CATAL\TIC EFFECT OF SMECTITE CLAYS IX ~DROCARROS GEXXRATIOX REVEALED RY P\-ROL\-SIS-GAS CHRO3UTOGWHY

J-B. DAVIS * and J.P. STASLEY

ISTRODUCIIOS

-4 recently developed practise in petroleum geochemistz is thtrmal anal>-- sis to determine the h>-drocarbon gene-rating capacity of sediments [l-6& .A commercial instrument now commonly used is the Rack Evd 131 which determine the amount of preformed \-olstiles including hydrocarbons (IOU temperature S, fraction) and the amount of volatilti including h>-drocarbons evoked durin_e the pyrolysis of a sample {hi@ temperature S, fractionk Comparative data acquired by the method are useful in asse&rg quality or potential of scdimtnts as sources of hydrocarbons Son-volatilt bitumms of the sdiment samples do not appear in the low temperature Sr fraction. but are subject to p_vrolyis by temperatures in excess of about 3oO=C. and their

p?rolyis products appear in the & fraction [S.9]. The preformed volatile and non-volatile bitumens. which often are in relatix-el- insimificant amounts in immature sediments. can be eliminated from consideration by a thoroufi solvent extraction of samples prior to analysis. Iming only the kerogen subject to thermal analysis. Pyol_\-sis-gas chromato_gaphy (Py-GC_J of the

products wolwd from heated sediment samples is used to a more Ii&mited extent because of the additional time and instrumentation requixd [IO]. @-GC can be coupled with mass spwtrometF for a precise identification ctf the hydrocarbons generated [I 1 J.

e-GC is useful for semi-quantitative and semi-qualitative analyses of pyoI_vz;ltes. Kerosen pyol>mtes and sediment p_vrol>zates are sometimes different. surpgestin_p 3 significant influence of the rock matrix [ 12Xj. There is 3 rcnbenq- for p\‘roIc\zed sedimentary rods. cornpar&& Mh isolated ker0gen-F. to yield smaller amounts of volatilti indudin_a h>-drwarbons of !o.lwzr molecular weight. This is noted particular& for argillaceous sediment szrnp!~~s with an organic carbon content of less than about 2%. Isolated ker+ens from such sediments yield a much broader range of volstiles including h_vdrocarbons of high moleculsr weight. Hou-ever. u-hen rhw samt kerosens arr: mixed in concentrations up to 2-34 with expanding tLr22-235~ cl3ys. including smectits and illites that contain smtztitc-iliitt mixed 13yr cl3y. p\-rolgatti consist primsrily of low molecuIsr weight hy&xarbws. indicative of catalytic cracking or excessive cleavage of carbon-carbon bonds. The cata&-tic cffecr of smectitic clays u-35 confimcd

using mixtures of such clays with long-chain aliphstic hydrocarbons. Pyre- Iy;?tsj of these mixtures consisted !arg+- cf ~;lst\)us hydrezarbons. So such rc4rs were obtin-ed when non-expanding sli~-s were used- As 3 conse- ~uenct. wgiltxsous wdimtnt~ that con&n less than approximately 3% organic carbon arc poor SOWLXFS of oil. although thex ma? be productive of gzw 1 i 2) and gas condcnsatc

Mixtures of kerogens with quartz. $Iica. Aumina. calcium carbonate. kaolin. or iliites not containins smectitc-i!litc mixed layer ~13~. yield pyre- I_\zates rmore similar to those of the rtipective kerogens alone. i-e. the range crf hvdrwarbons in the py+zat= is broad including those of hi& .molecu- _ hr xvti_ehr. This is due ostensibl:; to 3 lack of cstaI\;tic activit? of thee mir-cv& 3s compared with the cataI_vricalIy active three-Iayr ~135

