Organic geochemistry of shales—II. Distribution of extractable organic matter in the siliceous Mowry Shale of Wyoming

CJw&bnica et Co8mochimk.a Acta 1966, Vol. SO,pp. 416 ta 484. Pergamon Press I&d, XWnted in Northern Ireland

AM&--The avexage contit of extractable organic matter in the Lower Cretaceou~~ Mowry Sh&e is about 400 ppm in the northwestern pszt of Wyoming, aGndit inoressesaouthes&wardIy, Thue, the regionaXvariations parallel those sea&kg a v&e of 2100 ppm in the Douglm +XYXX. of the organ& carbon content reported previously. Vertical v&&ions at tb given locality likewise plarallel v~~t~o~ in the carbon content. The resulta suggest that the extractable matter is indigenous. Near Rawlins, however, the extractable content correlate poorly with the carbon oontent, s~~est~g that the extractable matter is not indigeno~~~ The exceptiona;i oharaeter of the extractable mmtterfrom the Rmwliosarea,is also ind.ioa&d by its rel&ively low oontent of hexane-insoluble matter. The ratio of the extractable content to the organic oaxbon content is higher, the higher the carbon aontent, Possibly the conditions favorable to the preservation of graattr amounts of orgsufc matter also favored the formation of gre&er pro-portions of bitumen, or organio aouroematerial which produced greater proportions of bitumen wa~ideposited in area8 in which greater amounts of organic matter were preserved.

Tl!ms is the second of a series of papers dealing with s detailed geochemicat study of the siliceous Mowry ShaIe of Wyoming. The purpose of the study is to determine the ~l&tions~ps of the ~st~bution and com~sition of the org&nic matter to the geologic history and to known occurrences of petroleum in the Lower Cretrtoeous rocks of Wyomiug. The tit paper of this series (SCHRAYEB, and ZBRRELLA,1963) described the region& and vertical ~st~bution of the total organic matter in this fo~~tion. This psper presents the seoond phase of the study-the dete~~tion of the distribution of thst portion of the orgcmic matter which is easily extrsctsbfe with organic solvents; i.e. the “bitumens” or “bituminous matter.” The extr&&bIe orgtic matter in flue-grGued rocks is psrticularly important in considerationa of the geochemistry of petroleum because this fraction contains most of the hydrocarbons, the compounds most probabIy having 8 genetic relationship to petroleum. Since P. V. SBLITH pubis his very secant work on Recent sediments in 1962, maay papers have appeared dealing with the ex~~bl~ content of selects, both Recent and ancient. In most studies of &ncient sediments, the published data represent at most only I few samples from any one formation at a given foc&ty, and usually very few localities are represented. As a result, reliable conchrsions regarding possible interrel&ionships of the content of extmetible m&ter, the extract ~rn~~~on, and geologic factors co~dse~dom be drawn. The locution presented in this psper is intended to remedy this situation. In most localities, the Lower Cretaceous Mowry formation is composed largely 415

G. 3. Semwv~l~

416

and W. M. ZAR~S~A

of hard, tight grey Q black, siliceous ma.rine sbaks. The f~~~~~n and its strati~rt~phic~lat~ons~~s were discussed in scmm detail in the fir& paper of this s&es, which also described ths region of inteze&, sampling looalities, and sampling procedures. For ~on~e~eu~e, the ~st~buti~~ of sampling Iocalities is again shown in Rg. t. Samples of shale from 22 outcrop suctions were analyzed for their content of extractable organic matter. Generally, eaoh sample represented a ten-foot straMgraphic interval or less. Extraction was carried out with a b~~en~~methau~l i3-_.__ ---- -..-2.\ .._I..” _I_.. -__. ._~ ij !~YELLOWSTON~~, /: I NATlONAi;-;i PARU ‘7

_...

iI*: ;6 BfG t-am BASIN

Ij

il

“_“..”_...__...._..-

.

