Subaerial weathering of sedimentary organic matter

Geochimlca et Cosmochlmlca Acta. 1978. Vol. 42, pp. 305 to 312. Pergamon Press. Printed in Great Britain

Subaerial weathering of sedimentary

organic matter

JERRY L. CLAYTON and PAUL J. SWETLAND* U.S. Geological Survey, Box 25046, Denver, CO 80225, U.S.A. (Received 28 July 1977; accepted in revised form 16 November

1977)

Abstract-Small diameter core samplei were taken from outcrops of the Permian Phosphoria Formation and the Cretaceous Pierre Shale of the Western United States to determine the effects of weathering on organic matter in shale outcrops. While the Pierre Shale core showed no evidence of weathering, the Phosphoria Formation showed significant reduction of overall organic content and pronounced changes in organic composition over the near-surface interval of the core. Total organic carbon is lower by as much as 60% over the upper 2 ft of the core. Chloroform-soluble organic matter and are 50% lower over this same interval. The ratio of saturated total hydrocarbon (C, 5 +) concentrations to aromatic hydrocarbons decreases steadily with core depth over the upper 2.6 ft of the core. Aromatic hydrocarbons are enriched in the stable carbon-13 isotope by an average of 1.7:& over this same interval. Shallow core samples also show a loss of n-paraffins relative to branched/cyclic compounds in the saturated C,,, fraction. Although the extent of weathering is variable, certain characteristic effects are recognizable and can be applied to the interpretation of outcrop data in organic geochemical studies.

INTRODUCTION

THE PURPOSEof this study is to determine the nature and magnitude of compositional changes in sedimentary organic matter as a result of surface weathering. Owing to the limited availability of subsurface materials, many organic geochemical studies must be based entirely or in part on outcrop samples, and the data obtained from them is inferred to be characteristic of the same interval at depth, below the zone of weathering (SCHRAYER and ZARRELLA, 1963, 1966; NIXON, 1973; POWELL et al., 1975; SWETLAND and CLAYTON, 1976; CLAYPOOL et al., 1978). It is critical in such studies, therefore, to be able to recognize both the qualitative and quantitative changes that weathering can produce in the organic content of outcrop samples, and the depth to which weathering-induced alterations may proceed. LEYTHAEUSER(1973) showed that surface weathering of the Cretaceous Mancos Shale of Utah resulted in a decrease in the amount of organic matter as well as changes in its composition. The present study documents some of these changes and provides new data on alteration due to weathering. Attention is focused on the geochemical measurements used commonly for petroleum source-rock evaluation.

SAMPLES Two 7/8-in diameter cores were taken with a portable drill from outcrops of the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation of northeastern Utah (sec. 25, T.lN.-R.9W.) and the Mitten Member of the Upper Cretaceous Pierre Shale of Colorado (sec. 1, T.lN.-R.71W.). The lengths of the two cores were * Present address: Gulf Science and Technology P.O. Drawer 2038, Pittsburgh, PA 15230, U.S.A.

16 and 22ft, respectively. The cores were drilled nearly perpendicular to the outcrop surfaces to allow a systematic study of the changes in organic composition as a function of distance from the surface exposure. Core recovery was about 80% for the Pierre Shale and about 65% for the Phosphoria Formation. The shallow core samples were collected with a 2-ft core barrel and the deeper samples (deeper than approx 5 ft) with a 5-ft core barrel. Because of less than lOO’% core recoveries the sample depths recorded on the two sets of core samples, and hence those given in this paper, may vary slightly from the actual depths. The ideal weathering profile for this type of study would be to drill along or close to bedding planes in order to minimize the possible effects of vertical change in lithology and organic content within the sedimentary sequence. The cores collected for’the present study were taken at localities where the outcrop surfaces dip at approx 30”. Bedding surfaces in the Pierre Shale dip at an angle of about 20” to the outcrop surface, so that the core was drilled nearly parallel to bedding (-20”). The core samples obtained were uniform in lithology, consisting of medium-gray, noncalcareous shale. However, because bedding planes in the Phosphoria Formation are oriented nearly parallel to the outcrop surface at the core locality, it was necessary to core the Phosphoria nearly perpendicular to bedding. Subsequent bulk powder X-ray diffraction analyses of the Phosphoria core samples revealed only minor mineralogical variation throughout the 16ft of the core. Analysis of the data seems to indicate that mineralogical variation bears no systematic relationship to changes in the geochemical data. Outcrop samples were collected at each core locality to compare with the core samples. In general, these sampleS were obtained from below the zone of highly fractured, obviously weathered materiai on the surface by digging into the outcrop.

