FACTORS IXFLLYESCISG THE EVOLLTIOS OF PETR0LEt.I AROMATICS FROM AX IMM_~TLRE CRLDE OIL DURISG P>-ROLSSIS OX A SHALE MATRIS

(Received December Sth. i9SO. rcttp’;t2 DtctzStr 22x?. 1933)

Samples of zn immawre crude oil hse been pyro!,\-red 2: Xtr’C QZ z zz?-gt of cask-P sediment mrtrices. The maturation of rhe aroxzaric frcczIo3 05 :I-.t ?t:roItxz3 5rzs zs,or.i- tored using proton magnetic resonance sgeccrroscopp. Fzc:ors \xtitS fs tit ~LZ~~X included the cation exchange capacity. the na?ure oi :)Lt erc~~pt Ions. ckp typt. k-t! or

elemental sulphur. and surf2tt zta. T3e vxia5lts :ozzd LO ka-t tie gpxctrr t%c; o=

maturation rate were the proton exchsqe capacity and The conctntrztion 0: eltzxx2? sulphur. In addition. removal of the’heteroqxSic compoxxs from ;ht crz.dt oil rtdrt2 in an enhanced rate of maturation. The imp!icatiors Of these restsllb for rhe processes involved in petroleum formation and mzrurztion ucder geological cor.Citiozzs 2re do- cussed.

ISTRODCCTIOS

The processes of petroleum formation involve the rezctios of compks mistures of organic compounds which tie place in the presezxe of a min- eral matris under mild thermal conditions_ The therma&--induced reactiolls take place during catagenesis where temperatures between 6O’C and 2OO’C are produced as the burial depth of the sediients increases [1,2]_ The overall process can be regarded as one of hydrogen redistribution in that the complex misture of all@ and cycloallq-1 aromatics which are present after diagenesis [3] ‘undergo hydrogen dispropotionation reactions [ 2] yielding increasingly hydrogen-rich alkanes and residual aromatic molecules which are increasingly depleted in hydrogen-

Roton magnetic resonance (PAIR) spectroscopy is ideally suited to moni- toring the changing proton composition of the petroleum during these pre ceses of formation ad maturation. Recently we reported a simple method of measuring the maturity of petroleum in source rocks by determining the percentage of aromatic protons in the aromatic fraction (PAP) of the soluble organic matter by PXiR spectroscopy [4-6]. Provided certain criteria are met, the PAP values increase in a regular manner with increasiig depth and sediment temperature, thereby providing a simple useful relationship for use

60

as a maturity index of the aromatic fraction from petroleum. In this paper we show that a similar change in PAP can be induced in immature petroleum by pyrolysing samples on a mineral matrix for varying times. Hence PZ\IR measurements on the aromatic fraction provide a means of monitoring the maturation of petroleum, and as one can distinguish factors which have an important influence on maturation processes during pyrolysis, by inference, factors important in a geological context can be distinguished.

Although many pyrolysis studies have been undertaken to gh-e informa- tion about categenetic processes, those which have used natural petroleum or petroleum precursors have been limited. Some of the latter trpe include pyrolysis of lignite [ 71, immature petroleum source rocks [S], and bitumens and recent sediments [9,10]. These workers examined the e\;olution of the different classes of organic compounds as thermal reactions proceeded. They observed that the products and product distributions obtained from the natural materials using pyrolysis temperatures up to about 3OO’C are sin&r to those obtained during catagenesis of organic rich sediments. This indicates that such temperature conditions are suitable for laboratory-based experi- ments designed to study catagenetic processes.

An attempt has been made in this study to obtain results which are rele- vant to petroleum formation processes occurring under natural conditions. Samples of petroleum source rocks have been used as the matrix for the pyrolysis reactions and an immature crude oil was selected as the organic substrate. In addition, reaction conditions were chosen to facilitate reactions im-olving free radical mechanisms rather than those involving carbonium ions to further parallel the natural processes [11,12].

This paper reports the results of a study in which immature petroleum or petroleum fractions, in intimate mixtures with a range of crushed shale samples, were pyrolysed and the degree of maturation nas monitored as a function of time. Increasing the proton exchange capaciv of the sediment or the amount of elemental sulphur in the sample was found to enhance the factors which enhanced the rate of maturation, while the polar XSO fraction of the oil was found to retard the rate of maturation.

ESPERDIESTAL

Crude oil sample

All esperiments were performed using the Flinders Shoal crude oil, or various fractions of this oil, as the organic substrate. Details of its location, geological environment, and properties have been published 113:.

Rock samples

The rock samples used to prepare the mineral matrices for pyrolysis experiments were sediment samples recovered during drilling operations on the Rarrow Island Oil&Id in North Western &stmlia The samples were clay&ones, siltstones and shales of Cretaceous age.