-,------------:

‘16 @*OUTCROP

. ,f \

, ..7

:

POWDER RIVER BASIN

i

GREEN RIVER BASIN

~AS~A~IE 3ASlN

BASIN

CHEYENEld BASIN ! I

Organiageochemiat~of shales--II

417

Small amounts of elemental sulfur were found in many Mowry samples. The sulfur is extractable with benzem+methanol and appears in both the benzene-soluble and hexane-soluble extracts. Since primary interest centered on the organic portion of the extractable matter, the elemental sulfur content of each rock sample was measured and subtracted from the measured total contents of benzene-soluble and hexane-soluble extractable matter to give data for the extractable organic matter. EXPERIMENTAL METHODS Materials

Solvents used include benzene (J. T. Baker Chemical Company, reagent grade), methanol (Baker and Adamson, reagent grade), and hexane (Neville Chemical Company, commercial gmde). The benzene and hexane were redistilled before use to remove relatively non-volatile impurities. The 2 : 1 mixture (volume) of benzene and methanol used for extraction contained O-2mg/l. non-volatile residue, an amount considered negligible. Clean quartz sand used in extraction was calcined in air at about 500°C prior to use in order to remove organic impurities. The calcined sand contained O-4 mg/kg benzene-soluble extractable matter, an amount which could be tolerated. Procedures

Fifty-gram samples of pulverized shale ( -00 mesh) were mixed with lOOgram portions of sand. This facilitated extraction by increasing the permeability. The mixtures were placed in grease-free extraction thimbles (Whatman, double-thickness, 43 x 123 mm) and covered with 30-g portions of sand to prevent loss of sample by splashing during extraction. Each sample was extracted in a Soxhlet extractor using 250 ml of 2: 1 (volume) benzen+methenol mixture. Extractions were usually carried out for six hours in two stages. The samples were allowed to soak in the extraction solvent during an intervening 16 hr period. If at the end of the sixhour extraction period the fluid issuing from the thimbles showed appreciable color, extraction was continued until very little color was being extracted. Most of the solvent was distilled from the extracts. The reminder was evaporated under nitrogen on a water bath at 80°C. The solvent-free extracts were dispersed in benzene, and the benzene-soluble portions were recovered by centrifugation and washing. The benzene solutions were collected in weighed 6 x 6 in. lipless Byrex test tubes. The benzene-soluble extracts were recovered by vacuum evaporation of the solvent at 30°C in a Rotary Evapo-Mix (Buchler Instruments) fitted with Teflon connectors to avoid contamination from plasticizers. The tubes containing the extracts were then weighed. For samples containing 250-2000 ppm extractable matter, the coefficient of variation for the determination is four to six per cent. Five milliliters of hexane was added to each tube containing a benzene-soluble extract. The extracts were dispersed by immersing the tubes briefly in an ultrasonic cleaning bath using water as the coupling medium. The hexane-soluble portions were removed by centrifugation and washing to a second set of weighed test tubes. After again being freed of solvent in the Rotary Evapo-Mix, the hexane-soluble extrscts were weighed with the tubes. The coefficient of variation for the determination is about six per cent.

418

G. J. SCHRAYEB and W. M. ZABRELLA

For the measurement of elemental sulfur, samples of unextracted pulverized shale were dispersed in a 3: 1 mixture of benzene and methanol buffered with acetic acid-ammonium acetate, shaken well and allowed to settle overnight. The supernatant liquid containing the elemental sulfur was scanned polarographically from -0.3 V to about - 1-OV versus a silver-silver sulfide anode. The apparent diffusion current, after correction for the residual current, was converted to the concentration of elemental sulfur using a calibration curve. Calculations

All extract contents are expressed as milligrams of extract per kilogram of airdried sediments (ppm). A quantity termed the “extractability” (E) of the organic matter in a rock is herein defined as the number of grams of benzene-soluble extraotable organic matter in an amount of organic matter equivalent to 100 g of organic carbon and is expressed in per cent. The extractability is calculated according to (1) E = BSE x 1O-2 c ON

(1)

where BSE = content of benzene-soluble extractable organic matter in parts per million, and Corg= content of organic carbon in per cent. Average contents of extractable matter for individual outcrop sections have been weighted for the footage represented by the individual rock samples. EXPERIMENTAL RESULTS Regional relationships

