butane 2,2-Dimethylpropane Isopentane n-Pentane Cyclopentane 2,2-Dimethylbutane 2,3-Dimethylbutane 2-Methy lpentane
Abbreviation CO C,
i-C4 n-C4 Neo-Cb i-CE n-CK CP 2,2-D hIB 2,3-D11B 2-LIP 3-hIP n-Cc MCP
Chloronaphthalene 1st run 2nd run 0 IO10 0,009 0.184 0.174 0.191 0.187 0.813 0,825 0.00 0.00 0,642 0.639 1.128 1.100 0.103 0.100 0.014 0,013 0,072 0,069 0.516 0.528 0.330 0.312 1.133 1.160 0.533 0.518
Isoquinoline 1st run 2nd run 0.011 0.009 0.177 0.180 0.188 0.185 0.852 0.832 @
0.651 1,132 0.107 0.018 0.075 0.504 0.326 1.151
0.636 1.116 0.108 0.013 0.075 0.527 0.318 1.128
Av. Value and Av. Deviation 0.010 f 0.001
0.179 f 0.003 0.188 f 0.002 0.83 f 0 . 0 1 0.00 0 . 6 4 f.0 . 0 1
1.12 f 0 . 0 1 0.105 =t0.003 0.014 f 0.002 0.074 xk 0.002 0.52
* 0.01
3-Methylpentane 0.32 f 0.01 n-Hexane 1.14 f O . O 1 a LI 0.53 f 0 . 0 1 Methylcyclopentane Methylcyclo entane b b 0.535 3,3-Dimetiylpentane RICP 3,3-DMP 0.540 0.008 f 0.001 3,3-Dimethylpentane 0.007 0,009 3,3-D1\IP LI 0.43 f 0.01 Cyclohexane 0.431 0.423 CH 2,2-Dimethylpentane D 0.013 0.014 0.014 f 0.001 2,2-DhlP 2,2-Dimethylpentane 2,2,3-trimethylb 0.016 0.018 butane 2,2-D3IP 2,2,3-TMB 2,2,3-T?*IBc = 0,004 f 0.002 2,4-Dimethylpentane 2,2,3-trimethylb b 0.036 0.037) butane 2,4-DRIP 2,2,3-TMB 0.034 a 2,4-Dimethylpcntane 0.033 f.0.002 2,4-DMP 0.031 0.112 D 2,3-Dimethylpentane 0.109 f 0.003 0.106 2,3-DMP 0.286 a a 0.28 f O . 0 1 2-Methylhexane 0.273 2-MH 2,3-Dimethylpentane b b 0.385 2-methylhexane 0.394 2,3-D1\IP 2-MH 1,l-DimethylcycloO.Oi7 i 0.002 0 . 085d+ 0.075 0 . OTOd*a . 0.078 pentane 1,l-DRICP I-cis-3-Dime thylcyclo0.183 f 0.001 0.182 0,184 0. 180d+ 0 . 200d,e pentane l,c-3-DMCP 1 3-Methylhexane b b 0.810 cyclohexane 3-1IH t CH 3-RIHc = 0.36 f.0.02 3-Methylhexane 1trans-3-dimethyl0 . 52Sd 0,549d b cyclopentane 3-MH l,t-3-DMCP 1-trans-3-dimethyl0.177 f 0.003 D 0 . 174d 0 . 1SOd lJt-3-DMCP cyclopentane I-trans-2-dimethyl0.32 fO.O1 a 0 . 31Sd 0. 330d 1,t-2-DMCP cyclopcntane 1-trans-2-dimethyl3cy10 entane 0 . 364d 6 b 1,t-2-DMCP 3-EP 0 . 34gd et h$pentane 0.028 0.031 0.030 rt 0.002 3-EP 3-Ethylpentane 0.005 f 0.001 0.006 a 0.004 2,2,4Trimethylpentane 2,2,4TMP / f 0.050 0.00i 0.051 0.049 Benzene n 1,214'' 1.22 xko.01 1.228 n-C? n-Heptane 1 n-Heptane 1-cis-2-di1.251 1,262 b l,c-2-DhICPc = 0.04 methylcyclopentane n-C, + l,c-2-DMCP I . 024 1.059 / I 1.04 i 0.02 hICII Methylcyclohexane 1,1,3-Trimethylcyclof / 0.111 f 0.003 0.108 0.114 pentane 1,1,3-TMCP Ethylcyclopentane b b 0.104 ECPc = 0.089 f 0.004 2,2-DMH 0.097 2,2-dimethylhexane ECP D n0.01 - . n_i -d 0.02d 2,2-DhIH 2,P-Dimethylhexane 10.26 10.23 10.27 0 9 Total Eluted with another component on this column. S o t eluted together on this column. c No individual peaks from these components obtained; values given were calculated by comparing results from two columns. d Peak not completely resolved from neighboring peak. N o t fieured in averaee. f Elutedo in %carbon region and not included. 0 Total not given because 2,2,4TMP, benzene, MCH, lJc-2-DMCP, ECP, and 1,1J3-TMCPwere eluted in 8-carbon region.
