Upper Devonian Black Shales of the Eastern ... - ACS Publications

10 U p p e r D e v o n i a n B l a c k Shales o f the E a s t e r n U n i t e d States Organic Geochemical Studies—Past and Present 1

Downloaded by UNIV OF ARIZONA on October 14, 2016 | http://pubs.acs.org Publication Date: August 1, 1983 | doi: 10.1021/bk-1983-0230.ch010

IRVING A. BREGER, PATRICK G. HATCHER, and LISA A. ROMANKIW—U.S. Geological Survey, Reston, VA 22092 FRANCIS P. MIKNIS—U.S. Department of Energy, Laramie Energy Technology Center, Laramie, WY 82071 GARY E. MACIEL—Colorado State University, Fort Collins, CO 80523 Upper Devonian black shales of the eastern United States have long been known as major gas producing shales. Our solid-state C nuclear magnetic resonance (NMR) studies of the kerogen in shales of the Lower Huron Member of the Ohio Shale have demonstrated a regional gradient in carbon aromaticity that parallels both the known metamorphic gradient and trends normal to inferred paleoshorelines. The kerogen is essentially coal-like throughout most of the basin with aromaticities of 0.50 or greater. Large changes in carbon aromaticities are observed along an east-west line in our study area, the western half of the basin where maturation levels are low. We believe that the eastward increase in aromaticity is related to increased contributions of vascular plant remains as one approaches paleoshorelines. It is likely that a large proportion of the natural gas in these shales evolves from low-rank maturation of the coaly kerogen. 13