ESPERXMESTAL _4SD RESULTS

&--GC w= performed with an Envirochem Inc. (Kemblesville. PA. U_S...._J Cnxon Model 78CB-2. in which u-as cmplo?ed a SCOT 30 m X 0.5 mm I.D. g&s capillary column. coated vrith SP 2100 (Supelco. Bellefonte. PA. L.S.X.): or a 6 ft.x 1,)‘s in. stainless-steel packed column coated with 24 UC1V 9SZ r Hewlett-Packard. dvondsle. PA. U.S.A_)_ A small sample

(1-100 mg) of sediment. kerogcn. or kerogen-mineral mixture was pyo- l>zed. ttxmperature-programmed from ambient temperature to 6WC at 2S=C/min. in the instrument tube furnace in an 8 x I,.!4 in. quip tube through uhich helium flowed at 35 mi/min. The p>-rol>zate was transferred as formed through a heated (240cCj inlet tube to Lnacon Trap 1 which consists primarily of a column of Tenau-GC snd contains a small amount of activated charcoal or Ambersorb (Rohm 6: Haas. Phi!addphia. PA. L.S.X. I

at the effluent end of the trap to adsorb low molecuiar weight hydrocarbons. During the pyol\-sis and trapping processes a small amount of pyrol>zate is shunted to the t-nacon flame ionization detector \FID,. Trap 1 of the L’nacon is automatically hwted. back fhushcd. and the p~~ol>ztt is tram- ferred in helium to Trap 2 for further concentration of the \.AxiIe~ and final& the concentrate is transferred from Trzq 2 to the Unacon g= chromsrtognph column for anal>-& (Fig. 1 .L A rewrding of rht Py-GC analysis consists of a thermogram (total volzltiItsJ. and 3 chromarogr~m showing the molecular weight range of the vo!atiles. The rrr?p ztt raxn

temperature adsorbs and concentrates effectively hydrih-arbczns higher In molecular weight than ethsne. Methsnt. carbon dioxide ard &most of tht ethsne pyrolytic+- generated are not adsorbed by the trqx but can be

1 TUBE FURXACE

collected in the helium effluent at the trap vent with a gas bag for separate analyis. The thermo_Pnm includes both the low temperature St fraction. if any. and the hi_eh temperature. pyrolyis. & fraction. as in the Rock Eva1 anal_vsis. An_\- temperature increment iraction can be chromsto_gaphed inde- pendentIy if desired. or the total volatiles can be chromato_gaphed as shown in Fig. 1. In the Fig_ 1 example an S: fraction is essential&- absent.

Kew_eens were isolated from sediments by rock dissolution with HCl and HF. folIowed b>- hesLy Iiquid (acidified ZnBr,) flotation recoveq of the residual kerosen. extensive washing of the kerogen with distilled water and iinall~- with mtth+xx chloride-acetone t 50 : 50).

A’erogen-mineral mi_xrwec

Experiments with mixtures of v-srious minerals and the Eocene Green River Formation kerogen or other ktrogens. have shown that on&- the expanding three-layer clays. including smectites Qxntonite. montmorillonitc) ar.d illites containing smectite-illite mixed la?;er clay cause a marked dif- ference in the composition of kerogen-mineral p>rol>zates- Expanding three-layer clays are more catalyticall_v active. causing extensive cleavage of carbon-carbon bonds resulting in a preponderance of low molecular weight hydrocarbons, as compared with hydrocarbon products of kerogen pyol>zed in the absence of expanding three-layer clays (Fig 2)- Heating the smectite or expanding clays to XO’C and holding them 3t thst temperature for 15 min deactivates them. so that the- lose their c3talytic 3ctivity. For exsmple this applies to Wyoming bentonite as well 3s to tw-o monrmorillonites (So. 19 3nd So_ 25) and two illites cont3ining smecrite-il!irc mixed la-tr clay (So_ 35 and So. 36) described by Kerr f II]. However. when these clays were each heated from ambient temperature to 65U=C ar,d cooled immedistel\-, two of them (So- 19 ;ind So. 36) were not desctivzttd and retzined their catalytic acti\it_v. indicating that some expanding ~13~s 3re quite rektsnt to the destruction of their crystal lattices.