Table 1 lists average values for the organic carbon and extractable contents for the outcrop sections. The relationship of the extractable content to the organic carbon content is graphically represented in Fig. 2. In general, the organic carbon content and the extractable oontent are directly related, both varying in the same direction. The dashed trend line in Fig. 2 was fitted visually. Figure 3 shows that above an organic carbon content of about 1.2 per cent, the extra&ability is higher, the higher the organic carbon content. Note that the extra&abilities are plotted against the reciprooals of the organic carbon contents in order to obtain a linear trend on the regression diagram of Fig. 3 for samples containing over 1.2 per cent organic carbon. The position of the dashed trend line was calculated from the linear portion of the trend line of Fig. 2. Mapping the values of the average content of benzene-soluble extractable matter (Fig. 4) shows that the regional distribution of extractable organic matter closely resembles that of the organic matter reported earlier (SCHILAYERand Z-ELLA, 1963). There is a regional southeastward increase in organic carbon content, extractable content, and extra&ability with a maximum in the general area of the Casper Arch. The average chemical composition of the extractable organic matter might be expected to show a corresponding regional trend. While such a trend cannot be ruled out on the basis of the present extraction data, evidence for it is meager or absent. Figure 5 shows the relationship between the organic carbon content and the asphaltene content of the benzene-soluble extractable organic matter for individual samples from six localities ranging from the extreme western portion to the

Orgtmiegeochemistry Of shslea--II Table 1. Average contents of organic oarbon and extra&able organic matter

Section numb%r 1 2

3 4 f 7 8 10 11 13 14 16 16 17 18 19 20 21 22 23 24

Section name Aksva Arminto Bargee Beaver Divide Bull L&k% Cody Canal Crazy Woman Douglas East Owl Creek E&on Ramoh Gro6 V%ntre Hat Six Little Bonanza Lovell North GreybuU Nowood Owl Creek RiltWk3

Sand Draw Shamrook Ran& Three T Ranah West Thermopolia

% Cars 2-80 2-55 1.60 1.64 l-73 0.86 2.05 2.94 1.79 f-85 1.19 2.61 2.03 1.33 1.32 2.07 l-55 2.0% l-68 2-20 2-75 l-48

Benz%ne~solubl% extra& fppm) lSS0

1631 720

1089 9m 317 1x04 212f 894 1052 541 1710 1128 688 881 1016 824 1224 930 3436 MB9 697

‘RELATION OF CONTENT DF * BENZENE-SOLUBLE EXTRACT TO ORGANIC CARBON CONTENT-ALL SAMPLES

---=TREND

LINE FITTED VISUALLY 4

PER&

ORGANI: CARBON

Fig. 2

fr

430

G. 3. SCBBAYJZRand W. Y. Zm

0 PERGENT

PORTION OF ____I-T".__--_ TREND LINE OF FIGURE2 .._ ._.,. _. m-r ----' 1.5 .B .75 1.0 .9 ORGANIC CARlON

Fig, 3. R&&ion of ~~~~t~~~ty

to organic carbon oontent.

.__..-..

1

Organic geochemistry of shales-11

421

extreme eastern part of the region of study. Though there is some indication of a direct relationship between the two variables below an organic carbon content of about l-2 per cent, no regional trend is evident. It should be noted (Fig. 2) that most samples studied contain more than l-2 per cent organic carbon (Fig. 2) ; hence, they would lie in the organic carbon range where no relationship to asphaltene content is evident (Fig. 5). 100

xGROS

VENTRE 113) .BULL LAKE(S)

* *WEST

fr 8 a

+

A. l

THERMOPOLlSl24) i

RELATION OF GROSS COMPOSITION OF BENZENE-SOLUBLE EXTRACT TO ORGANIC CARBON CONTENT OF PARENT SHALE

OO

5

I PERCENT ORGAN,:

CARBON I;

