137044473 aapg methods in exploration volume 9 douglas waples tsutomu machihara biomarkers for...

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Biomarkers for Geologists PracticalGuide to the pplication of Steranes andTriterpanes in Petroleum Geology

oDouglas W. WaplesTsutomu Machihara

Technology Research enterJapanN ational Oil orporationChiba Japan

AAPG M ethods in Explora tion No. 9

The American Association of Petroleum GeologistsTulsa Oklahoma U.S.A. 74101



FOREWORIn recent years, analysis of cyclic biomarker molecules has had a great

impact on petroleum geochemistry. The development of simple, relativelyinexpensive gas chrom atogra phy -ma ss spectrom etry gc-ms) systems hastaken biomarker technology out of specialized research laboratories andplaced it within the reach of many exploration geologists , who are nowfrequently asked to use biomarker data as one of their exploration tools. Atthe present time, however, few documents exist that can serve as guides forexplora t ion geologis ts seeking to in terpre t b iomarker da ta . This bookattem pts to fill that v oid.Our scope will be intentionally somewhat narrow, focusing only on thetwo group s of saturated cyclic com pou nds steranes and triterpanes) mostfrequently applied in exploration. These compounds contain three to s ix

r ing s and usu a l ly f rom 21 to 35 ca rbo n a tom s . Em pha s i s wi l l be oncommunicating those ideas about steranes and triterpanes that are generallyaccepted by biomarker specialists and that are most useful for exploration.The many biomarker problems currently of interest mainly to researcherswill not be discussed.Because we have written this book for exploration geologists rather thanas a review for researchers, the references are intentionally not completelycomprehens ive . No a t tempt has been made to t race sys temat ica l ly thehistory of biom arker science, nor to give special credit to the pioneers. Mo stof the references are inc luded primari ly to summarize the most recentthinking on the subject of biomarkers and to guide a reader inexperienced in

biomarker in terpre ta t ion to recent ly publ ished good sources of fur therinformation on a particular topic. Those interested in more detail about thechemistry of biomarkers can consult specialized sources, such as Philp1985), John s 1986), an d Petro v 1987), or articles cited he re and elsew here.Finally, we caution that the subject of biomarkers is considerably morecomplex th an this book indicates. In an effort to mak e bioma rker technologyaccessible to geologists, we h ave ch osen to generalize an d oversimplify a bitmore than most researchers would prefer. To present all possible variations,ramifications, exceptions, and uncertainties about biomarkers would defeatthe purpose of th is book, which is to provide a working handbook fornonspecialists. We ho pe th at specialists will not be too offended by so me of

our gene ra l i za t ions , and tha t nonspec ia l i s t s wi l l recogn ize tha t somestatements herein will have to be modified as knowledge of biomarkersimproves in the future.DouglasW WaplesTsutomu M achihara



Dou g Waples received his B.A. in chemistry from DeP auw University in 1967 and his Ph.D.in chemistry from Stanford in 1971. He beg an his career in petroleum geochem istry in 1971with a post-doctoral fellowship under Prof Dietrich Welte in Germ any followed by anotherresearch and teaching fellowship in Chile. He sp ent three years in research with Chevron fouryear s as a faculty m em ber at the Co lora do School of M ines four years in research an dexploration w ith Mobil and since 1983 has been an inde pen den t consu ltant. He is currentlyengaged in research on basin modeling with the Japan National Oil Corporation in ChibaJapan.Dr. Waples participate d as a shipb oard scientist on Legs 58 and 80 of the D eep Sea DrillingProject. He received the 1982 Sproule Award from the AAPG. He is the author or coauthor ofabout 60 publ ica t io ns inc ludin g two other books on petroleum geochem is try . His maininterests today are basin modeling exploration applications of petrole um geochem istry andtechnology transfer includin g teaching of semin ars.

Dr Tsutomu Machihara received his B.S. in chemistry from the University of Saitama in1975 and M.S. and D.Sc. degrees in organic geoc hemistry from Tokyo Metropolitan Universityin 1977 and 1981 respectively. Since 1982 he has worked at the Technology Research Center ofthe Japan National Oil Corporation. Dr. Machihara also participated as a shipboard scientiston Leg 87A of the Deep Sea Drilling Project. His main research interests include studies ofbiomarkers in bitumen s and oils s tudies of hydrocarbon generation by compaction pyrolysisof kerogen and stud ies of the chemical structure of kerogen hum ic substances and coal by avariety of chemical and geochemical techniques. Dr. Machihara is the author of about 35publications in the field of geochemistry.



h pter 1>

Chemical Structures and Nom enclatureThe basic structural building block of the biomark-ers is the isopren e un it (Figure 1A). The end closer tothe me thyl branc h is called the he ad /' and the otherend is the tail. Co mp oun ds formed biosyntheticallyfrom isoprene units are called isopreno ids. Two isoprene units joined head-to-tail (with minor modifications, such as hydrogenation of double bonds) form amon oterpan e (F igure IB). Two mo noterp anes ( fouri soprene uni t s ) l inked together form a d i terpane,wherea s six isoprene u nits can be joined either to forma s terane or a t r i t erpane, depending upon how thelinking is accom plished.Steranes usua l ly contain four rings, the D-ring ofwhich always contains five carbon atoms (Figure 2A).Triterpanes contain three to six rings (Figure 2B), withfive-ring species being m ost comm on. The E-ring usually contains five carbon atoms, as in hopan e in Figure2B ,but com pou nds having s ix carbon a tom s in theE-ring are also kno wn (e.g., gam mac erane in Figure 2B).During early diagenesis it appears that one or moreof the rings in steranes and triterpanes sometimes canbe opened by bacterial (?) act ivi ty (Peakman et al . ,

1986) . F igure 2B show s de- A- lupa ne, a t e t racycl ictriterpane with the A-ring destroyed.The preci se number of carbon a toms in a g ivencyclic biomarker varies considerably as a result ofdif-ferences in source material, effects of diagenesis andthermal matur i ty , and b iodegradat ion . Thus namesl ike t ri te rpa ne tel l us only that there are appro ximately 30 carbon atom s (3x10) in the comp oun d. Furthermore, the ring structures themselves usual ly donot account for all the carbon atoms. For example, afour-ring sterane con tains only 17 carbon atoms in thering structure ; the rem aining 10 to 13 carbon atom soccur in various groups or side chains attached to the

ring structure (Figure 2A).Each carbon atom in a biomarker molecule is numbered for easy reference. The numbering systems forsteranes and tri terpanes are shown in Figure 3. Thenumbering sys tem indicates where s ide chains areat tached to the ring system and w here small but s ignificant changes in molecular architecture occur. Forexam ple, the two triterpa nes in Figure 4 differ only bya s i n g l e m e t h y l g ro u p , p res en t i n co m p o u n d Aattache d to carbon atom n um ber 10, betw een the Aand B r ings (des ign ated hencefor th in th i s book as

C-10). In compo und B, in contrast , the methyl grouphas been replaced by a hydrogen atom. Compound Ais a me mb er of a c lass of com pou nd s cal led ho-pan es, and itself is called the C 3 0hop ane or oftensimply hopane.The carbon atom in the methyl group attached to C-10 bears the designation C-25 (see Figure 3B). In onenaming system, therefore, compound can be referredto as the 25-norhop ane, whe re the prefix nor me ans

that one methy l gro up is missing, and 25- indicateswhich methy l is absent. The prefix nor is only usedto refer to methyl groups.Another m ore general way to indicate the absenceof some grou p is to use the prefix de s. In this system ,however , we mus t ind icate the pos i t ion where themissing group should be attached to the carbon skeleton , ra ther than the number of the miss ing carbon,since the missing group is not always a methyl group.Thus the 25-norhopane in Figure 4 could also be called10-desmethylhopane. Note that the type of group thatis missing must be indicated using this system.The numbering system also indicates where stereo

chemical chang es occur. Stereoch emistry refers tothe spat ial relat ionship of atoms in a molecule. Thering systems of cyclic biomarkers often are reasonablyf la t , resembl ing a p iece of corrugated sheet metal .Wherever two rings are joined, each of the atoms atthe junction is attached to three other carbon a toms inthe ring stru ctu re (see Figure 2, 3, or 4). I ts fourthbond (usually to a hydr ogen atom or to the carbon of amethyl (CH 3- ) g roup) can poin t e i ther up or downwith respect to the plane of the rings. When b iomarkers t ructure s are dra wn on paper , the d i rect ions upand do wn are often referred to as out of the pag eand into the pag e, respectively.Substituents that point dow n are called alpha (a);those that point up are called beta (13). The differencebetwee n alpha an d beta stereochemistries is impo rtant,because the orientat ion of the subst i tuent at a ringjunction can greatly affect the molecular geometry, andhence the stability an d p roperties of the m olecule. Thetwo different ways in which alpha and beta orientations can be indicated are shown in Figure 5.In one sys tem, we indicate an upward d i rect ion(toward the viewer) by drawing the bond as a sol idwedge; i f the group points down, the bond is drawn