-l-he two illites and two montmorillonites were tested mixed individuali> with Green River Formation kerogcn 3i cozce~trzti3n~ rezlring in O-5 znd 5.05 tot31 organic c3rbon !TOC! iT3blt 11. Higher yields of v&stilt prod-

uc:s occurred with the mixtures contsining the higher concentration of ktrogcn as previously reported [ IZ. 131. Py-GC csxistently show that Iowcr mo!tsuIar weight h>-drocxbons art prtiuced predominsntl_v by mixtures with a high retie of ckq- surfice to orgmic matter. and these lower mo!ecuZar weight hydrocarbons must be lx&- due to a greater dqret of catalytic ~rxking (ckavage of carbon-carbon bonds). As a result. due to 3 greater h-cirogen imbalance more sarbonizztion occurs. 3s N-S determined b_\-

1

_-AZlECASE

* _ _ . t: ; - __ _ _ _ __;&_ --_- iI1

c~mpsring the residual carbon in btntonite-kerogen mistures ~olkwing p?;r&sis_- For exampie. 60% oi the originally sv;lilsble oqznic csr’bon remained as graphite in a pyrolyzed ktrogen-clay (’ 15 TOCi mis:ure. while only 284 of the carbon remained in a pyrolycd kerqcn-clay i 10% TOCr misture. Thse data sugest that the cl3>- has 3 limited catalytic capacity. perhaps due to “coking-. of the catalytic surfaces. These obser\-ations ~-ith herosen-clay mixtures have helped in interpreting results with sctual se&- ment srumples.

Tht carbon-carbon bond cle~vag,s effca of smcctitc c3itd-:ic x~fvity ~3s

confirmed bv comparing the pyolysis of ;? n-~~t3dt~;inc-sms~titc miatzrc with the pvrolvsis of a n-Lwtsdecanc-illite misrux The illits u3t.l ~XYXC‘~: Beavers B;nd._OK_ L.S.X.) has no sspand;rb!t sniectite-illite mis& kq-sr~ and has little catalytic r?cri\-it>-_ The r3tio of rs-ckxak~as to Ai_\- 1x-a tI;s

same in both mixtures t 1 : 1000~ and there tibviousl_\- is 3 strong Jiffcrexc in catalytic activity of the two clays which must be ;rttributtd to ;? scyt<ik catzxlytic carbon-carbon bond &a\-ag.+ effect of the expsndhg thxe-layer clq. smecrite ( Fig 3 I_

_A comparison also was msde bttwsen the amounts of voWi& including hydrxarbons pyol_\-ticall_\- gensrxcd from the Cretzceous Ezg!cford Fomx~ tion shale_ isolated Ea_nleford Formation kcrclgn. 2nd 3 s-rr,tctit,c- E3gle~od

ktrogtn misture. The Eqleiord shalt s;lmpIc wntrtintd J-25 TOC. Tht kerogen. psrriall>- purified. contained XCC; TOC. The yie7d of tx;l: vAt2c products generated tthcrmogams)_ a ~~11 YS tht ~lxztrz of h~&xxk~n~ {chramatogams) \v\-erc ~tntially tht s;lme Lx he s!zle sr?J dx idsr’2 kcrogen [Fig.. 4). This indicates that cxalyti: rictfvi~~ LX_ the E@tfxil shAe is not discernible- attributed to the rkrivel- high ratio L$- organic mztxr w mineral matter includixq smcctitc in the shrilc. In <:ontr;t~:_ 6s ~-i&i of volatile products and spectra of h_\-drocarbans _rsntrzted frxn the misture of EqIeford kerogen and a smectite rmontmorillonirt. -WI So. 141 w-ere different in magnitude and character t,Fig. 4,. The yield of pyrolyic pr~5.~t~

Awas-----65n*c 0 10 15 20 2s 30

c=mONwMaER

fram the mixture. which contained OS XX. on the basis of carbon was reduced to IS%. one-half rhat of either thr: isolated kerogen or the shale. 315 and 30%. respectively {Table 2,.