SHALE

Fig. 6

Certain of the relationships noted above are apparent at individual outcrop sections. Most sections exhibit the same relationships among the various observables under consideration, but deviations are evident in at least one. section. The results from several “normal” sections will be discussed first to illustrate the general patterns observed. Next, the results from a special section will be used to demonstrate the effects of lithologic variations. Finally, a deviant section will be described. At most localities, the close relationship noted previously between the extractable content and the organic carbon content is, if anything, even more striking (Fig. 6 through 11). The usual pattern is one of approximately linear variation, but the linearity should not be taken too seriously. In the overall pattern of variation (Fig. 2) curvature is apparent only below about l-2 per cent organic carbon. Samples in this range sufficient to define the curvature at any single locality are available from only a few outcrops. Whether or not one considers the extractable matter-organic carbon relationship linear, it is implicit in the observed pattern of variation that the extractability of the organic matter increases with increase in the organic carbon content. Relationships in the vertical direction are shown in Fig. 12 through 1’7. Normally, the variations in the organic carbon content correlate well with the fluctuations in the contents of benzene-soluble and hexane-soluble organic extractable matter, which

422

G. J.

&RRAYER

and W. M. ZARRELLA

~-~__.___._~--.~---_-~--

0

2

I PERCENT

3 ORGANIC

4

CARBON

Fig. 6. Douglas section: Reletion of benzene-solubleextract content and organic carbon content.

is in accord with the observations cited above. The correlation between the organio carbon content and the extra&ability, though still evident, is not nearly as good. This was to be expected from the rather low coefficient of correlation apparent in Fig. 3. The gross composition of the extra&able matter as expressed by the asphaltene content shows no well-defined vertical trends.

PERCENT ORGANIC CARBON

Fig. 7. Aloora seation: Relation of benzene-soluble extract content and orgmnic carbon content.

30

OO

I

Fig. 8. Amninto m&ion:

2 PERCENT

ORGANIC

3 CARBON

4

Relation of benzene-soluble extract cwbon content.

PERCENT

ORGANIC

5

content to organic

CARBON

Fig. 9. West Thermopolis section: Relation of benzene-soluble extra& and organic carbon content.

content

An additiona Mowry outcrop wan sampled southeast of Thermopolis. Though the ar&ytieal results for this locality were not used for mappiug purposes, the vertical va,ri&ions are of some interest (Fig. 18). In the lower portion of the formation, composed almost exclwively of Sue-grabed ahalea, the data vazy iu a manner kmilar to that seen iu Fig. 12 through 17. A silty zone occurs in the upper pa& of

G. J.

424

&.XilU??ER

and W. M. Z~ULRELL~ __-__._ -

I i

PERCENT ORGANK

CARBON

Fig. 10. Bull Lake motion: Relation of benzene-sohble extract content and organic carbon content.

3

PERGENT ORGANIC CARBON

the section. In pwsiq from the sbalo into the siltstone thm oe~urs an abrupt, marked deoreaw in the content of both organic -bon 4 extra&a&~ ax-g&c matter. At the same time, the compotitiion of 6he extra&able matter cluwges. Whereas the asphalten~ co&w& of the bemwne-s&ble organic mat&x of the sb&s is about 50 per centc,t&at of the siltstone extra&s is O-20 per cant. Uerestingly,

oqpnicgeo&emistry %

SOLUBLEEXTRACT IPPM

of Shal~II % 02468iO

Fig. 12, Douglas section: Vertical variations.

9% ORGANIC CARBON

Fig. 13. Alcovfbs&ion:

Vertical variations.

425

G. J. SCERAY~ and W. M.

426

hRRELLA

%

% 500

t500 2!mo

"ASPHALTENESn :XTRA~TABlL~TY 4 8 20406080 ’

*

7

i

‘k I *

.

Fig. 14. Arminto swtion: Vertioe3variation.

% % SOLUBLEEXTRACT % '~s~ALTEN~~" EXTRACTABILITY (PP~~ ORGANSCAR~N 320 0

, 2/

2

t 1% 30 150 b I' 70 6 1, 500 t c1500 b j IO

s

'-~HEXANE &%ENZENE ?t PJ *.