2 Waples and Ma chihara

^ ^A) ISOPRENE

B) MONOTERPANES

Figure 1Chemical structure for isoprene (A) andtwo ways of drawing a monoterpane (B) formedfrom two isoprene units. From Waples andMachihara (1990); reprinted with permission fromBulletin of Canadian Petroleum Geology

as a dashe d or dotte d line (Figure 6, left). (A plain line,as in Figures 2-4, indicates either that one is uncertainabout the stereochemistry, or that for the purposes ofthe illustration it doesn't matter.) This system can beused for any grou p attached to the ring system.The other system is only used to indicate the stereochem istry of hyd roge n atoms (Figure 5). In this sys

tem, a hydrogen atom in the alpha position (pointingdown) i s shown by an open c i rc le a t the poin t ofat tachment , whereas a hydrogen in the beta posi t ion(pointing up) is shown by a black circle. As Figure 5show s, the two system s are often m ixed.Stereochemistry can also be important at certainpositions in the molecule away from the ring structure.If four different substituen ts are attached to a particular carbon atom (for example, C-22 in the molecules inFigure 6), that atom is called an asym me tric or crural carbon atom. Because carbon atom s at ring junct ions are usual ly asym metr ic , b iomarkers general lyhave several chiral carbon atoms.Chemical com poun ds that differ only in the configuration of one or more of their chiral centers are calledstereoisome rs. Stereoisomers that are mirror imagesof each other (each chiral center is of oppos ite configuration in the two molecules) are called enan tiome rs,whereas s tereoisomers that are not mirror images ofeach other (one or more of the chiral centers are thesame in both molecules) are cal led diastere om ers.Petroleum geochemists general ly do not work withenantiom ers, but they do often use pairs or quartets ofdiastereomers.

ch o e s tane d i a c h o l e s t a n e

S T E R A N E S

G a m m a c e r - a nT R I T E R P A N E S

Figure 2C hemical structures and names for typical steranes (A) and triterpanes (B). Rings are ide ntified by letters. From W aples and M achihara (1990);reprinted with permission from ulletin ofCanadi-an Petroleum Geology

Where two diastereomers differ only in the configurat ion of a s ingle chiral center they are cal ledep im er s (Figure 6). Ma ny of the pa irs of cyclicbiomarkers s tudied by geochemists are epimers. Further d i scuss ion of these terms and concepts canbe found in any in t roductory tex tbook on organicchemistry.The configuration at any chiral center outside thering stru ctur e (Figure 6) is referred to as R or S(from the Latin wo rds rectus for right and sinisterfor left ). The terms R and S are used be cause the Rand S epim ers are mirror imag es, just as your right



Biomarkers for Geologists 3

STERANES

TRITERPANES

23 24

Figure 5 Structure of a typical steran e, the C27species 5a(H),14oc(H),17a(H)-cholestane, sh ow ingthe stereochemistry at each ring junct ion using thetwo systems. Ope n circles indicate alpha configurat ions (point ing dow n); wed ges and dark circles indicate beta configurat ions (point ing up). Stereochemistry at C-8 and C-9 is not designa ted in the n am ebecause hyd rogen s at those posi t ions are always inthe beta and alpha positions, respectively. See textfor further discussion.

Figure 3Num bering systems for s teranes (A) andtriterp ane s (B). Ad apte d from M acke nzie (1984);reprinted w ith permission of Academic Press.

C O M P O U N D AH o p a n e

3 0 C a r b o n a t o m s

C O M P O U N D B2 5 W o r h o p a n e


Figure 4Two hopane s that differ only by the absence of the methyl gro up (C-25) in com pound B. Compo undB can resul t (probably indirect ly) from seve re biodegrada t ion of com pound A. From Waples and Mac hihara(1990); reprinted with permission from Bulletin of Canadian Petroleum Geology



4 Waples and Mac hihara

l

r 2 2 R

w h e r e

f / / fw x - 1 1

E X T E N D E D H O P A N E SX = C H 3 , C 2 H 5 , C 3 H 7 , C 4 H g o r

r - X t / X ^ \ L i >

2 2 S

C 5 H 1 1

KX

Figure 6Structures of two diastereomeric 17a(H)-extended hopanes (designated 22R and 22S) which areinterconverted in a reversible reaction. These comp oun ds are also called hom ohop anes. This illustrationsho ws the stereochemistry at the ring junction s and at the chiral carbon atom C-22. From W aples andMachihara (1990); reprinted w ith p erm ission fromBulletin of Canadian Petroleum Geology

a pR and a p p S

and left hands are mirror images. The most importantcon figur at ion al difference s in R and S epim ers arethose at C-20 (20R and 20S) in steranes (Figure 7) andat C-22 (22R and 22S) in triterpanes containing morethan th irty car bon at om s (Figure 6). Each of thesegroups of compounds will be discussed in much moredetail in later ch apters.

Figure 7Reversible interconversion of 20R and20S steranes (epimers) and 5a(H),14a(H),17a(H) and5a(H)/14fJ(H),17P(H) steranes (diastereomers). Inone shorthand notation they are called a a a anda(3(3. The w avy lin e in the app structure at C-20indicates that the stereochem istry at that site isunspecified. From Mackenzie (1984); reprinted withpermission of Academic Press.



6 Waples and Machiha ra

H O H OC h o i e s t e r o l

2 7 C a r b o n a t o rE r g o s t e r o l


H O H OS i t o s t e r o l

2 9 C a r b o n s a t o m sC 3 0 S t e r o l

Figure 8Structures of som e important C 27to C30 sterols in photosynthetic organisms.

sal ine environments. The 3 3 structures are shown inFigure 7. Both the act and pp steran es are called regu lar steranes.In add ition to the regular stera nes, a family of rearr an g ed s t e ran es o r d i a s t e ra n es is co m m o n l yencountered (Figure 10). These compounds differ fromthe regular steranes by having methyl grou ps attachedto C-5 and C-14 instead of hydrogen atoms, and having hydrogens at tached to C-10 and C-13 instead ofmethyl groups. The transformation from regular steranes to diasteranes is believed to occur during diagen-esis under certain condit ions and during catagenesis(thermal maturation) in other cases. Diasteranes willbe discussed in more detail in Chapters 4 and 5.

Bes ides the regular s teranes , 4 -methyls teranes(steranes with an additional methyl group attached toC-4 in the A ring) have also been com monly reported .They appear to form two distinct families (Figure 11).One family, ca l led d ino s teran es , i s der ive d f romdinoflagellates (de Leeuw et al., 1983; Goodwin et al.,

1988). They exist only as the various stereoisomers ofthe C 30 homolog . A s F igure 11 show s , d inos teranescan be thought of as cholestane (the regularC7ster-ane) with three additional methyl groups, at positions4,23,and 24.The other family can be considered as the three regu lar s teranes (C 27 , C 28 , and C 29 from Figure 9, alsoknow n as choles tane, 24-methylcholes tane, and 24-ethylcholes tane) conta in ing an addi t ional methyl

group at C-4. They therefore form a homologous serieswith 28, 29, and 30 carbon atoms. In this book w e w illcall them the 4-me thylcholestane s. Figure 11 show sthe small difference in structure between the C 30 formsof the two famil ies . The 4-methylcholestanes are ofunce rtain o rigin (G oodw in et al., 1988).TRITERPANES

In cont ras t to the s teranes , which come f romsteroids in algae and higher plants (and to a lesser


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Biomarkers for Geo logist s 7

Figure 9Structures of C27-C 30 steranes derived from sterols. C27 = cholestane; C28= ergos tane o r 24-methylc-holestane; C29 = sitostane or 24-ethylcho lestane; C30= 24-propylcholestane. From Waples and Mach ihara(1990); rep rinte d w ith pe rmis sion from Bulletin of Canadian Petroleum Geology

Table 1. Various way s of referring to the Cstereochemistry at carbons 5,14, and 17 asNumber of Common-N ameCarbon Atoms System

27 Cholestane28 Ergostane29 Sitostane

27' ~ifya n d ^29 regular steranes. For each name one could also designate thealpha a) or beta(f3).Substituted-CholestaneSystem

Cholestane24-Methylcholestane24-Ethylcholestane

AbbreviatedSystemCholestaneMethylcholestaneEthylcholestane

V A A /Figure 10Structure of the 20S epimer of the C 29rearranged sterane (a diasterane). Fi l led circle represents a hydrog en atom in the beta configuration-open circles represent hydrog ens in the alpha configurat ion. By draw ing the methyl grou ps at tachedto C-5 and C-14 as straigh t lines w e are not specifying wheth er they are in the alpha or beta po si t ion.Both oca an d PP forms exist, bu t m ixed a[3 or (3aforms are absen t or very rare. From W aples andMach ihara (1990); reprinted with pe rmission fromBulletin of Canadian Petroleum Geology



8 W a p l e s a n d M a c h i h a r a

Figure 11Structures of theaaocC30 20R) mem bers of the two fam ilies of 4-methylsteranes. From Waples andMachihara 1990); reprinted with perm ission from Bulletin of Canadian Petroleum Geology

Transformation of triterpenoid s to triterpanes pro bably occurs along much the same lines as does transformation of sterols to steranes, although recent evidence f rom carbon- i so tope ra t ios of ind iv idualcompounds sugges t s that t ransformat ions in sed i ments are extremely complex (Freeman et al., 1990).As with steranes, the general m olecular architecture oftriterpanes is usually little affected by diagenesis. Thefirst important stereochemical transformation that weneed be concerned with is the formation during veryearly diagenesis of 17a(H),21(3(H) isomers. Thisgeometry, which is particularly stable, has the hydrogen attached to C-17 in the alpha configuration, andthe hydrogen at C-21 in the beta configuration (Figure12).Ho pan es with the 17p(H ),2ip(H) configurat ion(PP hopanes) are present only in very immature samples and, like some very im matu re steranes, are therefore unim porta nt in petroleum geochem istry. They aretherefore not discussed further in this book.