ANALYTICAL

PROCEDURE

The core samples were composited over depth intervals sufficient to provide samples large enough for analysis by the standard techniques used for evaluation of petroleum source-rock potential. The composited samples were

Co., 305

J. L. CLAYTONand P. J. SWETLAND

306

ground to approximately 100 mesh (- 153 pm particle diameter) and extracted in a Soxhlet apparatus with chloroform for 20-24 hr. Activated copper was used to remove free sulfur from the extracts. An aliquot of the chloroform extract was evaporated under a nitrogen flow to remove the chloroform and to determine the total amount of chloroform-soluble organic matter expressed as parts per million relative to the dry rock sample weight. The chloroform-soluble organic matter was then separated by column chromatography into three fractions: saturated hydrocarbons, aromatic hydrocarbons and asphaltic compounds [i.e. n-heptane, benzene and benzeneemethanol (1: 1) eluates respectively]. Each eluate was evaporated under a nitrogen flow to remove the solvent and then weighed. The saturated hydrocarbon fraction was further characterized by gas chromatography. The gas chromatographic analyses were performed with a 1.8 m x 2 mm packed column (3% GC SE-30 on 100-120 mesh Gas Chrom Q) which was programmed from 80 to 320’ at 12” min-’ for 10min and at lo” C min-’ for 12 min. Organic carbon values were determined gravimetrically by weighing the amount of CO* evolved during the combustion of the previously acid-treated powdered rock sample in a Leco induction furnace. The solvent-insoluble organic matter (kerogen) was isolated by successive HCl-HF-HCI digestion of the rock matrix followed by heavy liquid separation of the acidinsoluble heavy mineral residue. The isolated kerogen was freeze-dried and analyzed for carbon and hydrogen by combustion and measurement of the amounts of CO1 and HZ0 produced. Global Geochemistry Corporation, Monrovia, California, measured the stable carbon isotope ratios of the saturated and aromatic hydrocarbon fractions.

RESULTS

AND

DISCUSSION

Organic richness LEYTHAEUSER(1973) showed that surface weathering of the Upper Cretaceous Mancos Shale of Utah resulted in a maximum loss of 25% of the total organic carbon and up to 50% loss of the dichloromethane-soluble organic matter. Similarly, RADCHENKO et al. (1951) reported that outcrop samples of coal from the Perm region of the USSR contain 39”/, less chloroform-soluble organic matter than equivalent, unweathered samples from the subsurface. Our results for the Phosphoria Formation indicate that the surface and near-surface samples (O-2.6 ft) contain an average of 50% less organic matter soluble in chloroform than the average for the remainder of the core. Also, the samples from the upper 2.Oft of the core have an average of 60% less organic carbon than the deeper part of the core (Fig. 1, Table 1). In addition to the core samples collected, one outcrop sample of the Phosphoria Formation was collected from near the core locality and was analyzed for organic carbon (Fig. 1). As noted previously, the mineralogy is fairly uniform throughout the core, and therefore, the lower amounts of chloroform-soluble organic matter and organic carbon found in the nearsurface interval are not related to changes in mineralogy. Surprisingly, the Pierre Shale core showed almost no variation in either total organic carbon or chloro-

form-soluble organic matter (Table 2). Apparently, at the core locality, no appreciable reduction of the amount of organic matter of the Pierre has occurred as a result of surface weathering. Two outcrop samples (S-17 and S-18) were collected from loose material on the surface of the outcrop and one (S-19) was collected by digging into the odtcrop a few inches. These samples showed no loss of either organic carbon or chloroform-soluble organic matter when compared with the core samples. Hydrocarbon