+
+
(I
0
cl
(I
+
+
+ +
+
+
1
bt
+ +
+
+
+
(i
(I
(1
G
+
+
+
0
~
1956
ANALYTICAL CHEMISTRY
(1
*
average deviations demonstrate the high precision t h a t can be obtained. T h e average of the deviations for all components was about 2% of the amount present. Precision varies for different components and in general is poorest for those present in smallest amount and for the four components determined by difference, For example, the amount of 1-cis-2-dimethylcyclopentane is obtained as a difference between two large numbers. A direct check on the precision and accuracy of the method was made by analyzing a synthetic sample composed of crude-oil bottoms and 14 components of five to eight carbons (Table 11). Standard deviations from six determinations for the 14 components averaged 1.8% of the amount present. T h e difference between the amount added and the amount found averaged only 1.1% of the amount present, slightly exceeding the standard devintion for four components. The high accuracy and precision of these data partially result from the complete or nearly complete resolution of all 14 components. Components in the sevencarbon region that are incompletely
Table II.
ber 1
9
I
3 4
5 6 7 8 0
10 11
12 13 14 15
16
17 18
19 20
Source: Counties,
States, and Countries 347, Abqaiq, 36% Ain Dar, 7% Shedgum, and 237, Uthmaniya, Saudi .\rabia El Dorado Conipos- Union, Ouachita, and Schuler, Ark. ite Claiborne, Union, and Webster, La. Iiu w ai t Kuwait Hot Wyoming Compos- Fremont, Springs, Park, ite Natrona, Big Horn, and Washakie, Wy. Beaver Lodge Williams, N. D. Venezuela Tia Juana Garza, Tex. Teas
Name Saudi -4r:tbian ( Aramco)
Analysis of Synthetic Crude Oil
(.4verages of 6 determinations) Per Cent by \\'eight Added Found 0.766 0.i 5 7 1,094 1.087 0.159 0.159 0.239 0,240 1.034 1.024 0.672 0,673 1 399 1 399
Component Isopentane %-Pentane 2,2-Dimethylbutane Cyclopentane 2-Methylpen tane 3-lIethvl~entane n-Heuahe hlethvlcyclopentane 2,4-Dimethylpentane Cyclohexane 2,3-Dimeth>lpentane 2,2,4-Trimethylpentane n-Heptane Meth) IC>clohexane Total
Table 111.
Kum-
Each of the possible paraffins and the five-and six-carbon ring compounds was found and determined. Traces of 2,2,3-trimethylbutane and 2,2,4-trimethylpentnne, which have not previously been reported in crude oil, were found. Other difficultly detectable components, such as 2,2-dimethylpropanc, 3,3-dimethylpentane, and 1-cis-2-dimethylcyclopentane, also were found.
resolved or determined by difference were not available. Table I11 identifies 20 crude oils that have been analyzed. Analyses of these crude oils for hydrocarbons through seven carbons are given in Table IV in order of boiling point. The crude oils in both tables are listed in order of decreasing paraffin content in the fiveto-seven-carbon fraction.