Triterpanes can be divided into three distinct families based on the number of rings. The most commonand mos t thoroughly s tudied t r i t erpanes have f iverings (e.g., Figure 12), and a re therefore called pen ta-cyclics. Most of these com pou nds contain from 27 to35 carbon atom s, al tho ugh they have been r epor tedup to C 40 (Rullkotter and Philp,1981;Farrimond et al.,1990). A less comm on and more poor ly unders toodgroup of t r i t erpanes , the t r i cycl ics , has only threerings (e.g., Figure 2B). They range from about 21 tomore than 40 carbon atoms, but those with fewer than25 carbon atoms are dominant. The third family, thete t racycl ics , i s the l eas t s tudied and mos t poor lyunderstood family.

Figure 12Structure of 17oc H),2ip H) C 30hopane.

degree from animals), the source organisms for mosttriterpane biom arkers are believed to be bacteria. Various t r i t erpenoids conta in ing such features as -OHgroups and double bonds have been characterized asimportant constituents of cell membranes in bacteria.A wide variety of triterpenoids is probably producedamong the many types of microorganisms present indifferent deposi t ional environments, al though manydetails are still missing. In particular, there may be sign i f icant d i f ferences between aerobic bacter ia andanae robes , especia l ly me than oge ns . A rev iew byOurisson et al. (1984) provides a very rea dable discussion of the bacterial origin and importance of triterpenoids.




Figure 13Structure of the C 31 20R) moretane.

The pentacy cl ics often are divid ed into ho pa n-oids and nonh opan oids . The hopan oids includeboth the 17oc(H),2ip(H) hopanes (simply cal ledhopanes : Figs. 4, 6, and 12) and the 17B,(H),21a(H)hopanes (called moretanes: Figure 13).The most common pentacycl ic t ri terpanes are thehopanes. The hopanes most frequently analyzed contain 27 to 35 carbon a tom s and form a hom olog ousseries with the 17a(H),21(3(H) configuration (Figure12).They are comm only referred to as both hop ane san d 1 7 a (H ) -h o p a n es . As n o t ed p rev i o u s l y , t h e30-carbon member shown in Figures 2B, 4 (left), and12 is also often called simp ly ho pa ne .Each homolog differs from the next by a s ingle- C H 2 - group attached to the side-chain on the E-ring.

m

The C 29 and C3() 17a(H)-hopanes have no chiral carbon atom s in their side chains (Figure 12). The C ^- Q ,,17ot(H)-hopanes (often referred to as hom oho pan esor exten ded hop ane s ) , howe ver, al l hav e a s inglechiral carbon atom (C-22) in the side chain, and thuscan exist as both the 22R and 22S epimer s (Figure 6).Because all biologically produced hopane precursorsexis t on ly in the R form, new ly formed exten dedhop ane s in sediments all have the 22R configuration.A pair of C 27 hop anes (17a(H)-22 ,29 ,30- t r isnor-hopane and 18a(H)-22,29,30-trisnorneohopane, commonly called Tm and Ts, respectively) are also presentin virtually all sam ples (Figure 14). Tm is believed torepresen t the biologically produ ced structure; Ts is generated in sediments and rocks by diagenetic or thermalprocesses, or both. In the pas t Ts was bel ieved to beformed from Tm, but that conclusion has not been verified. The source forTs thus remains unknown.Recent ly i t has been shown that a homologousseries of moretanes containing 29 to at least 35 Carbonatoms exists , just l ike the series of 17a(H)-hopanes(Larcher et al., 1987; Kvenvolden and Simoneit, 1990).The paleoenvironmental significance of moretanes isnot yet fully understood. They may have a microbialorigin like the hopanes, but at least some are thoughtto be der ived f rom hig her p la n ts (Rul lkot ter an dMarzi,1988;Raman amp isoa et al., 1990).It is not unusual to encounter a few other pentacyclic triterpanes in significant concentrations in oilsand rock extracts, although the structures of most ofthese molecules have not been determ ined . Am ong

those that have been identified are two hopanes (28,30-bisnorhopane and 25,28,30-trisnorhopane), and several

Figure 14Structures of Tm an d Ts.



10 Waples and Machiha ra

2 8 3 0 - B i s n o r h o p a n e G a m m a c e r a n e

2 5 2 8 3 0 - T r i s n o r h o p a n e 18a H)0leananeFigure 15Structures of 28,30-bisnorhopa ne, 25,28,30-trisnorhopane, gam macerane, and 18oc H)-oleanane.From Waples and Machihara 1990); reprinted with pe rm ission from Bulletin of Canadian Petroleum Geology

nonhopanoids (gammacerane and a fami ly of compou nds called oleananes). Structures are show n in Figure 15 . The o leanan es are thou ght to come f romangiosperms (terrestrial plants); the other compoundsin Figure 15 are all believed to come from microorganisms. Further discussion of the paleoenvironmentalsignificance of these compounds is found in Chapter 5.The tricyclic and tetracyclic triterpanes (e.g., Figure2) do not appear to be degraded pentacycl ics , bu tinstead appe ar to be mem bers of separa te genetic fam

ilies. They are probably ei ther generated in smallerquantities by the same bacteria that produce the pentacyclics, or by other species of microorganisms thatsynthesize them instead of the pentacyclics. However,Phi lp (1985) has sugges ted that t r i cycl ics may beformed by partial aerobic oxidation of bacterial membranes. If he is correct, then their abundance in rocksand oi ls may be related more to diagenetic factorsthan to d i rect b iosynthe t ic prod uct i on by specif icorganisms.



h pter 3

Analytical ProceduresSAMPLE PROCESSING

Samples for b iomarker analys i s can come f romman y sources: well cut t ings, s idewall cores, conventional cores, outcrop material, produc ed or tested oils,solidified bitum ens, tars, or dead-oil stains. Max imumcare should always be taken, part icularly with rocksamples, to avoid contamination by fuels, lubricants,or other petroleum-based products. Because even conventional cores can be stained by mud additives, samples should be taken from the interior of the core tominimize problems.Because b iomarkers normal ly are not present inlighter distillation fractions such as gasoline or diesel,d iesel contaminat ion presents no problem for b iomarker analysis . However, i f crude oi ls are used asdr i l l ing-mud addi t ives , removal of contaminat ionprior to analysis of rock samples is both crucial anddifficult. Coals (indigenous or in the form of lignosul-fonate mu d addit ives) or gi lsonite addit ives couldalso cause severe contam ination prob lems if present ineven minor quantities in cuttings samples because oftheir high organic-carbon contents and high concentrations of biomarkers.

Th e m i n i m u m am o u n t o f s am p l e r eq u i r ed forb iomarker analys i s i s h ighly var iab le , because i tdepends direct ly on organic richness. Rock-Eval Sjdata (see, for example, Tissot and Welte, 1984) can beused as a roug h guid e, because the Sj yield is approximately equivalent to the quantity of hydrocarbons inthe solvent-extractable material. Because a minimumof 50 mg of hydro carbon s is normally required to permit gas chromatography-mass spectrometry (gc-ms)analysis, the quantity of samp le required is that whichwould give 50 mg of Sj. For example, if the S t valuefor a given rock sample is 2.0 mg/g rock, a minimumof about 25 g wo uld have to be extracted to pe rformbiomarker analyses.

Rock samples are air dried, crushed, and extractedwith an organic solvent such as dichloromethane. Thesoluble material (extract) is then recovered by evaporation of the solvent. From this point onward extractsand oi ls (including tars and o ther sol idified organicsubs tances) are handled in the same manner . Theasphal tenes are remo ved f rom the ex tract or pet roleum by precipitation w ith a light solvent such as pen-

tane. The asphaltene-free material is then separa ted,using liquid chrom atography, into fractions consistingof sa turated hydrocarbons , aromat ic hydrocarbons ,and polar (NSO) compounds . Al though some l igh tcompounds (up to about C15) are lost during evaporation of the solvent in this step and the previous one,steranes and triterpanes are unaffected by evaporativeloss.The saturated-hydrocarbon fract ion commonly istreated further with molecular s ieves to remove then-alk anes , whic h migh t in terfere wi th s ubse que ntanalysis of the steranes and tri terpanes. The rem aining fraction contains the branched and cyclic alkanes,including the steranes and triterpanes.

GC-MS ANALYSISA typical gc-ms system used for analysis of bioma rkers cons i s t s of a capi l l ary gas ch rom atog raphconne cted to a mass spect rom eter and a com pute rwo rk station (Figure 16). The mixture to be ana lyzedis injected into the gas chromatog raph, w here the vari

ous compounds are separated according to the speedat which they move through the gas-chromatographiccolumn. Separat ion of the saturated hydrocarbons isachieved pr imar i ly accord ing to molecular weightand vola t i l i ty , a l though molecular shape may a l soplay a role. The separated compounds leave the gaschromatograph in sequence and enter the mass spectrometer ' s ion chamber, where they are analyzed inthe same sequence.Each com poun d entering the mass spectrometer isbombarded with a high-energy electron beam that ionizes the molecules by knocking off one electron. Themolecular ions formed in this manner are unstable,

how ever; most break ap art to give a variety of smallerfragment ion s. The molecular and fragm ent ions produced in this ma nner differ in mass, but most bear a +1charge. Because of the differences in their mass/chargeratios (m/z; also sometimes called m/e) caused by thedifferences in mass, they can be separated by a magnet ic field or a qua drupo le. The separated ions movesequential ly to the detector where the relat ive abundances of each mas s are recorded. The complete recordof the quantities and masses of all ions produ ced froma com poun d is called its mass spectrum.