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Each sample was air dried and crushed to a particle sire of 0.10 mm_ The soluble organic matter was removed by eshaustive extraction with me-Ayl- ene chloridmethanol (10 : 1). Rlank pyrolysis experiments, without -&e organic substrate, were carried out to determine the extent of generation from kerogen of additional soluble organic material_ Only those samples producing insignificant amounts of soluble organic matter af-ier pyrolysis were used.

q.-mlpis ScTnples

Samples used for pyrolysis were prepared by adding a solu5on of the organic substrate plus 0.5% (w!w) elemental sulphur in J;-pentie to the &J- mineral matrix. The pentane was removed on a ro+%ry aaporator to produce a -uniform coating of the organics on the mind surfaces. The rario of organic substrate to mineral matrix was I : 100 in 211 esperimexs, Th% mixture was placed in thick-walled glass ampouk, cookd with Eqxuid nitiogen, aacuated for 4 h (ca. IO’= Torr) and se&d wEh a g&s-blowing torch. Q-rolysis samples were placed in a protective steel rube and hea+& in a furnace at 3OO’C for varying times_

Extmction end separation of stunpies

After pyrolysis, samples were estracted with methylene chloride-merha- no1 (10 : 1) and the recovered soltable organic mat+zr subjected to chzoma- tography on silicic acid. Eluting the column with n+en&ne, &en n%h 6ieshy-l ether-n-pentane (1 : 1) and finally with methanol-methylene chloride (50 : 1) provided saturate, aromatic, and SSO frzctiors respecrirely [ 6 ] _

Proton mcgnetic resonwzce spectroscopy

The aromatic dictions were analysed using a high resolution, 60 M3z, Hitachi ?erkin-Elmer R20B proton magnetic resonance spectrometer. _. spectra were recorded using carbon tetrachloride solvent and Losford yzci- sion SMR tubes.

Prepam tion of ccidic shale

The air&&d, crushed, and sokent-estra cted sediment vms conrinuously stirred in a beaker with a volume of buffer (acetate-acetic acid, pH 4.05) equal to twice the weight of sediment. After settling (20 min) the superna- tent liquid was decanted off and the solid residue was washed repeatedly with deionised water until the washings were free of buffer. The sample was then stirred (24 h) with a 25% (w/w) aqueous slurry of cation -change resin in the H’ form. The misture was wet sieved (0.5 mm sieve) to remove the exchange resin from the sediment and then dried (60°C).

6 2

M

r

.=. ~

= I E

."~1 C"~. C'~. C<I

"~. ,-~ ~. =. ~

!

l

!

I ! . - - =

= " " ' " ' ~ E

j : == , ~

= , :=='~ L - -

K

RESULT6 XSD DISCUSSION

Table 1 contains details of the properties of the rock samples used as a

63

matrix for the pyrolysis reactions_ These samples were also used as the starb ing material in experiments where the exchangeable cations swe varied- In all expeximents the Flinders Shoal crude oil, or various fractions of this oil, was used as the organic sub&ate_ The crude petroleum or petroleum frac- tion was mised intimately with the mineral matrix in a ratio of 1 : 100, sealed in an evacuated glass ampoule and heated at 3OO=C for the specified time. AfM pyrolysis, the organics were estzacted with mezhylene chloride methanol (10 : l), the aromatic k&ion was isolated by column chromatog- rephy on silicic acid, and the PAP due was determined by PMR spectros- copy.

The characteristic curve obtained when an immature petroleum sample is pyrolysed at 300sC is shown in Fig. 1, The plot of PAP value against time shows an initial rapid change eventually levelling out and showing little change upon further heating. Both the initial slope of the curve and the limiting value change with variation in properties of the inorganic makix and with changes in the type of organic material. Khen blank esperimen+s which did not contain the mineral matxis were conducted under othme identical conditions, unchanged petroleum was recowred. This shows that the pres-

Fig. 1. Plot of percentage of aromatic protons in the axomatic famion (PAP) against rime for pyrolpis of z&r crude oil on a shale matxis (Sample So_ 4. Table l)_

Eg. 2_ Distribution of the gaseous hydrocarbons obtained from pyrolysis of the crude oil on a shale matrix (Sample So. 4. Table I ); (A) no added sulphur, (B) 0.5% (a.+) of ele- mental sulphur added.

ence of the mineral matris is essentiai for the reaction to proceed at these te_mperatures.