Crude
0 792 0 293 0 776 0 336 0 183 1 342 0 848 9 924
0 805 0 296 0 773
0 342 0 185 1 386 0 862 9 9'37
Standard Deviation 0,007 0.017 0.002 0.005
0.016 0,008 0 018
0 010 0 0 0 0 0
012
014 012
004 009
0 013
Oils Analyzed
Geological Data Era Period Formation RIesozoic Jurassic Jurassic
Mesozoic Cretaceous
Color Dark brown
Sulfiir, Gravity, w/ C ".\PI 1 ,i 2 8.1.1
Sacatoch, Tokio, Dark brown Woodbine, and others
1,16
38,9
hlesozoic Cretaceous Dark brown and Embar a n d Tensleep Dark brown Paleozoic Permian Pennsylvanian
2.62 2 50
27.8 26, 6
0.11
1.42 0.85
43 3 26 7 40.0
1.47
29.2
Paleozoic Silurian Cenozoic Tertiary Paleozoic Cambroordovician Pa 1eoz oic Permian Kidilrr Winkler, Trx. Paleozoic Rlainly PennWest Kansas Com- Trego, Ellis, sylvanian and Russell, Ellsposite Mississippian worth, Barton, Rush, Pawnee, Stafford, Reno, and Sedgwick, Kan. Eola Garvin, Okla. Paleozoic Ordovician Beaver Lodge Williams, N. D. Paleozoic Devonian 85% Slaughter, Yoakum, Hockley, Paleozoic blainly Permian 1.57" 12-lb. Casand Cochran, Tex. inghead Gasoline Southeast Saskat- Paleozoic lIississippian rllida chewan, Canada Cenozic Tertiary Lagunillas Quince Venezuela Alberta (Edmon- Paleozoic Devonian Redwater ton), Canada Paleozoic Permian Lubbock, Tex. Lee-Harrison Hockley, Tex. Paleozoic Pennsylvanian North Smyer Cenozoic Tertiary Brazoria, Tex. Hastings Los Angeles, Calif. Cenozoic Tertiary Wilmington Harris, Tex. CenoAoic Tertiary South Houston
Silurian Green Xliocene and Eocene Dark brown Ellenburger Dark bi-ovn Guadalupe
...
n a r k brown Dark brown
0 42
35 0
hlcLish Devonian Alainly San Andres
Brown (;reen 11:ii k brown
0 01 0 13 1 i0
36 6 40 7 36 4
llission Canyon
Brown
0.81
88.1
lliocene and Eocene Black Dark brown D-3
2.23 0.47
16.3 83 3
Clear Fork Ptrawn Frio LGcene
Dark brown
3.21
Green
0.1%5
23.3 43.2 31.1 19.5 21.3
Frio
Bronn Dark brown Brown
0.19 1.59 0.26
VOL. 31, NO. 12, DECEMBER 1959
1957
1958
ANALYTICAL CHEMISTRY
Methane and low-boiling sulfur or olefin compounds were absent or below detectable limits. The crude oils analyzed represent extremes in composition. They range from highly paraffinic for S o . 1 through highly cycloparaffinic for S o . 20 to highly aromatic for No. 16. The paraffin compounds vary from mainly normal paraffins for No. 5 to mainly branched pa,raffins for KO, 8. Similarly, the cycloparaffins vary from predominantly compounds with five-membered rings for S o . 19 t o predominantly compounds with six-membered rings for S o . 20. The compositions of four of the crude oils are particularly noteworthy: No. 8, W n k l e r ; No. 16, LeeHarrison; S o . 19, Wilmington; and S o . 20, South Houston. The sample of 11.inkler crude oil was probably produced from a higher stratum than that analyzed by Forziati et al. (8). S o t only do t’hese crudes rcpresent. extremes of compound classes, but the isomer distributions within the classes do not follow the trends coninion to most of t h e other crude oils. The analyses of these 20 crude oils support many of the conclusions on hydrocarbon composition reached by the American Petroleuni Instit,ute Research Project, 6 (8.1 4 ) and others. Basically, crude oil is not a random collection of hydrocarbons. Even though the amounts of the various classes of compounds differ greatly, the isomer distribution xithin a given class sliom some siniih r i t y from one crude to anot1it:r. Thus, some generalizations about, isomer distribution can be made. I n general, the relative abundance of tlie isomers decreases as branching increases. For esaniple, the amounts of the six-cwbm branched paraffins usually decrease in the order: Z-MP +.3-1IP --L 2,3-DlIB + “P-DMB T h e only e s c q h o n is S o . 8; n-liere 3methylpentane predominates over 2methylpentane. The seven-carbon branched paraffins most often decrease in the ordcr: 3-3IH + 2-1111 + 2.3-DlIP + 2,4-D1II’ + 3-EP + 2,P-DlIP + 3,3-DlIP + 2,2,3-T1IB
However, exceptions to t’his order are numerous toward the middle of this sequence, where the isomers are present in similar amounts. The 2,3-dimt?thyl isomer predoniinates over the other dimethyl isomers in both the hexanes and heptanes. On the other hand, the 2-methyl isomer predominates over the 3-methyl in the hexanes, but the 3-methyl predominates over the 2-methyl in the heptanes. This reversal in favored isomers makes prediction for higher carbon numbers difficult. Although the isomers from a given class may be arranged in a typical