11



12 Waples and Mach ihara

GAS CHROMATOGRAPH( G OS \

HSinp OINJECTOR COLUMN

(SEPARATION)

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The various classes of biomarkers all fragment incharacteristic ways in the mass spectrometer, depending upon their molecular structures. For example, thedominant f ragmenta t ion pa t te rns for s te ranes andtriterpanes are sho wn in Figure 17. Nearly all steranemolecules (except 4-methylsteranes, discussed later)will yield a large amount of the fragment ion with amass of 217 daltons ( m /z 217). Similarly, triterpane s(except bisnorlupanes and 25-norhopanes, also discussed later) will generally yield large quantities ofthe m/ z 191 fragment ion. Both types of co mp oun dsyield many other fragments in lesser quanti t ies aswell.

In practice, however, we normally do not record theent i re mass spec t rum for individua l compounds.Instead, we monitor each compound coming out ofthe gas chromatograph to see if it gives the ions character ist ic of the most common types of biomarkers:e.g., m /z 217 for steranes or m /z 191 for triterpa nes.Normally we scan each compou nd emerging from thegas chromato graph for the presence of several preselected fragmen t ions. This process is called selectedion monitoring, single ion monitoring, or SIM.SIM is the key to using g c-ms effectively in pe troleu mgeochemistry, because it allows u s to classify q uickly alarge numb er of different m olecules in each sam ple.

The outp ut from an SIM analysis is called a mas schroma togram or mass f ragmentogram. Severa ldifferent ions can be monitored for each sample. Thefollowing generalizations serve as the basis for mostof our interpretations of SIM data: m / z 217 mass chro-matog rams show steranes (Figure 17B), whereas m / z191 mass chro ma tog ram s show tr i terpane s (Figure17C). Of course, other types of compounds can alsogive small amo unts of m /z 217 or m /z 191, and thuswill appear as minor peaks in these mass fragmen-tograms. In most cases, therefore, we only interpretthe data for the major peaks in each mass fragmentogram.

Othe r fragment ions besides m /z 217 and 191 canalso be valuable. The m / z 177 mass chrom atogram isuseful for looking for an important class of triterpanesthat have lost the methyl group attached to the A/Bring junction. Examples include 25-norhopane, show nin Figure 4, and the bisnorlupanes. The m /z 231 masschromatogram can be used to search for 4-methylsteranes, since the dom inan t fragment ion is now 14 daltons larger than the typical sterane fragment. Thesecompounds are covered in more detail in Chapter 6.As discussed next, the m/ z 218 mass chromatogramcan be a useful alternative to the m/ z 217 in looking atthe regular steranes.



BiomarkersforG eologists 3

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Figure17Examplesofa total-ion-current trace A) and ma ss chromatogram s for steranes B) and triterpanesC). Dom inant fragmentation patterns for steranes and triterpanes are also sho wn . The lower-left po rtionofthe molecule yiel ds the charged fragment ioninboth cases. The m /z 217 and m/z 191 mass chromatogramsused for analyzing steranes and triterpanes, respectively, are chosen because they represent the most abundantion from each typeofmolecule. From W aples and Machihara 1990); reprinted with perm ission from ulletinof Canadian Petroleum Geology.

Although compounds withinasingle class (suchassteranes) all have similar m ass-spectral c haracteristics,the actual probability (or ease)offorming any particular fragment ion forexample,m/ z217) willbediffer

entforevery indiv idual com pound in theclass. Co nsequently,thedis t r ibu t ionofpeaks observedinanymass chromatogram is not anabsolute indicat ionofthe t rue re la t ive concent ra t ionsofthec o m p o u n d s .




B100.0 R MA TUR E, CRETACEOUS CRUDE OILSG

Figure 18M ass chromatogram of a mature oil show ing the comb ined s ignals from the m/z 217,218, and 259fragment ion s, representative of steranes. See Table 2 for identifications of p eaks indicated w ith letters. Thistype of data handlin g is claim ed to give a more realistic picture of the actual abundances of the various typesof steranes. The relative peak heights are not directly comparable to those from chromatograms for single fragment io ns, and can only be compared with other chromatograms of the same typ e. This particular sample lacksdiasteranes. From Grantham 1986b); reprinted with perm ission of Pergamon Press PLC.

Furthermore, the distribution of, for example, regularsteranes obtained from the m /z 217 fragm entogramwi l l be s l igh t ly d i f feren t f rom the d i s t r ib u t ionobtained from the m /z 218 fragmentogram.Therefore , i f one wishes to obta in quant i t a t iveinformation about the relative concentrations of individual comp ounds, one must (a) always use the same

mass fragm entogram (i.e., do not mix data from m / z217 and m /z 218), and (b) establish res pon se factorsfor each compound for that part icular fragment ionand use them to correct the observed ion intensities.Furthermo re, if one wishes to compare a bsolute concentrat ions from sam ple to sample one m ust includean internal standard.For mos t appl icat ions , however , we s imply takeratios of observed peak intensities from a single fragmentogram. These rat ios may not represent the t ruerelat ive concentrat ions, but they wil l be comparablefrom sa mple to sample. True quanti tat ive t reatmen ts

are rather unusual in most applications today.The contribut ions from al l fragment ions can besummed to show al l the material emerging from theg as ch ro m at o g rap h . Th i s t o t a l - io n -cu r ren t t r ace(TIC) looks very much l ike a normal gas chromatogram (Figure 17A).Grantham (1986a,b) has used a technique for displaying sterane da ta that combines SIM and TIC. Hesum s three fragment ions (m /z 217,218, and 259) thatare characteris t ic of the three types of s teranes a aregular stera nes, p*P regu lar steranes, an d diasterane s,respect ively) to give a s ingle chromatogram, cal ledm /z (217 + 218 + 259). An ex ample is show n in Figure18.Granth am feels that this display is the most objective way of describing the true distribution of steranesi n a s am p l e . Ho w ev er , q u an t i t a t i v e d a t a o b t a i n edfrom such a mass chromatogram cannot be compareddirectly against the standard forms of data using individual fragment ions, such as m /z 217.




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Figure 22Series of m/z 217 (sterane) mass chro-matograms showing the gradual conversion of 20Rsteranes to 20S andocasteranes to PP from leastmature (top) to most m ature (bottom). Identities ofpeak s are given in Table 2.

C2H , or C 29 s teranes. In pract ice, however, we deriveour most-accurate data from the C 29species (peaks Eand H) , which are l eas t suscept ib le to over lappingpeaks in the mass chromatograms. The main problems wi th 20S/ (20R+20S) measurements occur insamples in which all sterane conc entrations are low orin which the C 2 9 s teranes are scarce. In such cases,basel ine noise or the presence of coelut ing components can introduce unacceptable error.Al though mos t workers assume that the in i t i a l20S/(20R+20S) ratio at the start of therm al matu rationis 0.0, there may be minor variat ions in the ini t ial20S/(20R+20S) rat io caused by early-diagenetic processes (Moldow an et al., 1986; Peak man and Maxw ell,1988), or by l i tholo gic differenc es such as th osebetween coals and shales (Strachan et al., 1989). Thisphenomenon could in some cases introduce error intom at u r i t y d e t e rm i n a t i o n s m ad e u s i n g t h e 2 0 S /

(20R+20S) ratio, particularly at low degrees of transforma tion of 20R to 20S.Figure 24 shows a plot of 20S/(20S+20R) versusdepth for the same well represented by the five samples in Figure23.There is considerably m ore scatter inthe data from the shallower, less-mature samples thanin the deeper ones, perhaps as the result of diageneticvariations or analytical error.There are a number of other possible causes fore r ro r s in b i o m a rk e r p a r am et e r s , i n c l u d i n g t h e20S/(20R+20S) sterane ratios. Natural contaminationsometimes can occur, although often it is easy to recognize. Figure 25 shows steranes from five samp les in

the Higashi Niigata NS-6 well in the Niigata basin, inwhich the shal lowest sample shows an anomalouslyhigh sterane maturi ty compared with the other samples . The mos t common explanat ion for such ananomalous maturi ty would be ei ther oi l s taining orcontamination by some kind of drilling additive containing m ature steranes. In this case, howev er, lack ofevidence for staining or contamination suggested thatthe shal lowes t sample conta ined main ly reworkedo rg an i c m a t t e r e ro d ed f ro m d eep er , o l d e r , m o re -mature rocks. Figure 26 shows the depth trend of the20S/(20S+20R) sterane ratios for this well.In samples containing large amoun ts of the t ri ter-

pane 28,30-bisnorhopane, quantification of the oca-20Sform of the C 29 s terane may be difficul t by SIM,because the two compounds coelute, and because bis-norhopa ne gives a minor m /z 217 peak in addit ion to

Figure 23M/z 217 (sterane) mass chromatograms of five rock extracts from the Nosh iro G S-1 well in theAkita basin of Japan. Sam ple d epths are given in m eters. Formation nam es and ages are: SB = Shibika wa (latePleistocene); SA = Sasaoka (late Pleistocene); U.T. = upper T entokuji (early Pleistocen e); L.T. = lower T entokuji(Pliocene); and FU = Funakawa (middle Miocene-Pliocene). See Table 2 for peak identifications. The poorerresolution here compared with the mass chromatograms in Figure 22 is due to poor separation on the gas-chro-matograph colum n.