Sedimenxs and is&ted kerosem

The Miocene Baonrp Formation shale of Indonesia. believed to be rhe source of gas and gas condensate deposits in the as&ad -Arun Formarion. Fields IOU- molecular u-eight hydrocarbons when p:Tol>zed_ ln contxxst, tht sediment-- organic matter &erogn) isolated from rht Baons shalt yields on p,\rolysis a broad range of hydrocarbons including those of hi& moku-

EAST CAMERON BUC 286 YERYZLZL13N BLK 212

KEROGEN

SEDiYENT SEDiMZBii 0.51 f.0.c 0.7% T-O-CL

I .I ;: ‘ii -

i GAscoN3ENsATE ii

GAS coNxNsAtE 1 SERY. BLOC% 21s

lrtr w-ci&t in the oil range. This contrast is depicted in Fi_p. 5. along with a chrL.mstogrsm of the Arun Field gas condensate. The concentration of .xgani< matter in the Bxq shalt is relativtl_v low. with a TOC content of abwt I C;. The apparent catal-tic effect noted in the example of Bsong shale ~rl~b~blv is due to its linw-n wntcnt of cxpandiq three-layer clays. Further . esampk arc: xgi1Ixekx3s Sew,, e-ne sediments of the northern Guif of Mexico

brr?;in . zs shown in Fig. 6. Sow that ialatcd kerogens fram these sediments. Lvhen pyr+zd. _\-ie!d 3 broad range of h_\-drcxarbons. but the sed.Tents do nix: that is. thee xdimsnts which ccrntain expanding threelq-cr clsys yiidd primr?rIl_\- ILN- mokeukx wei@ hydrxarbans. -4s in the USC of the Baong &rrk LJ,~ Ir.don&;l. the Seo_esnc sediments I Fig. 6) are law in organi< kxKbkYL

Mixtures of smeaitic or sspanding clqs and kerogcrx in which the

I KEROCEN

SED:IYEHY 4% T.O.C.

OiL A.P.L GRAVITY. 35’

ZreroSen concentration is relatively hi_& TOC > 2%. yield pyd_\zates similar to the pyrol>zates of isolated kerogens. This indicates the importance of the ratio of mineral matter to organic matter in such mixtures. and suggests the Same for sediments. For e..arnple. Jurassic sediments fro-m the Sorth Sea containing 4% TOC yield on p_\rol_vsis a broad mnge of h_\-drocarbons in tht oil range. as dots the kerogen isolated from Wse xdiments (Fig. T!.

Petroleum production suppo.rts these obstn-at&k. G3s and Sss ccn- densate are produced from the .Arun (r-en-air) Fomxtion u%h.in .the Bzong Formation snd from r-n-ok asjocistcd with the SeaScne sediment esxn- pies (Figs. 5 and 6): oil is produced from rsbt=oirs awsistcd uith t:h?r Sorth Sea exsmple (Fig. 3). If one were to determine the carnpzitian Lx. pyroll_\2atti of onlF the i.wlsted kcrogtns. ?;uch cxrektion \\-_,,-&I nl;;t be obser\-ed.

One of the moat obvious effects of smtctite satslytic 3stixity is tsce&x-t cle3vage of carbon-c3rbon bonds in high molecular lveight hydrLx3rbons to produce hydrocarbons of iowr moleculsr u-cighr. This effect can be ob- sen-cd by comparin_e the chromatogmms in Fig. 4: for ts3mpls. by compsr- ins the difference in the pyrol_\zz;tc chromstognm for the ktrogen 2nd the pyrol_\zste chrom;rto_gmm for the kerogen-smectite mixture. The high molcc- ulsr weight h>-droc3rbons gencntcd from the kerogtn 3re presumsbl_\- intsr- msdi3tc products in the presence of smtitite. These intermediatti become further pyrol_\zrd to IOU- mokular weight hydrocxbons rind 3 gmphitk residue. resulting in 3 lower tot31 h_\-drocxbon >-ield.