400.

0 :

/: ii*. : ?’

440.

I

,

, /

680

,' ; I_--.-

1

Fig. 15. West Thermopolis section: Vertical vmi&iona.

Organic gcochemietryof shales--II

427

SOLUBLEEXTRACT % % % ORGANICCARBON (PPMI 'ASPHALTENESI'EXJRACTABILITY 6 IO 500 1501 20 40 60 60 too HEXANE

520

L

Fig. 16. Bull Lake section: Vertical vmietions.

the extractability does not change abruptly at the lithologic boundary but rises gradually to a value of about ten at the top of the silt zone. Near Rawlins, the Mowry Shale does not exhibit the relationships noted at most other sections. Though still evident, the correlation between the organic carbon content and that of the benzene-soluble extractable organic matter is very poor (Fig. 19), a fact which is also apparent in the vertical variations (Fig. 20). The asphaltene content of the extracts is very uniform and abnormally low, ranging from 10-15 per cent. Many samples, in the upper two-thirds of the formation at least, have abnormally high extra&abilities. DISCUSSION AND CONCLUSIONS

The most marked correlation observed is that between the organic carbon content and the content of extractables. Similar correlations between the extractable hydrocarbon content and the organic carbon content have been reported by P-PI (1956) for the upper Telissa shales (Miocene) of South Sumatra, the Cretaceous Graneros shale of the Denver Basin, Permian (Leonard) rocks of West Texas, and the C&aceous Second White Specks zone of Alberta, and by BAKER (1962) for the 5

428

G. 3. &XRAYER and % O~ANle ~~

w.

M. Z~R~EELLG

SOLUBLEEXTRACT % % (Pi) '~SP~LT~NESn EXTRA~TA6lLlTY

Fig. 17. Gros Ventre motion: Vertical variations.

Pennsylvanian Cherokee group of KLUWMand Oklahoma. PEXLIZWchose to interpret his correlations as proof that the hy~~r~~ were phenol to the rooks and that the rooks were ~t~le~-so~~ rocks. BAKER, though agmeing that the extra&able hydrocarbons were probably Ahigenic, suggested the possibility that they are allogenic, haviug been piaked up and retained chiefly by highly absorptive arid adsorptive orgsnic matter. Hence, they occurred in amounts proportional to the organic cmbon content of the rocks. If sorption were responsible for the MXNmulation of hydrocarbons in the finegmined rocks, however, one would expeot to observe either a constant ratio of extractables to organio carbon or a fixed vertic& trend in the ratio reflecting the direction of flow of the fluids from which the extraotable matter WBSsorbed. If the migmting flaids always contied a large excess of sorbable extracta;ble organic matter, one would expect to observe a oonsW& ratio of extractablea to orgsnio carbon across the formation-the orgmia mat&r b&g &ur&ed with extraotables throughout. On the other had, if i&a migra&q &aids oontied very small amounts of sorba.ble extr&able matter in the case of, say, upwani fluid m&r&ion through the fine-grained rocks, one would expeot to find ts high ratio of extrz&ables to orgtie csrbou at the bottom of the fofmaton aa.d

Orgsaia geochemistry of shaks-II %

429

% % SOLUBLEEXTRACT EXTRACTABILITY 'IAsPHALTENES" lPPMl

ORGANICCARBON

0

20 40 6080

Fig. 18. Southeast Thersnopolisa&ion:

0

2 PERCENT

8

Vertial varistions.

3 ORGANIC

‘4

4

CARBON

Fig. 19. Rawlins section: Relation of benzene-solubleextra& content and organic carbon content.