BiomarkersforGeologis ts 21

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22 Waples and MachiharaTable 2. Identities of common steranes in

Number ofDesignation Carbon Atoms

A 27B 27C 27Q 28R 28S 28D 28E 29F 29G 29H 29I 30J 30K 30L 30

M 27N 27O 29P 29"Stereochemistry of hydrogenatomsat positions1

the m/z 217 mass chromatograms in most of the illustrations in this book.Ring Side-chainStereochemistry* Stereochemistry

Regular Steranesa a 20SPP 20R+20Sa a 20Ra a 20Spp 20RPP 20Sa a 20Raa 20SPP 20Rpp 20Saa 20Ra a 20SPP 20RPP 20Sa a 20R

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Figure 24Plot of 20S/(20S+20R) for C 29regularsteranes in rock extracts in the Nosh iro G S-1 we ll,Akita b asin, Japan. See Figure 23 for mass chromatograms of selected samples. Data scatter isworse for low-maturity sam ples, where diageneticeffects and analytical errors may be m ore important.Formation abbreviations are explained in the caption for Figure 23.

i t s dom ina nt m / z 191 f ragmen t ion (Dahl , 1987) .Using SMIM one could easily distinguish betw een thetwo different sources for the m/ z 217 peak.The work of a numb er of autho rs (e.g., Mackenzieand M cKen zie, 1983; Ma ckenz ie, 1984; Rullkotter andMarzi, 1988) indicates that sterane epimeriza tion doesnot occur at the same rate as does kerogen maturation(e.g., vitrinite reflectance), nor does it precisely paralleloil generation. Grantham (1986b) concurs, presentingevidence that the role of time is significant in steraneisomerization. Th us the application of 20S/(20R+20S)ratios to estimate either kerogen m aturation or oil generation is only approximate. Nevertheless, it has beenpopu lar to attempt such a correlation.Figure 27 shows trends of the 20S/(20R+20S) ratiofor the regularCgsteranes ve rsus vitrinite reflectance(%Ro) for samples from three different studies. Thedata on which the line of Zum berge (cited in Bein andSofer, 1987) is based were not specified. Bein and Sofer(1987) used Zumberge ' s l ine for samples wi th20S/(20R+20S) ratios up to 0.52, but their preference forthe equilibrium ratio w as not given. We therefore havenot ex tende d their line beyon d 20S/(20R+20S) = 0.52.

The other two lines, interpreted by us from limiteddata sets, generally agree rather well with Zumberge's



Figure 25M/z 217 (sterane) mass chromatograms for five rock extracts from the Higashi Niigata NS-6 well,Niigata basin, Japan. Sample dep ths are given in meters . Formation names an d ages are: Ny = Nish iyama(Pliocene-Pleistocene); Sy = Shiiya (late Miocene-Pliocene); uTd = uppe r Teradom ari (middle-late M iocene);lTd = lower Teradomari (m iddle Miocene); and N t = Na natan i (early-middle Miocene). The shal low est sa mp leshow s the highest biom arker m aturi ty, proba bly as a resul t of the presence of rewo rked o rganic matter erodedfrom older rocks. See text for further discussion, and Table 2 for peak identifications. As in Figure 23 ,the poorresolut ion is due to the qual i ty of the gas-chromatograph colum n. From Om okaw a an d Mac hihara (1984).



24 Waples and Mac hihara10 20 30 40 50 (%)

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Figure 27Plot of 20S/(20R+20S) for C 29regularsteranes versu s vitrinite reflectance (% Ro) for threedata sets. (A): Deriv ed from an equation attributedto Zumberge by B ein and S ofer (1987). The data setused in deriving the line was unspecified. (B): Interpolated by us from m easured data of Goodarzi et al.(1989) for the Triassic Schei Point Formation, Sver-drup ba sin, Canadian Arctic. (C): Interpolated b y u sfrom measu red data of Sakata et al. (1987) for N eo-gene rocks of the Niigata Basin, Japan. Adaptedfrom Waples and Machihara (1990); reprinted w ithpermission from theBulletin of Canadian PetroleumGeology.

(1) developm ent of a local Ro-sterane relationship th atis already calibrated for the rocks of interest, and (2)use of a kinetic model for sterane epimerization (discussed later in this chapter).p p / aa ratios

Another maturity parameter derived from steranesis the pr op ort ion of 14(3(H),17P(H) an d 14



26 WaplesandMachihara

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(1986b) for low levels of sterane and moretane isomer-ization in young (Tertiary) samples support the viewof Mackenzie.However, the problem of lack of universal validityis intrinsic to all kerogen-ma turi ty p aram eters wh enthey are applied to predict ing hydrocarbon generation, and is probably best viewed as a complication inbiomarker in terpreta t ion ra ther than as a completeconde mna tion of the technology. Because of these limi tat io ns, the m atu ra t io n l ines in Figures 27 and 30should be used with part icular caut ion in est imatingm at u r i t i e s wh erev e r t i m e o r t em p era t u res a re ex treme. A kinet ic approach, in which calculated biomarker ra t ios are compared wi th measured data totest proposed burial and thermal histories , is real lypreferable to dia gram s like Figures 27 and 30.

Several cr i t i c i sms of the use of b iomarkers asmaturi ty indicators that we discussed earl ier in thischapter include contaminat ion, reworking, possiblemigrat ion effects , diagenesis , analyt ical error, lowconcentrat ions, and coelut ion. Other potent ial problems a l so ex is t . For example , Tannenbaum et a l .(1986a) pointed out tha t the presence of mineral catalys t s , par t i cu lar ly smect i t e , can affect the ra te ofb i o m ark e r t r an s fo rm a t i o n s i n l ab o ra t o ry h ea t i n gexperiments . However, because the effects of suchcatalysts in natur e are likely to be quite different fromthose in the laboratory, laboratory experiments maynot be relevant to modeling natural hyd rocarbon generat ion . We therefore should base our models forb iomarker matur i ty parameters more on empi r icaldata from natural geological situations than on datafrom laboratory simulations.

An o m al o u s m a t u r i t y b eh av i o r o f b i o m ark e r s i swell documented in natural samples. Figures 25 (top)and 26 showed an example of natural contaminat ionof s teranes by mature biomarkers in reworked sedimentary organic mater ia l . Other examples of suchcontam inat ion w ere ci ted by Farrim ond et al . (1988,1989,1990), who noted that many of the organic-leansamples analyzed in studies of the Toarcian and Ceno-manian-Turonian anoxic events in Europe showedanomalously high hopane maturities. They concludedthat the hopanes in the lean samples were dominatedby reworked mater ia l , whereas in the r i ch samplesindigenous h opan es were dom inant . Rullkot ter et al .(1986) noted that various maturity parameters used ina study of oils in the Michigan basin gave discrepant

resul ts , and at t ributed this problem to mixing. How ever, it is quite possible that they w ere simply observing a variety of facies-caused differences in parameters that at that time were still believed to be purelymaturity controlled.Figure 39 shows retardation of biomarker transformations in an organically lean carbonate rock. The twosamples show n are from the same w ell in the Michiganbasin, U.S.A. Measured vi trini te reflectance for theorganic-rich Antrim sam ple (TOC = 3.7%) is 0.7% Ro, avalue with which the sterane biomarkers are in agreement . The more deeply buried Al Carbonate sample(TOC = 0.31%) is much mor e m atur e (1.7% Ro), yet thesteranes (e.g., peaks E and H) indicate the rock to beimmature. Tri terpanes (not shown) yield maturi t iesthat are in agreement with those of the steranes.Two exp lana t ions are poss ib le for the a ppa ren tretardat ion of biomarker t ransformations in the Al

Carbonate: either the absence of clay catalysts in thecarbonate has prevented formation of the more-stablebiomarker epimers from less-stable ones, or the poorsource quality of the lean rock has prevented generat ion of new hydrocarbons that would bear the signature of mature biomarkers. Unti l these quest ions arean s wered we s h o u l d b e cau t i o u s ab o u t u s i n gbiomark er m aturities derived from clay-free rocks andfrom very lean rocks.Ano ther quite different but intrigu ing exam ple of adifficulty with biomarker maturities is shown in Figure 40. Four genetically related oils from the Gipps-land basin of Australia were analyz ed by gc-ms (Philp

and G i lber t , 1986) . The Ha puk u o i l has a norm alappearance compared with most other oils in the area,but the other three show very high concentrations ofthe C 3] extended hopane s (peaks f and g) and theC29hop ane (peak d). More alarmingly, the 22S/(22R+22S)ratio in the Perch and Dolphin oils is extremely low,and disagrees with the rat io calculated from the C 32extended hopanes (peaks h and i) in the same samples. This result indicates either that these oils wereformed at very low levels of maturity, or that there issome problem with the biomarker analysis.Philp and Gilbert (1986) explained these changes intri terpane distribut ion, the immature appearance of

the C 3] extended hopanes (peaks f and g), and the discrepancy in 22S/(22R+22S) for the C 3] and C32 extended hopanes by sugges t ing that these three o i l s hadleached (dissolved) biomarkers out of rich, immature

Figure 39Sterane distrib utions (m/z 217 mass chromatograms) in tw o rock extracts from a single w ell in theMichigan basin. The Devonian-age Antrim Shale (A) is at a much lowe r maturity (as measured b y vitrinitereflectance) than is the deep er Silurian-age S alina A l Carbonate sam ple (B), yet the sterane distributions indicate the A l C arbonate to be m uch les s mature. Identities of peak s are give n in Table 2. See text for further discussion . From Waples and M achihara, 1990; reprinted with perm ission fromBulletin of Canadian PetroleumGeology.