.An inter-tins difference exists between the p_\-rol~zst of the ktrogxr-smectitt mixture shown in FiS. 4 snd the p>-rol>z;itt of the J:-LX-

tadecsne-smcctite misture shown in Fig. 3. n3mcly the presence of higher molecular weight products in the lsttcr pyrol_\zate (Fig. 3). One cnn spsu- 1st~ that hydrogen-rich n-octadecane ( 15% HI is more readily converted to hi_eher molecular weight hydrocarbons than is the less hydro_een-rich Eagle- ford Formation heroSen (3.7% H). A second possible f3ctor is the reiati~ely IarSt amount of 3 single component in-octadecanc, subjected to pyolysis. compared with a uide distribution of intcrmedistes generzted from the kerogen. The latter may be more prone to polymerization and graphitizkon in the pyolyis process. In 3ny event. 3s shown by the thermogram in Fig. 3, the ma_tirude of the total products from the n-Ktadecane-smectite mixture is about equal to the amount of E-octadecane p>~ol_\zcd. wherezrs the yield of products from the kerogen-smectite mixture ~3s less than one-hslf that of the pyrolyed ksropen (Fig 4).

Espitalie et al. [lZ] su=ested thst high molecular weight t,intermediare! hydroc3rbon products are retained by adsorption on mineml includin_e cl3y

surfaces until hi&x pyol-tic temperatures crack them to produce low rnc~!~exiar weight hvdrLxarbons_ I’hilt this mechanism is no doubt operative to wms extent. C& data indicate a more specific (catalgic) acti\it> with r-pa-t CQ smetite (threeia-cr expandablei cIq-s. This is shoL\n in Fig 3. 2nd is ~r?gge~ted further b>- 3 relrttivt lack of activity of silica gel which has a B.E.T. surfax area tl?f abwt XKI crn’..‘g compared with 3 btntonite *with a BET_ surixt arti of 0n1y rtbcrut 43 cm’..‘_e (Fig. 21. The bentonitc is by far the met a&\-e in the pr&uction of kxv m&xulsr weight prcaiucts.

The ro!r‘ of smectitcs in carbon-csrban bond cls3vs_ee and in hydrocarbon sxkins 3s related to petroleum formrttion is 3 subject of long standing HenJcrsL~n et 31. [ 151 determined that smeaite t bcntonite! caused 3 ten-fo!d in<rcL1.\= ; _n the thermal Je,-~~rnpl~sitI~w of t:-xtawsanc (C,,J to pwduce 3

high ~l~nxntrxi~vl of I~w-er 3Ik3tr,es inA,xiing both branched and qxlic r?,s ws:! 3s n,vrnA riih;tn~-5. Sxkr’tt [ 163 ~~xmprrrai the carbon isotcrpis campr-‘si- ti\Jn ,ri methane thtmxdI_\- praiuad from c-~xtrtdwans in the presence and in rhs rrbssx~ t>f ;I sm;!ei:e. l\fsthrrne pr~tiused in the presence elf smectke .\?l\av<J !itrls CarLIn iwt3jx frxtiax&.n. attributed to 3 carbonium ion nx~hrtnism induaz by the ~13~ and its sdsorbcd w3ter.

0 ns xv-ho has studk~ clr?y cat3lysis in det3il in recent yc3rs. Johns [IT!.

ZlcEirve~ :h+ mechanism 6 smsaitc carrrlytk ac;ivity is that these clr?_vs act zs xi3 sat;zlysts thr.wgh Ji_so&ti~n of inter@-cr xva~cr. thus promoting

~-arbonIum i&m re;rutiL-ns. TO wnfirm Ihis 3s tk 3cru31 mechanism. cstrcful kfnstk studies probably r?re required with 3 scrks ef smeaitc catal_\-sts tvith JifGrezt exchan_eeab!s carions. dificrenr inter135cr chxses. and \-xyin_e xwun:~ ctf Lvt3hek31 3nd tttnhedral substitution.