The mechanism of the reaction was investigated by ana@sing the saturated gaseous products of the pyrolysis reacCon. It has been shown that the pres- ence of branched chain akanes is evidence of a mechankm involving carboni- urn ions while the absence of such compounds suggests a mechanism invok- ing free radicaI processes [11,12 J_ The distribution of gaseous products ob- tained from the ana&sis of two pyrolysis experiments is shown in Fig_ 2: Sample B was the same as SampIe .A escept that 0.5% (w,%-) elemental suI- phur was included in Sample B. From these results it can be seen that the addition of elemental sulphur to the mineral matris gives rise to enhanced levels of straight chain alkanes in the products and we infer that it also encourages the reactions to proceed by a free radical mechanism_ Since free radical processes are believed to predominate in petroleum forming processes under geological conditions [ll] all other experiments in this study were

65

Fig. 3. Plots of percentage of aromatic protons in the aromatic frzaion (PAP) ag&sa time for pyrol>%is of the crude oil on shale matrices without &Ced prozou. shale ma- trices 1 (I), 2 (A) and 3 (0). ad with a proton exchange mauis. samples IA [z). 2-a (-I-) and 3A (Z). together with plots of tie percentage of soh22e organic zzmtter [SOM) recovered after pyrolyti

condxted with elemental sulphur (O-6% TIF~W) added to the mined matrix_ Tko sets of experiments showed that both the tit&l xate of maturation

and the limitig lxlue of the curve are affected by the proton exchange capacity of the rock matris. Firstly, an increase in the rate of maturation of aromatic compounds, as judged by PAP values, was obsen-ed with three different. minezal matrices when matrix cations were replsed with protms

prior to pyrolysis. The resulti of these esperiments are shown in Fig. 3 where the maturation profiles for tie three acidic mat&es are compared with those obtained tirn the original non-acidic matices. Secondly, a series of samples with increa~9gproportions of eschangeabie protonsxas prepare5 fiorn one sediment sample (Sample Xo. 1, Table 1). The maturation pro-

I 1 I I I

-7 . _. -_

.._. _.

Fig. 4. Ploxs of percentage of aromatic protons in the arolratic fraction (PAP) agskst

time for p57olysis of rile cxde oil on a &ale marrir wirh increasing proton exchar?ge capacity. ShA sample 1 with increasing proportions of acidified shale (IX); (m) ms:ris of l-4 only. (I) msrix of 1 and 1-A in the ratio 1 : 2. (=) matrix of 1 and 1A in tke

ratio 3 : 1.

Nes obtained using these samples are shown in Fig. 4; it is apparent that as the proportion of acid clay in the matrix is increased, both the maturation rate and limiting value increase_

The catalytic effects of acidic clays in catalytic reforming and other simi- lar processes are well known [la], and the catalytic role of clays in petro- leum genesis is also well recognised [15,16]. Recen+.ly it has been suggested that the Bronsted acidity of clays plays a part in the diagenesis of organic compounds in sediments [ 17 J. It is therefore not surprising that acid catal- ysis on clay surfaces facilitates reactions which result in an increase in the aromatic character of the system, Our curves of PAP versus time for wrious mistures differ in two wws: the limiting value of the curve and the initial slope of the curve.

There appear to be two likely reasons for the variation in the limiting values for the curws. Firstly, the limiting value may result from deactivation of the catdlyst by coking, and the different limiting values result from differ- ent catalysts coking at different rates. However, since the plots of recovered soluble organic matter which are included in Pig. 3 do not show a marked

SF

Discontinuity at the point where substantial change in the slope of the PAP versus time curves occurs, and the general shape of these curvy does not seem to approximati the exponential decay cun-e expected to result from a process such as coking, the limiting wlues are probably not due to s simple physical phenomenum such as catalyst poisoning by coEng_ The second, and we believe more likely, explanation is based upon the well-established fact that different catalysts can be give rise to different product mixtures from the same starting material [14]. Since the catalyst is the \xriable in our sys- tern,, we probab’g obtained different product mixtures and correspondingly different limiting PAP values in the pyrolyses.

The most probable proce~es responsible for the variation in the initial por- tion of the PAP versus time curves are those involving dehydrogenation, par- ticularly in view of the effect of sulphur which is discussed next in this paper,

The role of sulphur in promoting dehydrogenation rezcticns of q-do- alkyl wstems under mild thermal conditions to produce a range of aromatic ring systems is well-recognised [IS]. The results obtained from pyrolysing a

Fig_ 5. Plots of percentage of aromatic prozorrr in rhc aromatic fraction (PAP) againsi, time for ppolpi~ of the crude oil OE a shale matrix containing differenr concentrztionr; of elemental sulphur. Curves were obtained using smp!es prepztled from shle sample 1, without added sulphur (0). with 0.1% (a!~) added sulphur (z). with 03% (~;a) z.Cdeti sulphur (I)-

Fie. 6. Plou of percentage of aromatic protors in the aromatic fraction {PAP) agai_mf tizw for pyrolysis of t‘ne saturate (0. I) and aromatic (I , =) fractions of rhe crude oil ai-2 (o_ I) and without (r , Z) the heterocyclic fraction. Shale matrix So_ 4:.