5

430

G. J. SCHELAYERand IV. M. ZARRELLA % % SOLUEiLE EXTRACT % ORGANIC CARBON [PPMI '~SP~IALTENESI' EXTRACTABILITY

260 280-

320c $3340. E Ei n360380 -

420440460'

I/

=-HEXANE ' ' " * * ' ' ' Fig. 20. Rawlins section: Vertical vmiations.

successively lower ratios at stratigraphically higher levels. The lowermost strata would have been exposed to the highest concentrations of sorbable extractable matter, while overlying strata would have been exposed to solutions which had been depleted of sorbable extractable matter by contact with the fine-grained rocks below. An examination of the present data as well as BAKER’Sdata reveals, however, that the ratio is not constant, nor does it vary in a single direction in the forma~on. Rather, the ratio seems to be directly related to the organic carbon content, one ~ne~~g with the other. BITZIERLI(1963)has cited similar instances from the European Gas. These considerations, as well as others concerning the diffiaulties of transporting high molecular weight organic matter in the requisite quantities through fine-grained rocks, indicate that where the extractable content of fine-grained rooks correlates well with their content of organic carbon or organic matter, the extractable matter is indigenous, as PIXILXW~ suggested. At most localities, the extractable matter in the Mowry Shale is in~genous. The increase of extractabi~ty with mareased organic carbon content may be due to regional variations in organic source material. This possibihty cannot be ruled out, but supporting evidence is lacking. Considering the present data one might

Organic geoc~mist~

of shales--II

431

expect that regiona ~ffe~nce~ in eource rn&~~al would be reflected in a. regional trend in the gross ~m~sition of the extracts as expressed by their asphal~ne contents, or perhaps as a correlation in vertical profile between the organic carbon content and the ~p~ltene content of the extractable organic matter. The data provide no good animation of either expectation. Possibly the ~lations~p between ~ph~~ne content and organic carbon content below l-2 per cent organic carbon (Fig. 5) indicates source material differences where the Mowry Shale contains very little orals carbon; but within the region of study, such areas are not large, and are found only in the extreme no~hwest. It is also possible that the westwardly decreas~g extrac~bility of the shales is due to progressively greater losses of bitu~nous material toward the west. Although this ~ssibility is not to be excluded, sup~rting evidence is lacking. If it were true, and if a loss of ‘~bituminous material” be equated to a loss of petroleum, Lower Cretaceous oil accum~ations might be expected to be most n~e~us in the western and no~hwestern parts of the region of study. Instead, most Lower ~taceous ac~um~ations occur in the east, where both the organic carbon content and the e~ractable content are highest (Fig. 4). F~thermore, the extent to which the shales lost their bituminous matter to adjacent carrier beds p~bably depended largely on the maximum depth of burial of the shales and on their compactibility. At a given locality, for the relatively thin Nowry Shale (150450 ft), the maxims depth of burial was approximately equal for all parts of the fo~ation, so that on the basis of depth of burial alone, all parts of the fo~ation would be expected to have lost bituminous matter in the same propo~ions. Extra~tabilities would be expected to be constant t~oughout the rock unit. This is contrary to the present obse~ations. Compactibi~ty, on the other hand, might be expected to vary within the formation. The compactibility of clayey sediments is lower, the greater their average particle size (SPECTOR, 1953). On this basis, one would expect silty zones to exhibit higher extractabilities, Our obse~ations confirm this in the case of observably silty zones. Most of the samples studied, on the other hand, contain little or no silt observable with a hand-lens in the field, altho~h vacations in their organic content may parallel undetected variations in average grain size (SCH~AY~Rand ~A~E~A~ 1963). The fact that in these fine-grained samples the extractability of the organic matter is low, rather than higher in samples containing less organic carbon, suggests that in the fine-grained shales the average grain sizes bears no significant relationship to either the organic carbon content or the extractability. Finally, differences in se~entation con~tions could have been ~s~nsible for the variations in extractability. Both the organic matter content and the extraotability may be higher in the Casper Arch area because of a lower E, at the de~sitional in~~ace during sedimentation. A relatively more reducing de~sitional en~~nment is ~~~ated by a greater prevalence of pyrite in the ~o~ formation in that area. An inde~ndent suggestion that more reducing segmentation conditions lead to greater bit~inization of the deposited organic matter can be seen in EMERY’S (1960) studies of the basin sediments off southern California. Cal~~ations using EHERY’Sorganic carbon and organic extract data show that in the Santa Catalina Basin extra~tabilities range from 2.6 to 3.9 ; in the Santa Cruz Basin, from 44 to 5.0 ; and in the Santa Barbara Basin, from 7.3 to 8.1, Only in the Santa Barbara Basin