concentration

and composition

Table 1 and Fig. 2 show that in the upper part of the Phosphoria core ((r2.8 ft) the hydrocarbon fractions are depleted in both the absolute concentration of aromatic hydrocarbons and in the amount of aromatics relative to saturated hydrocarbons. The concentration of aromatic hydrocarbons increases from about 200 ppm at the surface to about 780 ppm at a depth of 2.8 ft. The average concentration over the remainder of the core (2.X-16.Oft) is about 750 ppm. Saturated hydrocarbons also have been lost over the s’ame near-surface interval, but are relatively less depleted than are the aromatic hydrocarbons, resulting in a high ratio of saturated to aromatic hydrocarbons among shallow core samples compared to the deeper part of the core (Fig. 2). Similarly, LEYTHAEUSER(1975, written commun.) found high ratios of saturated to aromatic hydrocarbons among weathered outcrop samples of the Cretaceous Mancos Shale of Utah. The relative concentrations of saturated and aromatic hydrocarbons are sensitive also to both the thermal history of the sample and the type of parent organic matter (BAKER, 1972). Previous investigations have shown that thermal alteration can result in an increase jn the ratio of saturated to aromatic hydrocarbons by thermal generation of saturated hydrocarbons (catagenesis) or preferential destruction of

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4.0 v 6.7

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Fig. 1. Weight % organic carbon vs cored interval depth, or distance from the surface exposure of the outcropPhosphoria Formation.

Subaerial weathering of sedimentary organic matter

Table 1. Geochemical data for the Phosphoria de&, (feet)

&bon (WC.%)

P-l P-2 P-3

0 - 0.5 0.5-0.7 0.8-0.9

1.04 1.38 0.99

663 1050 643

P-4 P-5

2.02.8-

2.6 3.6

2.59 2.74

1094 1649

P-6 P-7 P-8 P-9 P-10

X.84.75.46.16.8-

4.5 5.3 5.8 6.7 7.3

2.38 3.09 3.19 2.62 1.63

1x2 1663 1168 1353 1502

756 1181 807 882 1190

0.61 Il.33 0.43 0.35 0.95

.B.*8.8 9.19.8 10.7-11.3 12.0-12.9 14.5-14.8 15.1-15.9

3.89 3.65 6.70 2.30 2.72 2.32

2095 2479 2868 1384 1858 __

1582 1847 894 1007 1301

0.51 0.30 0.29

number

P-11 P-12 P-13 P-14 P-15 p-16

extract (PPm)

Formation. Duchesne

307

County,

Utah

ratio

@pm) 1.03 0.95 1.20 0.82 0.54

6.82 __ -_

71.52 __ __

1.14 -_ __

6.71 -_

69.42 __

1.16 __

6.49 __ __ __

68.87 __ __ __

1.1, __ -_ __

6.08

65.20

1.12

65.35 -__ __

1.16 __ __

__

__

6.30 __ __

0.61 0.61

__

__

__

__

- indicates not analyzed. aromatic hydrocarbons at higher temperatures (organic metamorphism) (LOUIS and TISSOT, 1967; BAKERand CLAYPOOL,1970; CONNAN, 1972; LE TRAN et al., 1974; CONNAN et al., 1975) so that in cases of extreme weathering it may be difficult to separate the effects of weathering from those of thermal maturation or “incipient metamorphism”. It is possible that some variation in saturated to aromatic hydrocarbon ratios in the Phosphoria core is due to primary differences in the original organic matter deposited with the sediments. This probably explains the variability’in the saturated to aromatic ratio in the deeper part of the core (2.8-14.8 ft). However, it is significant that except for sample P-10 (Table l), variability in the deeper part of the core is limited to a relatively narrow range of values (0.29-0.61), most of which are only about one-half of the average value of the saturated to aromatic ratio over the near-surface interval. It is unlikely, therefore, that high values among the shallow-core and outcrop samples are attributable only to differences in the oriTable Sample number

2. Geochemical Sample depth (feet)

data

ginal organic material from which the hydrocarbons have been derived. Because the loss of total hydrocarbons is accompanied by a relatively greater loss of aromatics than of saturated hydrocarbons, the samples lowest in total hydrocarbons (most thoroughly weathered) also have the highest saturated to aromatic hydrocarbon ratios. Figure 3 demonstrates a linear relationship between these two variables for the shallow Phosphoria core samples (t&2.8 ft); there is no correlation between these same two variabies among the deeper (2.8-16.0 ft) Phosphoria samples (Fig. 3). Apparently, weathering of the shallow samples has overshadowed any variability in the saturated to aromatic ratio which may have been inherited from the original organic matter. The organic carbon content and the concentration OfC 15+ hydrocarbons are depleted by about the same percentage in the weathered zone of the Phosphoria core (approx 50%). Therefore, even though the absolute concentrations of both organic carbon and C,,,

for the Pierre

Organic carbon (wt. X)