series of samples with increasing proportions of elemental sulphur are shown in Fig. 5_ An increase in the initial rate is apparent in each case as the propor- tion of sulphur is increased. The effect is probably due to increasing ease of dehydrogenation as the level of sulphur in the system increases. Small dif- ferences in the limiting value of the curves are observed although these dif- ferences are much less than those observed with increased proton avaiIabil- ity_

The influence of the nature of the organic matter on the thermal matura- tion of aromatic compounds yl’iis investigated using a constant mineral matris (Sample So. 4, Table 1) and carrying out pyrolysis experiments with different fractions of the crude oil. The results obtained are shown in Fig. 6. When the aromatic fraction is pyrolysed in the absence of the polar XSO fraction a significantly greater rate of maturation is observed together with a higher limiting value. Similarly, when the saturate fraction is p~-rolysed in the absence of the SSO fraction the rate of increase in the PAP value of the aromatics produced during the reaction is significantly greater than when they are present, Again the presence of XSO compounds results in a decrease in the PAP of the limiting value of the curve. The reduced catalytic effect of clays in the presence of heterocyclic compounds is well known [14] and

appears to be due to the polar molecules being absorbed onto active sites on the clay surface in preference to the less polar organic substrates_ The ob- servation that the less polar saturates are more sensitive to the presence of the SSO fraction than the aromatic compounds is consistent with this inter- pretation.

The application of these results to the interpre-Ation of maturation effects on petroleum oc curring under geological conditions must be done with cau- tion- To take resulk and conclusions obtained from reactions which hare been studied at 3OO’C in the laboratory and infer from them processes which have taken place over a geological time scale and under less severe thermal conditions ixwolves a long and uncertain estrapolation- Furthermore the esperiments in the laboratory are deliberately designed to gi-e masirrum effects. The geological conditions are generally much less esrreme and *&e effects are therefore comparatively less prono~unced, Despite these Lmirations some worthwhile pointers may be obtained which can help one -unders*%nd the geological processes.

The effects of the presence of eschangeable protons on the mineral matris may be relevant to petroleum formation processes in early diagenesis and possibly during catagenesis. _-- recent reporr: of simulated geochemical rrans- formations of sterols by acid clays [17] indicates thar. acid ca-Ay_& is involved in early diagenesis. The obsemxion rhar. iso_meric C-S al>\-lbenzenes in petroleum are sometimes near their equilibrium concentrations [IS] sug- gests that protonated intermediates hare formed to allow isomer equilibra- tion: this suggests acid-lay effects 2t the catagenetlc stage. Tne occurrence of immature crude oils in carbonate rocks often appears anomalous in rerms of the more mature crude oils in elastic rocks which have been subjected to similar thermal conditions. These differences have been at+tibuted to the presence of clay minerals with surface-attire proper&s in the &szric emon- ment [15] _ The absence of significant proton actEQ-_ in the carborate matrix reIative to that of the elastic matrix couId be a s~gnifrcani factor in causing these differences.

The large effect of sulphur on the maturation of the aromatics ;n these esperiments suggests it probably also plays an important role in geological processes_ If the hydrogen required for the formation of lower carbon numbered alkanes by C--C bond cleavage is provided from increased aromati- sation of cyclic aTstems, the rate-determining process in petroleum forma- tion and maturation could be dehydrogenation. The presence of elemental sulphur, or compounds capable of producing elemental sulphur under cata- genetic conditions, would then determine the rate of formation and matum- tion.

The deactivating effects of the heterocyclic fraction on the catalytic prop- erties of the mineral matris suggests that source rocks rich in such com- pounds would require more severe thermal conditions for maturation tha those with a lower concentration of similar compounds. It is interesting 10 observe that heavy heterocyclic cornpour& are released as intermediates during pyrolysis of kerogen [9]_ If this process has a parallel in the formation of petroleum in sediments, it may result in substantial deacti\Mion of the clay catalyst. Organic matter released at an early stage may mature very

rapidly due to the active catalytic effects of the clay which is subsequently deactivated by the heavy heterocyclic compounds during the main phase of generation and maturation. This effect could esplain the obsen-ation that small quantities of mature petroleum are produced under milder thermal conditions than the main body of petroleum [2].

ACKSOWLEDGEMESTS

\Ve wish to thank West _-Iustralian Petroleum Ltd. for providing the sam- ples to carry out this work. Financial assis-tance from Xest AustAian Petro- leum Ltd.. Woodside Petroleum Development, The Australian Research Grants Committee. and The Sational Energy Research, Development and Demonstration Council is also grateful& acknowledged.

REFERESCES