432

G. J.

SCHRAYERand W. M. ZARRELLA

do anaerobic monitions prevail from the se~rnen~~ interface do~w~d. It is seen that the sediments from this basin exhibit the highest ~xtr~~bilities. REDFIELD (1958) described a more convincing case in his study of the Recent sediments of the Maracaibo Basin of Venezuela. He found that both the organic carbon contents and the extractable contents of the sediments were highest in the central portion of the lake where the oxygen content of the bottom waters was lowest; in fact, zero. Maps of the oxygen content of the bottom waters, organic carbon in the sediments, and extractable matter in the sediments all exhibited

Fig. 21. Relation between content of organic extra&ables and organic carbon content in Reoent sedimenti of Lake Maraoaibo. (Based on data of REDWIELD, 1968.) similar patterns, suggesting that anaerobic conditions on the bottom and in the sediments were largely ~nsible for the p~se~ation of the organic matter. A regression plot of Redfi eld ‘s carbon and extractable data (Fig. 21) shows a pattern which is compatible with that found for the Mowry Shale. Similarly, in the Maraoebibo sediments the organic carbon content and the extractabihty of the organic matter increase together (Fig. 22). Considering that no information is given regardiug the texture of the Maracaibo sediments and that REDEIELD'Bextract data may not have been corrected for elemental sulfur, the correspondence with observations of the Mowry Shale is good. We ~n~tively conch&de, therefore, that the regional difFerences in e~r~~bi~ty in the Mowry Shale are due primarily to di%renoes in the oxidation-reduction potential under which the sediments were deposited. The available data indicate, however, that other as yet unidentified fa&ns are also important, The abrupt decrease in the organic carbon content in passing from shale to siltstone at the Southeast Thermopolis section (Fig. 18)is in accord with the observations of others who have reported that, in general, the concentration of organic matter in elastic sediments de-see with increasing gram size si5Elllrr9. and PATXODZ,

Organic geochemistry of shales-11

1942; R~NOV,

433

1968; T~OFEEV, 1968; BORDOVSKY, 1967; EMERY, 1960). Inter-

pretations of the extraction results must be regarded as very tentative owing to the very real possibility that the organic portion of the silty samples has been signifkantly affected by weathering. Bearing this in mind, let us consider first the observation that higher extractabilities are encountered in the silty zonea. In the light of the preceding discussion of regional extractability variations, this observation is unexpected. In the presumably more oxidizing environment in which the silts were deposited, lower extra&abilities would have been expected. Similarly high extra&abilities for silt&ones and sandstones have been reported by others (BAEER,

Fig. 22. Relation between extra&ability and organic carbon content in Recent from data of REDFIELD, 1968.) sediments of Lake Mrtracaibo. (cdCd8ted

1962; BORDOVSKY, 1967; KOZLOV and TOKAREV, 1967; MAIMIN, 1968; PHILIPPI, 1966; KIDWELL and HUNT, 1968; TIMOFEEV, 1968), and have usually been attrib-

uted to the allogenic character of the bitumen. The same explanation may well apply to the data for the Southeast Thermopolis section. The lower asphaltene content of the siltstone extractables suggests greater fluidity of the materials, which is in accord with their possible migratory origin. Moreover, in their lower asphaltene contents, the siltstone extractables resemble the Lower Cretaceous crudes which contain very small amounts of asphaltenes (HUNT, 1963). Although the data seem to indicate that the siltstones contain non-indigenous bitumen, this interpretation must be tested by detailed chemical studies of fresh siltstones from the sub-surface, as well as by comparisons with petroleum. The poor correlation between the extractable content and the organic carbon content at the Rawlins section suggests that here, too, the bitumens ha,ve undergone some migration. As at the Southeast Thermopolis section, abnormally high extra&abilities and low asphaltene contents are observed, although silt&ones are absent in the Rawlins section. The Rawlins bitumens, or a significant portion

434

G. .J.

SCHRAYER

and W. M.