Shale, Boulder

Chloroform extract (PPrn)

County,

Total c15+ hydrocarbons (PPm)

Colorado Saturated to aromatic ratio

S-17 S-18 s-19 S-l

CXltc?Xp outcrop outcrop outcrop (0 - 0.5)

0.86 0.75 0.92 0.86

660 640 -621

--485

1.05

s-2 s-3 s-4 s-5 S-6 s-7

l.O2.03.05.07.08.0-

1.5 2.5 3.5 5.5 7.5 8.5

0.85 0.83 0.84 0.89 0.84 0.90

9.0-

9.5

561 499 540 553 504 557 535 586

1.08 0.92 1.15 0.98 1.01 1.31 1.15 1.09

603 607 584 504 536 451 517

1.26 1.37 1.23 1.02 0.95 1.21 1.43

s-9

10.0-10.5

0.97

616 614 603 684 561 582 641 690

s-10 s-11 s-12 s-13 s-14 s-15 S-16

11.0-11.5 12.0-12.5 13 -14 15 -16 17 -18 19 -20 21 -22

0.92 0.89 0.91 0.91 0.86 0.81 0.78

693 705 651 633 573 543 565

S-8

indicates

not analyzed.

0.91

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J. L. CLAYTONand P. J. WETLAND

308

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Saturated/aromotlc ratio

hydrocarbon concentration, parts per million

1.5 hydrocarbon

Fig. 2. Aromatic hydrocarbon concentration (parts per million by weight) and saturated to aromatic hydrocarbon ratio vs core sample depth-Phosphoria Formation.

Similar to the ratio of saturated to aromatic hydrocarbons contained in the solvent extract, the atomic H/C ratio of a given type of kerogen is also sensitive to thermal alteration and is useful as an indicator of the degree of catagenesis or organic metamorphism (SEYLER, 1948; VAN KREVELEN, 1950; MCIVER, 1967; VASSOYEVICHet al., 1970; TISSOT et al., 1974). Because the selective removal of aromatic hydrocarbons during weathering mimics the effect of increasing thermal alteration, the presence of high saturated to aromatic hydrocarbon ratios in outcrop samples may indicate either thermal maturation or severe weathering or a combination of both. Therefore, in order to separate the effects of weathering from those of thermal alteration for studies based on data from outcrop samples, it is important to measure temperature-sensitive par-

hydrocarbons are reduced in the weathered zone, the hydrocarbon-to-organic-carbon ratio is fairly uniform throughout the length of the core. Kerogen

elemental

analysis

Kerogen, or chloroform-insoluble organic matter, was isolated for five Phosphoria core samples and analyzed for per cent. hydrogen and carbon. The atomic H/C ratio for these five samples varies between 1.12 and 1.16, and does not show any systematic change with core depth (distance from the outcrop surface) (Table 1). Although this represents a very limited number of samples, these data indicate that severe weathering does not result in preferential loss of either hydrogen or carbon during the degradation of kerogen.

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Total C,,,hydrocorbon concentration. In parts per million

.

.

.

1 800

Total

I 1000

I 1200

I 1400

I 1600

I 2 1600 20(10

C ,5+ hydrocarbon concentration In parts per milllon

Fig. 3. Relationship between saturated to aromatic hydrocarbon ratio and total Cts+ hydrocarbon concentration for weathered (c-2.8 ft, ‘on left) and unweathered samples (3-15 ft, on right)-Phosphoria

Formation.