JcJhns [I?] ha?; viewed the dearbox?-Isian of fatty acids. either free or c~cIS~~ in kerozsn. and the _esnsrk,?.,n of rxtlkmes zs prabzbly rht first srr?pt in petr&um fwmzti~n. Usins 3 C3 m~ntmorillonitc as catalyst. Johns &r~~mintct zn aztiv;lti~n energ? for tht de:rtrbos_\-htion of E-drk-osxzoic xid \c’-- I t&l bc 31.1 kczli mo!s. \Vhite the Cz3 r:Aks?nt N-S the mz.or produ;t:- produc:2\i n-&ants ranged from C:, to C,,. indicating both csrban-carban bond cle3x-sge and some incrcxse in chain length. Johns bslisves 3 tw0-st35e csaiytic prcwzss is invoh-cd. the first sts_ec bsin_e

&xzrb~x~l3tion of 3 fztt>- ;?cid to FrAx:c s n-rilkmc with one carbon Iess_ idIowJ bv c3tzlvtic crxkine of the ealkanc to shorter chain Iength

5rt!krint~. kith minor p&u&on of n-slkanes of longer chrrin length. IYork b? John [ 171 reve3led an unexpectedly strong c3tsl_vtic acti+ of

smstitss with regwd to E-alkanes, acruslly involving aaktion energies of I~SS th3n X5 kcal/mole_ This is remarkabk bwxuse arbon-bond clcsvs~c zntrgies hat-e bwn c&xlatcd to vxy bttwm 60 and 90 kcal_AnoIe [IS]- LWe bekvc it is this strong catalytic activity of smectitti thst reduces the amouns of high molecular weight E-alkyres in pyrolgates of kerogen-clay mixture. and resk in the formation of relstively 1-e amounts of low molecuizr w5g.ht hyirocarbons_ This is shown clwly in Figs 3 and 4 and ouggsrs a

signifkmt role of expanding thre4ayer clays in the ultimate generation of hydrocarbons from sedimentary orsanic matter. which is most evident for or_eanic-poor sediments containing these clq-s.

Horsficld and Dou&s [ 131 have sugceted that although high temperature pyrolysis of sediments would not be expectti to duplicate slow geothermal maturation procces. one would expect the distribution of p>-rolFtic prod- ucts of kerosen within sediments (that is. in whole rock) to reflect the type and distribution of hydrocarbon products resulting from indigenous kcro_ecn maturation. They concluded that sediment iwhole rLxl;! p>-rolyates and the pyol>zates of isolated kerogens or coal mscsrais. because of their dif- ferencti cannot be directly compared when ont is tn_ing to relate p>-rolytic data to organic matter or maturity_

Thermal analysis of sediments and kerogsns which is limited to 3 qusnti- tation of total pyrol>zate. such 35 thermal evolution r?n;llysi?; \c_s_ Rock Ev;lli. is insufficient because the p_\-roI!tic product, arc’ not determined. 11~ith P_\--GC. howa-cr. 3 sediment sample can be pur,eeJ of its prsformcd wlatile hydrocarbons at low tsmpcratura, up to XU’C, and rhe purged frxtion chromatographtd- This is followed by hsstiq the _emp!c through an_\- p_\-rol_\-tic temperature range bttwctn 250 and 50U’C q or hi&crr. and chro- matograph>- of the pyolytic fraction. S\;c\t on& are the quatitia af fractions dctcrmina3 but their r&tive compositions arc’ determined as ~v\-tll_ The wrsatilit- of this t_\pc of thermal an+-& and rhc CCIS~ with which it now tin be applied will permit a wider use of its definitive ;Idvantagts. and a dsterminstion oi the composition oi pyrcrl>-zats of ssdiments xi11 likely brsome an integral part of source rxk an;ll_\-scs_

The authors thank the msnqemtnt of Mobil Rtie:;irsh and Development Carp, for permission to publish this paper. The Green River Formation kerogen was supplied by \V_L_ Orr. and J-T. Edwards furnished the -X-ray diffraction data relative to the expsnding three-lq-tr clsys employed.

REFERESCES

" ¢ 0

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