order of abundance, they arc>not present in the same or even similar relative amounts in all crude oils. Actually, there are large differences in the distributions. For example, within the sixand seven-carbon branched paraffins, the ratio of monomet.hy1-branched isomers to dimethyl-branched isomers varies between 6.4 to 1 for S o . l i and 2 to 1 for KO, 18. The distribution varies extensively even within the dimethyl-branched isomers and the monomethyl-branched isomers. For example. the ratio of 2,3dimethylpentane t o 2,%diniethylpentane varies between 24 to 1 and 3 to 1. The ratio of 3-methylhesane t o 2methylhesane varies between 1.S t o 1 and 1 t o 1. There are also la,rge variations in the distribution of compounds of a homologous series. For example. the ratio of n-heptane to n-hexane varies betn-een 1.7 to 1 and 0.6 to 1. Similarly. for toluene and hcnzenc. the ratio varies between 7 to 1 and 0.8 to 1. These variations in distribut,ion within :I class are similar t o those reported 1)y o t h s (8,f6), but are somewhat larger. The authors’ complete data rea,dily show how broad these variations may be. T h r y may d l be important in studies of the origin of petroleum. Correlations of isomer distributions n i t h the propertks of crude oils, and the conditions under which they were fornicd are very cornples and require much further study. H o w v e r . these (lata indicate that the agc. or stage of maturity of the crude oil is an important consickration in predicting coniposition. Crude oils t,hat origiiiatetl in the Paleozoic era (Sl5.000.000 to 175,000,000 :.cars ago) \-ury tliff(wnt1y in composition than those froin the more recent, 3Icsozoic 2nd Cenozoic eras (175,000,000 yenrs ago t o the prescnt). For example, after cquxting tlie fiveto seven-carbon portion of each crude oil to loo%, the aromatic fr:rction of the eight-carbon portion froiii thc two reccnt eras is loxv and nearly coiistuit4.1% with an averagc deviation of only 0.6%. Hon-ever, the prrct.nt:ige of aromatics in thc 12 crude oils from the older Paleozoic era varies from 2 to 20%. Also, for the older crudf, oils the trend is for the aromatic percentage to increase as the ratio of cyclohexanes to cyclopentanes increases; this trend is not followed with the younger crudes. These facts may mean that the chaotic conditions and the higher and more variable temperatures in the Paleozoic era were more suitable in certain cases for tlhe formation of aromatics than were conditions in the Cenozoic and hlesozoic eras. Although contrary to some theories on the origin of petroleum, a further
interpretation of these data could be that one path for formation of aromatics was isomerization of cyclopentanes to cyclohexanes, which dehydrogenated to aromatics. The cycloparaffin content of t h c s crude oils tends to increase as the ratio of branched to normal paraffins iricreascs. If a starting material of n e d \ constant composition is assumed, this trend suggests that isomerization and cyclization reactions occurred togethtsr in the formation of petroleum. CONCLUSION
Ilt~tailcdcrude-oil analyses olt:iiiied easil). b!. t h neiv mcthod should prove valuable in many areas of petroleum research. One use is in estimating naphtha yields and naphtha quality for reforiiiing and isomerization (PO). dnothcr would be as a basis for appraising crudc-oil quality and value. Such data might also provide n new tool in geology and geochrniistry for studying the forniat,ion and migr:ition of petroleum. The method could be e s t e i i d d to determine compounds above seven carbons, but more elaborate experimrnt:d techniques \I-ould be necessary. The best approach might involve prcfractionation of several cuts into analyzer columns of high selectivity that would retain aromatics past cycloparaffins and cycloparaffins past paraffins in each cut. Because of the increasing number of compounds of each higher carbon number, complete separation of individual components above seven carbon atoms is improbable. Combining gas chromatography with analyses of trnppcd-out cuts by infrarcd and mass spectrometry should a t least partiall:- ovcrcome this obstaclr.
..1he
ACKNOWLEDGMENT
authors givc specid acknowledgiiient t o H. C. Oliver of the Btandard Oil Co. for obtaining many of t h r samplcs of (,rude oils, and for his helpful tliscussioiis. They thank Ralph 0. Clark of thc Gulf Rese:irch mid Devclopni(~nt Co. for supplying thv saniplc, of I