309

Subaerial weathering of sedimentary organic matter ameters which are relatively insensitive to weathering. Our data from the Phosphoria Formation core samples indicate that the H/C ratio of kerogen from outcrop samples may provide an estimate of thermal maturation which is not significantly altered by weathering. However, it should be noted that the level of organic maturity of a sample and the type of organic matter present may both determine to what extent (if any) the H/C ratio might be altered by weathering. Carbon isotope composition Figure 4 illustrates the stable carbon isotope composition of the saturated and aromatic hydrocarbon fractions for the Phosphoria core. The data are given as per mil deviations relative to the Chicago PDB standard. The isotopically heaviest aromatic sample is the shallowest one (-26.140/,,) and the samples become isotopically lighter with increasing distance from the surface of the outcrop to a depth of about 3 ft. Below 3 ft the isotopic composition of the aromatic fraction is less variable, ranging between -28.57 and -29.01%” and does not show any trend with increasing depth. Overall, the aromatic fraction in the upper 3 ft has an average 6r3C value of with an average value of - 27.06’:,,, compared -28.804;,, for the rest of the core. The near-surface interval over which this r3 C enrichment occurs coincides with the interval containing the highest saturated to aromatic hydrocarbon ratios. This indicates that the destruction of aromatic hydrocarbons during weathering is accompanied by preferential loss of compounds enriched in the light “C isotope and/or preferential cleavage of r2C-i2C bonds. In contrast to the aromatics, the saturated hydrocarbons do not show any enrichment in 13C among the shallow samples. This may be because the saturated hydrocarbons as a class are more homogeneous than the aromatic fraction (i.e. do not contain the variety of structural types that are in the aromatic fraction). The 613C values for the saturated fraction range from -28.79 to -27.79%, and do not vary systematically with increasing distance from the surface exposure. The average value for the saturated fraction is -28.36%,-about the same as the overall average for the aromatic fraction (-28.10%,). In the case of this particular core the extent of weathering over the shallow core interval, for any given sample can be roughly estimated by the difference in 613C values between the saturated and aromatic hydrocarbon fractions.

solved hump of branched/cyclic compounds as compared with the deeper samples. This difference between the shallow and deeper samples is most pronounced among the lower carbon number molecules (n-Clh to about n-C2J. The higher molecular weight normal paraffins, approximately C25 and longer, are present in about the same abundance relative to the branched/cyclic fraction in both the near-surface and deeper samples. This indicates that weathering results in preferential loss of normal paraffins relative to branched/cyclic compounds and the lower carbon number molecules (in this case
_

b

I

.

. . . -

.

.

-.

Aromatic hydrocarbons

Gas chromatography Typical gas chromatograms of the saturated hydrocarbon fractions from six Phosphoria core samples are shown in Fig. 5. The chromatograms show that the molecular distribution of the alkanes is different in the near-surface samples than in the deeper samples. The two shallowest samples (P-l and P-2) are depleted in normal alkanes relative to the unre-

I .

Saturated -hydrocarbon! r

Fig.

I

I

4. Stable carbon isotope composition of the C15+ saturated and aromatic hydrocarbon fractions vs core sample depth for the Phosphoria Formation. Data are given as per mil deviations relative to the Chicago PDB standard.

310

\ _:’

J. L. CLAYTONand P. J.

n- Cl7

P-l O-O.5

SWETLAND

P-8

feet

5.4-5.8feet

P-2 0.5-0.7feet

6.1-6.7

feet

n- Cl7 10.7-

Retention

time

II .3

feet

I)

Fig. 5. Gas chromatograms of C15+ saturated hydrocarbon fraction, Phosphoria Formation core. Note the relatively greater amounts of unresolved branched/cyclic compounds compared to n-alkanes in the shallow samples (P-l and P-Z). All of these samples were run on the same column under the same instrumental conditions although sample sizes and detector attenuatjon were varied to give comparable traces. Sample P-13 was run at a slightly faster chart speed than the other samples.

below the zone of weathering (high organic carbon content, apparently unaltered saturated hydrocarbon distribution). Possibly this sample is just beginning to undergo weathering so that the organic constituents which are most sensitive to weathering are the only ones which have yet undergone appreciable alteration. SUMMARY AND CONCLUSIONS The magnitude of weathering-induced compositional changes in sedimentary organic matter is variable and probably depends upon a variety of physical and chemical factors including the mineralogy, the type and volume of porosity and pe~eability of the rock, the type of organic matter and its stage of thermal maturation, the climate of the area, the amount

and character of biological activity in the rock, the tectonic history of the area. and the attitude of the bed. As noted previously, core recovery was substantially lower for the Phosphoria Formation core than for the Pierre Shale core. Loss of water circulation occurred frequently during the Phosphoria drilling operation and the samples obtained appeared to be highly fractured. In contrast, circulation loss occurred infrequently during the Pierre Shale coring procedure and the samples obtained did not show any appreciable fracturing. Also, it is significant that the Phosphoria Formation core was taken in an area which receives approximately twice as much precipitation annually as does the Pierre Shale core locality. The climatic differences between the two core localities and the difference in the physical character of the