ZARRELLA

thereof, may have migrated into the inowry Shale from adjacent permeable strata, or they may have originated in the Mowry formation itself and migrated to a very limited extent. A more complete explanation of the Rawlins data must await more detailed chemical studies. Acknowledgments-The authors extend their sincere thanks to Mr. JOHN SEBAK, who performed most of the extractions; t.o Mrs. LYDIA DAUUHERTYand Miss EILEENDERUSHA, who performed the sulfur analyses; to Mr. W. P. FIEHLERfor drafting most of the figures; and to Mrs. MARLENECOSTELLO for typing the manuscript. Their thanks are also extended to Gulf Reseamh & Development Company for permissionto publish this paper. REFERENCES B&EER D. R. (1962) Organic geochemistry of Cherokee group in southeastern Kansas and northeasternOklahoma. Bull. Amer. Ass. Petrol. Geol. 46, 1621-1642. BIT%!ERLI P. (1963) Classification of bituminous rocks from western Europe. Procee&ags, Sixth World PetroEeurn Congress, Section I, Geophy&cs aruEGeology, 155-165. BORDOVSKY0. K. (1957) Bitumen content of the sediments of the western part of the Bering Sea. DokE. Akad. Nauk S.S.S.R. (Geoche~~t?,g Seethe) ll& 1321 (Consultant’sBureau, Inc., tr~slation). ER~ERYK. 0. (1960) The Sea 08 Southem California. John Wiley. HTJNTJ. M. (1963) Composition of crude oil and its relation to stratigraphy in Wyoming. &II. Amer. Ass. Petrol. Geol. 57, 1837-1872. KIDWELL A. L. and HUNT J. M. (1958) Migration of oil in recent sediments of Pedernales, Venezuela. In Habitat of Oil, Amer. Ass. Potrol. Geol., Tulsa, Oklahoma. KOZLOVV. P. and TOK~REVL. V. (1957) Geochemical nature of the organic material and bitumens dispersedin deposits of the coal-bearing horizon of the Lower Carboniferousof the Se&en) Kuibyshev district of the Volga valley. D&l. Akad. ,Ya& S.S.S.R. ~~~~~~~~~ 113,391 ~~onsult~t’s Bureau, Inc., tr~lation). MAIMINZ. L. (1958) The possibility of identifying oil soume rocks in the Carboniferousand Permian of the Volga-Ural region. Trudy Vsesoyuz. Neftyan. &Tauch.-~88&?dov&el.Geelogorasvedoch. Inst., No. 117, 252-276 (Associated Technical Services, Inc., translation). PHILIPPIG. T. (1957) Identification of oil source beds by chemioal means. ColzgreaoCTeo~ogico Internaciona;l,XX” Sea&on, Ciudad de Mexico, Section III, Geologiade1 Petroleo, Editorial Stylo, Mexico, D. F. REDELELD A. C. (1958) Preludes to the entrapment of organic matter in the sediments of Lake Maracaibo. ~a~~ of Oil, Amer. Ass. Petrol. Geol., Tulsa, Oklahoma. Rowov A. B. (1968) Organic carbon in sed~ent~ry rocks (in relationto the presenceof petroleum) Geokhimiya 4, 510-636. SC~RAYERG. J. and ZAEEELLAW. M. (1963) Organic geochemistry of shales: I. Distribution of organic matter in the siliceous Mowry Shale of Wyoming. Geochim. et Comwchim. Acta 27, 1033-1046. SKEMPTON A. W. (1963) Soil mechanicsin relation to geology. Proc. Porka. Geol. SOC.29, 33-82. TIMOFEEV G. I. (1958) The distributionof organicmaterial in Bat-Boyos sedimentsof Dagestan. G~kh~~~ya 4,762-758. TRACKP. D. and PATNOREII. W. (1942) Source Beds of Petroleum Amer. Ass. Petrol. Geol., Tulsa, Oklahoma.