A

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RETENTION TIME MINUTES)



38 Waples and Machihara

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40 Waples and Machiharanearby immature Cretaceous rocks . Mat tavel l i andNovelli (1990) suggested leaching of organic materialfrom Tertiary rocks during vertical migration as a possible explanation for the presence of oleanane in Italian oils thought to be sourced from Jurassic-age sediments , where oleanane is un know n.

In spite of the wide p opularity of bioma rkers today,van Graas (1990) is generally skeptical about the utilit y o f b i o m ark e r r a t i o s a s m a t u r i t y p a ram et e r s ,because he did not find them very sensitive or reliablein the maturity range of his study (0.6-1.0% Ro). Hefound absolute concentrations of biomarkers (normalized to the saturated-hydro carbon content) to be morereliable, although he had to establish different empirical trend s for different facies. His view is still a mino rity one at present, but may well be an indication of afu ture t rend toward greater skept ic i sm regard ingmaturity information obtained from biomarkers.

SUMMARYIn our view, the following statements provide reasonable guidel ines for apply ing b iomarker data toestimate maturities of extracts and oils:1. Any application of biomarkers to predict vitrinitereflectance (i.e. , kerog en matu ri ty) or hydr oca rbo ngenerat ion m us t be v iewed as only approxim ate . Amuch better general approach is to use kinetic modeling to comp are predic ted and measure d b ioma rkerratios as a test of proposed thermal and burial histories.2.If one wishes to use biom arkers to estimate vitri

nite reflectance, the most useful results from 0.45% Roto about 0.75% are usual ly obtaine d by using Figure27, which plots the changes in 20S/(20R+20S) ratios(peak E/[p eak H + peak E]) for C29 regular steranes.3.Of lesser value is the plot in Figure 30, in w hichboth the 20S/20R and PP/acc rat ios for C 29 regularsteranes are plotted. The main prob lem w ith Figure 30lies in the effects of diag en esis on PP/ococ ratio s.4.At lower maturity levels, the 22S/(22R+22S) ratioin 17oc(H)-extended hopanes (peak f/[peak g + peakf]) is of qualitative value. However, once the transforma tion of 22R to 22S be gin s, this rat io cha nge s sorapid ly that i t canno t be used to es t imate m atur i ty

precisely. This ratio is thus mainly of value as a qualitative measure of im maturity.

5.The C32 ex tended hopanes (peaks h and i ) aremore reliable than the C 3] hopanes (peaks f and g) incalculating 22S/(22R+22S) ratios for hopanes.6 . M ore tane /ho pan e ra t ios (e .g . , peak q /p ea k d)are of some value, mainly in a qual i tat ive sense, asindicators of immatur i ty . However , they may beaffected by source as well as maturity.7. Tm /Ts ratios from m /z 191 mass chrom atogram s(peak b/pe ak a) can in principle extend the m aturi tyscale beyond 0 .9% Ro, but na tural var ia t ion s f romsample to sample general ly make them less precisethan the ste rane cu rve in Figure 27. They are of greatest value in estimating relative maturities of samplesfrom the same facies.

8. Oleananes and tricyclics show promise as qualitat ive or possibly even qua nti tat ive maturi ty indicators . At the present t ime, however, these parametershave not been adequately quantified and tested.9. All ma turi ty da ta obtained from biom arke rs inrock extracts should be comp ared carefully with kero-gen-maturi ty data because of the possibi l i ty of contamination of extracts by migrated m aterial. How ever,we should not expect perfect agreement, since the relative influences of time and temperature on biomarkers are different from those on kerogen maturat ion.Furthermore, biomarkers can show internal discrepancies for the sam e reason s.

10. Because of many unknowns about the chemicalprocesses involved in b iomarker maturat ion , weshould exercise care in interpreting biomarker-maturitydata, especially in carbonates and organic-lean rocks. Arecent d i scuss ion by Zumberge (1987) summarizesma ny of these points for a series of low-m aturity oils.11. Concentrations of all biomarkers decrease withincreasing maturi ty. Thus in highly mature samplesbiomarker concentrations may be too low for accuratequantification.

12. In addit ion to the saturated steranes d iscussedh ere , a ro m at i c s t e ran es an d m e t h y l p h en an t h ren eshave also been used for a num ber of years as ma turityindicators. The interested reader could refer to Mackenzie (1984), Riolo et al. (1986), Sakata et al. (1987),Radke and Welte (1983), or Tupper and Burckhardt(1990) for further information. Biphenyls are a newaddition to the list of maturity indicators (Alexanderet al., 1990), and their potential n eed s to be ev aluatedfurther.



hapter5Biomarkers as Organic-Facies Indicators

INTRODUCTIONBecause b iom arkers are der ived f rom bio logicalprecursor molecules in specific organisms, and because each of these organism s lives only unde r certainconditions, it is logical to attempt to use biomarke rs asindicators of those life conditions. Steranes are in general indicators for p hotosynthe tic biota, both terrestrial and aquatic. Triterpanes, which are derived mainly

from bacteria, m uch more frequently are indicators ofdepos i t ional and d iagenet ic condi t ions . Together ,therefore, steranes and triterpanes can provide information about most parts of the system we call organic fades.However, any application of biomarkers for organ-ic-facies interpretat ions should be made with a ful lawareness that organic facies (and hence biomarkerdistributions) can change rapidly, both vertically andlateral ly (Mo ldow an et al . , 1986; Curia le and Ode r-ma tt, 1989; Farrim ond et al., 1990). If one is w ork ingwi th a typ ical samp le of cu t t ings m ater ia l , the b iomarker signature will be an average for all facies pre

sent in the sample, and thus may be much harder tointerpret.Furtherm ore, if only a few samples of core m aterialare avai lable, one cannot be sure that these samplesare representative of the complete rock unit. In somesource rocks , par t i cu lar ly l acus t r ine beds , l a tera lchanges in organic facies can yield oils that look quitedifferent (Katz and Mertani, 1989). Oil samples themselves wil l almost certainly represent some kind ofaveragin g of input from v arious localities.Another problem is that we now real ize that thereis no definite a nd uniqu e facies interpretationfor.mostbiomarkers. It is likely that there are multiple precur

sors for many biomarkers, and that these precursorsmay com e from ve ry different kinds of organisms thatcan live in a wide range of environm ents. This poin t isdiscussed in detail later in this chapter. The pro per useof biom arkers in organic-facies interpretation s is analogous to the use of microfossils in age dating or pale-oenvironmental interpretation, in which the presenceor absence of a single variety is not completely diagnost ic. Just as micro paleontolo gists base their interpretat ions on assemblages of fossi ls , geologists andgeochemists interpret ing biomarker data should look

at assemblages of biomarker data. In this sense, theterm biom arke rs refers to ma ny other types of compounds than steranes and triterpanes: for example, n-alkanes, isoprenoids, di terpanes, sesquiterpanes, andporphyrins.Finally, biomarkers, even in the broad sense of theword, represent but one part of the total geologicaland geochemical information we have avai lable. Ifbiomarker data disagree with other information, they

may be revealing some new and hi therto unsuspectedt ru ths about the samples . On the o ther hand, theb i o m ark e r d a t a o r i n t e rp re t a t i o n m ay s i m p l y b ewrong. Any successful facies analysis must evaluateand integrate all available information to make a single, consistent story. Biomarkers should not be givenun due con s iderat ion s imply because they representrelatively expensive, high technology.These complicat ions have led to a certain a mo untof pessimism abou t the use of bioma rkers for organic-facies interpetat ions (e.g. , Moldowan et al . , 1985).However, if an assemblage approach is taken to interpret ing biomarkers, and if biomarkers are integrated

with all other geochemical and geological data, theyare very usefu l (e .g . , Mu rchiso n , 1987) . A cer ta inamount of restraint must simply be exercised in interpret ing biomarker distribut ions in terms of organicfacies.STERANES

C 27-C 29 Regular steranesThe steranes inherited directly from higher plants,an imals , and a lgae are the 20R epimers of the

5a(H),14a(H),17oc(H) forms of the C 27 , C 28 , C 29 , andC 30 steranes (Figure 9). The relat ive proport ions ofeach of these regu lar steranes can vary greatly fromsample to sample, however, depending upon the typeof organic material contributing to the sediment (Figure 42).Huang and Meinschein (1979) provided the fi rs tevidence that the relat ive proport ions of the C 27 -C 29regular sterols in living organisms w ere related to specific envir on me nts (Figure 43), and sug ges ted thats teranes in sed imen ts might provide valuable pale-

41



42 Waples and MachiharaC-28

Figure 42M/z 217 (sterane) mass chromatogramsof three very-immature sam ples sh owin g a varietyofC27-C29sterane distributions. Immature sam pleswere selected to avoid com plications caused b y maturation. Identities of p eaks are given in Table 2.From W aples and M achihara (1990); reprinted w ithperm ission from the Bulletin of Canadian PetroleumGeology.

oenvi ronmental in format ion . They proposed that apreponderance of C 29 sterols (or steranes) wou ld indicate a strong terrestrial contribution, whereas a dominance of C 27 would indicate a dominance of m ar ine

c-27 C-29

Figure 43Triangular diagram showing initial proposal of environmental depe ndence of the sterolcomposition in organisms. Further data indicate thatthe simple patterns implied by this diagram oftenare not valid , and that great care must be taken inusing sterol and sterane distributions for environmental interpretations. From Huang and Mein-schein (1979); reprinted with permission of Perga-mon Press PLC.

p h y t o p l an k t o n . C 2g was found in general to be thelowes t of the three s teranes , bu t where re la t ivelyabu nda nt i t might ind icate a heavy con t r ibut ion bylacustrine algae. Since these suggest ions were fi rs tma de, triangu lar d iagram s (e.g., Figure 44) have oftenbeen employed to represent the relat ive proport ionsof these three steranes.