Subaerial weathering of sedimentary organic matter sections sampled are of fundamental importance in explaining why the Phosphoria Formation shows significant weathering-induced alterations and the Pierre Shale does not. Also, the degree of thermal alteration of the organic matter in outcrops probably has an effect on its response to weathering processes. However, our analyses and previous investigations (CLAYPOOLet al., 1978) indicate that both of the sections sampled in this study have had similar thermal histories. Although the depth below the outcrop surface to which weathering may extend and the magnitude of the associated changes in organic composition are variable, weathering does result in characteristic qualitative effects which are recognizable and can be applied to the interpretation of outcrop data in organic geochemical studies. For the evaluation of petroleum source-rock potential two important qualities to be considered are the amount of organic matter in the rock and its thermal history. Outcrop samples provide minimum values for organic richness measurements. In the Phosphoria core, compared with the average value for the deeper, unweathered part of the core, the near-surface core interval averages 60% less organic carbon and contains an average of 50% less chloroform-soluble organic matter with an average 53% reduction of heavy (C,,,) hydrocarbon concentration. In this case organic-richness estimates based on weathered outcrop data would be low relative to unweathered material three feet below the surface. However, even the most weathered sample of the Phosphoria is still recognizable as above average in organic content, compared to shales in general (HUNT, 1961). In contrast, the Pierre Shale did not show any reduction of organic content at the outcrop surface. Outcrop samples of the Pierre at this particular locality would provide reliable estimates of organic content which are representative of the same unit at depth, that is, below the zone over which weathering might be expected to occur. Data from samples suspected of having undergone severe weathering should be compared with any available subsurface data, or data collected from known fresh surface exposures, to determine if anomalously low organic-richness measurements occur in the outcrop samples. Such a comparison also permits an estimate of the magnitude of weathering-induced alterations. Further, the measurements most sensitive to and diagnostic of weathering (S/A ratio, 613C of the aromatic fraction, gas chromatography (GLC) of the Cl5 + saturated hydrocarbon fraction) should be examined for samples suspected of having been weathered. One effect of weathering, the preferential destruction of aromatic hydrocarbons, mimics the effect of thermal alteration (both processes result in an increase of the saturate to aromatic hydrocarbon ratio). However, other measurements which are also sensitive to temperature history do not show any signifitwo

311

cant alteration due to weathering. The hydrocarbonto-organic-carbon ratio is fairly uniform throughout the length of the Phosphoria Formation core. This ratio has been shown to be affected by thermal alteration (PHILIPPI, 1965; CLAYPWL et al., 1978). Another temperature-sensitive measurement, the atomic H/C ratio of the kerogen, is unchanged in the Phosphoria surface and near-surface samples, although it should be emphasized that only five samples were analyzed for kerogen elemental composition. Further, the gas chromatographic analyses of the saturated hydrocarbons show that even the most drastically weathered samples of the Phosphoria core contain a full carbon-number range of molecules and high amounts of branched and cyclic alkanes relative to normal alkanes (Fig. 5). Extreme thermal alteration, or incipient metamorphism, results in an assemblage of saturated hydrocarbons with a more restricted boilingpoint range and a predominance of straight-chained molecules (CLAYPOOL et al., 1978). It is important, therefore, to analyze both the soluble and insoluble fractions of the organic matter when attempting to infer the thermal history of a sample based on outcrop data. W. KITELEYand E. K. MAUGHAN assisted in the selection of core sample localities. R. A.

Acknowledgements-L.

HILDRETH helped with the coring. Analytical assistance provided by J. P. BAYSINGER, J. M. PATTERSON and A. H. LOVEof the U.S. Geological Survey is appreciated. Illustrations were prepared by T. KOSTICK of the U.S. Geological Survey. We are grateful to G. E. CLAYPOOL, T. D. FOUCH and J. G. PALACAS for critically reviewing the manuscript. This research was carried out under project number 9410-01397 of the Branch of Oil and Gas

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NIXON

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SWETLAND

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