The proposal of Huang and Meinschein has beenused wi th some success . For example , F igure 45shows the sterane distributions for two facies withinthe Elko Formation (Eocene) of Neva da. The co ntinenta l l ign i t ic s i l ts tones show a very s t rong pre dom inance of the C 29 steranes, as expected. In contrast, theoil shales, which were deposited in a lacustrine environment , show much larger propor t ions of the C 27and C 28 steranes, which presumably are contributedby nonmarine algae. Robinson (1987) noted that inIndonesia, C29regular s teranes and diasteranes dominate am ong fluvio-del taic oi ls , reflect ing the stro ngcontribut ion of terrestrial plant material . Humic andwaxy coals general ly display a s trong dominance ofthe C29 steranes.In spi te of such successes, however, one must becautious when applying these oversimplified rules.Volkman (1986, 1988) has commented that mos tmarine sediments, including those deposited in pelag-




1 . GIPPSLAND OIL (LOCATION UNKNOWN)2 . GIPPSLAND OIL-LAKES ENTRANCE

Figure 44Triangular diagram sh ow ing interpretation of environment from sterane distributions.This type of diagram has be en adap ted from that inFigure 43, but is sub ject to the same cau tions dis cusse d for that figure. From Shanm ugam (1985).

ic env ironm ents far from terrestrial influence, sho wpredominances of C29 steranes (peaks E, F, G, and H).Furtherm ore, lower Paleozoic and Precambrian sediments often contain substant ial amounts of the C 2 9sterane, even thoug h land plants could not hav e cont r ibuted (e .g . , Grantham, 1986a [see Example 4 inCha pter 7]; Rullkot ter et al . , 1986; Vlierboom et al . ,1986; Lon gm an an d Palm er, 1987; Fowler an d Do uglas, 1987; Buchardt et al., 1989). Volkman conc ludedthat there must be unpro ven mar ine sources of the C29sterane; Matsum oto eta l.(1982) and Fowler an d D ouglas (1984,1987) suggest i t could come from cyano-bacter ia (b lue-green a lgae) . Nichols e t a l . (1990)showed that large amounts of C Msterols are producedby m arine d ia toms dur ing the spr ing b loom in co ldAntarctic waters.Recent ly an ex t remely in teres t ing and rad ical lynew interpretation of regular-sterane distributions hasbeen put forth by Grantham and Wakefield (1988).They showe d tha t the ratio ofC28/C29regular steranes(e.g., peak D/peak H) in marine environments is controlled not by facies but by geologic age. The C 2 8 /C 2 9ratio increases from past to present, as shown in Figure 46 . They a t t r ibu ted th i s change to evolu t ion arytrend s within living organisms. Use of this ratio offersa unique way to date oi ls , and thus to make a bet ter

Figure 45Triangular diagram showing proportionsofC27, -28/a n d -29regular steranes in tw o facies ofthe Elko Formation, Eocene, of Nevada . The terrestrially dom inated lignitic siltstones s how a strongpredominance of the terrestrially derived C29 sterane,whereas the oil shales, dep osited in lacustrineenvironmen ts where nonm arine algal material dominated, have more C27 . From Palmer (1984); reprintedwith perm ission of the Rocky Mountain Association of Ge ologists.

first guess about their source rocks. However, Grantham and Wakefield cautioned against overly enthusiastic use of this method for precise age dating, at leastat our present state of know ledge .In one example, the C 2 8 /C 2 9rat io was used to distinguish between possible source rocks of Eocene andAlbian age for oils in the Zagros Otogenic Belt of Iran(Bordenave and Burw ood, 1990). In a second exam ple,we see in Figure 47 the m /z 217 mass chrom atogramof an extract of a cuttings sam ple that w as believed tobe from a reasonably mature Paleozoic interval in theMiddle Eas t . The low matur i ty ind icated by20S/ (20R+20S) (peak E/ [ pe ak H + pea k E]) andBB/aa ([peak F + peak G]/[peak E + peak H]) rat ioswas very surprising, and suggested contamination bycaving. The high C 2 8 /C 2 9 sterane rat io (peak D/peakH) supported the interpretation that the steranes camemainly from immature Cretaceous or Tert iary strataabove the Paleozoic.

In a th i rd s tudy , bo th Cretaceous and Jurass icsource rocks were suspected to be present. We predicted on the basis of the C^ / C ^ sterane ratio (Figure 48:peak D/peak H, about 1.4) that an oi l found in thearea had come from the Cretaceous source, becauseFigure 46 indicates that Jurass ic samples are morelikely to have C 2 8 /C 2 9ratio s less than 1.0. This delicate



44 Waples and Ma chiha ra

P R E -CAMBRIAN

P A L A E O Z O I C

CAMBRIAN OROO-VICIAN DEVONIANCARBONIFEROUS PERMIAN

M E S 0 Z 0 I C

TRIAS-SK JURASSIC CRETACEOUS

E U G L E N I D S C =VSIUCOFLAGELLATES

DISCOASTERS

EBRIWAW

DIATOMS C

CENOZOIC

TERTIARY

650 600 550 500 450 4 0 0 350 300 250 200 150 CO

T t . 4

1.31.2

-1.1

1.0

-0 .9

0 .8

-0.7

0.6

0.5

0.4

0.3

0.2

--0.1

MEANc 2 8 / c 2 9STERANERATIOOFM A R I N E -SOURCE-ROCK-OE RIVEDCRUOE OILS

Figure 46Plotofthe ratioofC28 /C 29 regular steranes as a functionofgeologic time, sh owin g that as a resultof evolutionary changes in p hotosynthetic organisms, theC28/C29ratio has increased from past to present, particularly since the Jurassic. From Grantham and W akefield (1988); reprinted with pe rm ission of PergamonPressPLC.

d i s t i n c t i o n m ay bea p p r o p r i a t e forM es o zo i candyounger rocks, but because the C ^ /Q g rat io beginstochange dramatically only in the Mesozoic, it will be ofl imited valueindist inguishing one Paleozoic periodfrom another . For example , JonesandPh ilp (1990)found no difference a mo ng oils fromatleast two distinct Paleozoic sources in the An ada rko basin.The hypoth esis of Gra ntha m an d Wakefield a ppliesonly toma rine sed ime nts . Hall and Dou glas (1983)

reported lacustrine shalesofDevo nian a ge from Scotland withastrong predom inanceofC28 steranes overC 27 and C29- McK irdy et al. (1984) found similar d istribut ionsinoi l show s at t ribu ted toplaya-la ke sourcerocksofC am b r i an ag einthe O fficer b asinofAu s t ra l ia . Fur therm ore, except ionstothe hy poth esis ofGrantham and W akefield have been noted. For example,anextractofOrdovician rock from the Ana darkobasin showed avery hig h ratio (0.91) more typical ofJurass ic orC re t aceo u s s am p l es ( J o n es andP h i l p ,1990). Thus we s h o u l d be cau t i o u s in a p p l y i n g

C 2 8 /C 2 9 ratios until they are verified further.In spite of these recent advances in our und ersta ndin gof sterane distributions, any simplistic interpretat ionofC27-C29sterane ra t ios, especial ly interms ofpaleoenvironment, is still risky. Useofthe t riangulardiag ram in Figure 45 withou t reference to facies implicat ions is now common becauseit is auseful w aytod i s p l ay s t e ran e d a t a . Ho wev er , t h e re is at leas taminor concern that C 27-C 29distributions ma y be influenced b y m aturity as well as facies (Cu riale, 1986).Inany case, al l interpretat ionsofs terane d i s t r ibu t ionsmu st be consistent with othe r geological evidence andwith common-sense logic.C3DRegular steranes

Recently Moldow anetal. (1985) prop osed that thepreviously neglected C 30 steranes (Figure 9) are present only in p o s t -S i l u r i an s am p l es d ep o s i t ed inmarine environm ents. Melloetal. (1988a,b) hav e co n-




I o n 2 1 7 . 0 0 a m u . C

firmed these conclusions based on work in Brazilianbasins. I t is not yet known whether the converse ist rue; i .e . , that a l l mar ine en vi ronm ents con ta in C 3 0steranes. It has also been suggested that C 30 steraneswill eventually be identified in lacustrine algae as wellas m arine (Volkman, 1988).The precise time in the past at which the C ^ steranesevolved is also not known. The data set of Moldowanand cow orkers included samples of Cambrian age fromwhich the C 30 steranes w ere m issing, and samp les ofDevonian age in which they were present.The C 30 s t e ran es r ep o r t ed b y M o l d o wan , M el l o ,

and coworkers are bel ieved to conta in thei r ex t ramethyl group in the longes t s ide chain . They cantherefore be cal led 24-propylcholestanes, and can beseen in the m /z 217 mass chro matog ram (peaks I, J, K,and L in Figure 42, for exam ple). Howe ver, they aremo re un am big uou s ly ident i f i ed in the m /z 414 tom /z 217 metas tab le t rans i t ion us ing SMIM. In contrast, the two families of 4-methylsteranes mentionedprevious ly (d inos teranes and 4-methylcholes tanes)have their extra methyl group attached to ring A. Them / z 23 fragment ion is mor e domin ant for 4-methyl-

C

D

H

steranes than is the m/ z 217 ion, because the additional methyl group is attached to the fragment tha t bearsthe positive charge.However, 4-methylsteranes (see discussion below)also give a mod est m / z 217 peak, and thus can be confused with regular steran es in the m /z 217 mass chromatogram. In particular, some of them elute at aboutthe same t ime as the C 30 steranes (Moldowan et al . ,1985; see also Figure 49). Figure 42 (A and B) showsthe presence of minor amounts of C 30 steranes (conclus ively ident i f i ed f rom other mass-spect ra l ev i dence). 4-Methylsteranes are absent. Figure 42C, how ever, show s a very large am oun t of material that couldbe e i ther the 14a(H) ,17a(H)-20R epimer of the C 3 0sterane, or one of the 4-methylsteranes. More detailedanalysis of this compound by the mass spectroscopistwo uld be req uired for positive identification.

Jones and P hilp (1990) observed in their stu dy of theAnadarko basin that the ratios of the C 2 9 /C 3 0P|J regular steranes ([peak E + peak F ]/[pea k J + peak K]) varied between oils from different Paleozoic sources. Thisratio played an im portant role in grouping the oils intogenetic famil ies and in correlat ing oi ls with source

>-hzUJHZUJ>ua.

M

^A^^VAJWRETENTION TIME

Figure 47M /z 217 (sterane) ma ss chromatogram from an extract of a moderately mature Paleoz oic rock fromthe Middle East, sho win gavery immature sterane distribution (main ly craa-20R) and a dom inance of C28 overC29 , a characteristic normally associated with much younger samples. Identities of peaks are given in Table 2.See text for discussion.




I o n 2 1 7 . 0 0 a m u.

>fc(0zu>UJ

M

R E T E N T I O N T I M E

Figure 48M/z2 7(sterane) mass chromatogram of an oil with a dom inance of the CMsteranes overC29.Identities of peaks aie given in Table2.See text for discussion.

217.065vw 7/u_A^1

* JLws^-^JwWJl^U^r -i | i | i | rRETENTION T IME -

Figure 49M/z2 7and23 (sterane) mass chromatogram s of an extract from an imm ature U pper Jurassic sample of marine origin in which dinoflagellate cysts were ab unda nt. PeaksC,D , H, andLare, respectively, the



Biomarkers for Geologists 47rocks. However, s ince al l the plausible source rockswere marine , the cause for the va riation wa s not clear.

4-MethylsteranesUnt i l recent ly i t was thought that a l l 4 -methyl -

s t e ran es were d i n o s t e ran es , b u t i t h as n o w b eenshown that both C 30 dinosteranes and a series of C ^-C 30 4-m ethy lcho lestan es exist (Figure 11). Both arebest detected using the m /z 231 chrom atogram (Figure49).Th e d i n o s t e ran es , wh i ch a re fo u n d m a i n l y i nmarine environments, are believed to be derived fromdinostero l in dinoflagel lates , but the origin of the 4-methylcholestanes, which are common in both marineand freshwater sam ples, is s t il l a mystery. Dinosteranes and 4-methylcholes tanes f requent ly occur together in marine samples. Probable precursors for the4-methylcholes tanes have been repor ted in micro

s co p i c p ry m n es i o p h y t e b ro wn a l g ae o f t h e g en u sPavlova(Volkman et al., 1989,1990). Prym nesioph ytesdate back to the Carboni ferous a nd are comm on inboth marine and brackish waters (Volkman et a l . ,1990), but i t is not yet clear how wide ly distrib utedthrough space and time thePavlovagenusis .Goodwinet a l . (1988) sugg es ted f reshw ater d inof lage l la tes ,algae, or bacteria as possible sources for the 4-methylcholestanes found in nonmarine environments.Unt i l more data are avai lab le , C2g -C 30 4 -m et h y l cholestanes should not be used as paleoenvironmentalindicators , and in any case the 4-methyl -24-ethyl -cholestanes should not be confused w ith dinosteran es.

Much of the pre-1989 literature in which 4-methylster-anes were always interpreted as indicating a dinoflag-ellate origin should therefore be viewed skeptically.Diasteranes

Diasteranes (also called rearranged steranes: Figure10) are present in significant quantities in most samples that are at least moderately ma ture. They are wellknown as C 27 , C 28 , C 29 , and C 3 0 species. Figure 39Ashows the 20S and 20R forms of the C 2 7 an d C2 9diasteranes, which are usually the most dominant andeasily observe d. The C 27 diasteranes (peaks M and N)are well separated from other importa nt peak s, but theC 29 20S diastera ne (peak O ) frequently ov erlaps ba dlywith the PP C 27(20S + 20R) regular steran es (p eak B).The relat ive amount of diasteranes com pared withregular s teranes seems to depend on both sedime ntl i tho logy and matur i ty . Dias teranes seem to formmost rea dily in clastic sedim ents, wh ere clay catalystscan play a role in their formation from oth er sterane s.They are therefore frequently used to distinguish carbonate facies (low diasteranes) from clastic ones (e.g.,H ug he s, 1984; Zu mb erg e, 1984; Mello et a l , 1988b;

Czocha nska et al., 1988; Mattavelli a nd Nov elli, 1990;Ried iger et al., 1990; Alajbeg et al., 1990). Bro wn (1989)noted that Indones ian shales conta in ing mangrove-derived organic matter had lower diasterane contentsthan did closely related shale-poor coals.However, the facies dependence must be more sub

tle than simply one of clastic/nonclastic or of clay content. Moldow an et al. (1986) suggested there migh t besome redox cont ro l on the d ias tera ne/s te rane ra t io .Clark and Philp (1989) found significant differences indiasterane contents between oi ls sourced exclusivelyfrom evaporitic carbonates, and those containing a contribution from deep-w ater m icritic carbonates. Conn anet al. (1986) found diasteranes to be abun dan t in som eorganic- lean Guatemalan anhydr i tes . Palacas e t a l .(1984) observed abundant diasteranes in clay-free samples of the Sunniland Limestone (Florida) and saw nocorrelat ion between clay content and diasterane content in that formation. Clark and Philp (1989) havesumm arized the published occurrences of high diasterane contents in carbonates, and suggest that there m aybe other as-yet-unidentified m echanism s for formingdiasteranes in at least some carbonate e nvironm ents.

The application of diasteranes a s facies indicators isfurther com plicated by their depen dence on maturi tyas well as environment. D iasteranes seem to be m ores tab le than regular s teranes , and thus become moredominant with increasing maturi ty. Figure 50 showsthe increase in diasteranes relative to regular steranesin three extracts of different ma turities from the sam efacies. The rat io of C2 7 / C 2 9 ota d ias tera nes (peak s[M+N ]/ [0+P]) paral le l s the ra tio of C^ /C ^ a a regular s teranes (peaks [A +C]/[E+H ]), a t rend that is notsurpris ing in view of the proposed genet ic relat ionship between regular steranes and diasteranes.

Finally, diasteranes seem to be unusually abundantin many lower Paleozoic oi ls, com pared with regularsteranes (e.g., Mo ldow an et al.,1985;Vlierboom et al.,1986; Reed et al., 1986; Longman and Palmer, 1987),even whe re no c lay mineral s are ev ident . Al thou ghthese oils are not all of high maturity, it is possible thattime has play ed a role in the conversion. Alternatively,there ma y be some as-yet-unrecogn ized facies affectthat is not related to clay minerals.Resin-derived cycloalkanes

Compounds bel ieved to be cycloalkanes der ivedfrom resin have been observed in oi ls derived fromcoals and in ex t ract s f rom coals in the Indones ianArc hipela go (T hom pson e t a l . , 1985; Com et e t a l . ,1989; Jamil et al., 1990). These c om po un ds a re not steranes, but may be confused with steranes because theyyield large m /z 217 peak s (Figure 51). They are discussed more fu l ly l a ter in th i s chapter underTRITERPANES.



4 8 W a p l e sand Mac hihar a

> -HG OLUI -

>




ft

_ jj

ft

iU I A J

R

*J P-CM-

i Jibv' fRETENTION TIME

Figure 51M/z 217 mass chrom atogram of saturatedhydrocarbons in an Indonesian oil generated from acoaly source rock. Peaks marked R are due toresin-derived com poun ds. aa-20R and 20S forms ofC29 regular sterane are indicated, but are present invery low concentration. From Robinson and Kamal,1988; reprinted w ith perm ission of IndonesianPetroleum Association.

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137044473 aapg methods in exploration volume 9 douglas waples tsutomu machihara biomarkers for...

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