The Upper Devonian black shales of the eastern United States are part of an eastward-thickening deltaic wedge in the Appalachian basin. The black shales extend from central New York southward and westward to Pennsylvania, Ohio, Kentucky, West Virginia, Tennessee, and Alabama. Similar black shales are present in the I l l i n o i s and Michigan basins. Regionally, the more important black shales are known by several different names. In New York, they are the Geneseo, Rhinestreet, and Dunkirk Shale Members of the Genesee, West Falls, and Perrysburg Formations, respectively. The Ohio Shale is present in Ohio, Kentucky, and West Virginia; the Chattanooga Shale is in Kentucky and Tennessee. For many years these shales have 'Deceased This chapter not subject to U.S. copyright. American MiknisPublished and McKay;1983, Geochemistry and Chemical Chemistry ofSociety Oil Shales ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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e l i c i t e d much i n t e r e s t because t h e y a r e a m a j o r s o u r c e of n a t u r a l gas. I t i s of p a r t i c u l a r i n t e r e s t that these shales have r e l a t i v e l y minor amounts of known a s s o c i a t e d crude o i l compared w i t h n a t u r a l gas i n s p i t e of the f a c t that they have some of the c h a r a c t e r i s t i c s normally a t t r i b u t e d to source beds f o r crude o i l (e.g. o r g a n i c c a r b o n c o n t e n t ) . I t has been p r o p o s e d t h a t the o r g a n i c - r i c h b l a c k - s h a l e f a c i e s o r i g i n a t e d from s h a l l o w - water (200m or l e s s ) d e p o s i t i o n i n e u x i n i c e n v i r o n m e n t s (1.), though more r e c e n t l y a deep-water d e p o s i t i o n a l model has been suggested (.2). These black shales were deposited i n the Chattanooga Sea, a southwest t r e n d i n g marine b a s i n bounded i n the south and east by Acadian orogenic highlands. The b a s i n extended from Alabama and Tennessee northward i n t o Kentucky, V i r g i n i a , W. V i r g i n i a , Ohio, Pennsylvania, and New York. N a t u r a l gas i n and above t h e s e s h a l e s i s f o u n d t h r o u g h o u t the b a s i n . S t u d i e s by C l a y p o o l and c o w o r k e r s (3_) have shown t h a t the gas i s " d r y " i n the e a s t e r n p a r t of the b a s i n ( i . e . , contains mostly methane) and becomes "wet" towards the north and west ( i . e . , o t h e r h y d r o c a r b o n homologs such as ethane and propane c o n t r i b u t e to the gas). This data combined w i t h data on the s t a b l e carbon i s o t o p i c compositions of the gas l e d Claypool et a l . (4.) t o c o n c l u d e t h a t the c o m p o s i t i o n of the o r g a n i c m a t t e r i s e s s e n t i a l l y u n i f o r m t h r o u g h o u t the b a s i n and t h a t thermogenic processes were s o l e l y r e s p o n s i b l e f o r the production of n a t u r a l gas i n the s h a l e s . Deeper b u r i a l i n the e a s t e r n p a r t s of the b a s i n apparently l e d to e v o l u t i o n of "post-mature" dry gas (4). The model p r e s e n t e d by C l a y p o o l et a l . (3) i s i n d i r e c t c o n t r a s t t o the model p r o p o s e d e a r l i e r by B r e g e r and Brown (5,6). B r e g e r and Brown p r o p o s e d t h a t the s o u r c e of o r g a n i c m a t t e r i s a p r i m a r y d e t e r m i n a n t f o r the e v o l u t i o n of gas. S h a l e s c o n t a i n i n g o r g a n i c m a t t e r t h a t i s m a i n l y d e r i v e d f r o m t e r r e s t r i a l v a s c u l a r p l a n t s tend t o produce gas w i t h i n c r e a s i n g m a t u r a t i o n (7_) and t o o c c u r i n p a r t s of the b a s i n p r o x i m a l to the p a l e o s h o r e l i n e ( i . e . , t o the s o u t h and the e a s t ) . D i s t a l t o the p a l e o s h o r e l i n e , i n more "open" m a r i n e p o r t i o n s of the b a s i n , a l a r g e r c o n t r i b u t i o n of aquatic or a l g a l t y p e o f o r g a n i c m a t t e r i s o b s e r v e d (5,6). T h i s t y p e of o r g a n i c m a t t e r i s more l i k e l y t o produce o i l or l i q u i d h y d r o c a r b o n s h a v i n g m o l e c u l a r w e i g h t s h i g h e r t h a n t h a t of methane. These h y d r o c a r b o n s , when m i x e d w i t h methane produced f r o m c o a l y or "humic" kerogen, y i e l d "wet" gas. The Breger and Brown model does not, however, e x p l a i n the s t a b l e c a r b o n i s o t o p e c o m p o s i t i o n s of e i t h e r the gas or the kerogen i n the shales. Apparently, the kerogen becomes i s o t o p i c a l l y l i g h t e r (depleted i n C) as one progresses away from the p a l e o s h o r e l i n e s (_8). Maynard (_8) p r o p o s e d t h a t the i s o t o p i c t r e n d s r e f l e c t e d the m i x i n g of i s o t o p i c a l l y h e a v i e r v a s c u l a r p l a n t components w i t h i s o t o p i c a l l y l i g h t e r a l g a l components. Unfortunately, the carbon i s o t o p i c r e l a t i o n s h i p between v a s c u l a r and a l g a l components i n modern sediments i s the reverse of the

Miknis and McKay; Geochemistry and Chemistry of Oil Shales ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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above r e l a t i o n s h i p , but Maynard s u g g e s t e d s e v e r a l p o s s i b l e explanations that might account f o r the reversed trend. Though t h i s trend i s opposite t o expected trends based on modern analogs, i t i s c o n s i s t e n t w i t h the trends expected from Claypool's model (3_) which suggests that maturation e f f e c t s were p r i m a r i l y r e s p o n s i b l e f o r i s o t o p i c gradients. A s i m i l a r trend i s o b s e r v e d i n t h e s t a b l e c a r b o n i s o t o p i c c o m p o s i t i o n s of t h e gas ( 3 , 4 ) and i s used t o s u p p o r t the t h e r m o g e n i c h y p o t h e s i s . U n f o r t u n a t e l y , no simple explanations f o r the i s o t o p i c composit i o n of the gas a r e a p p a r e n t , and the q u e s t i o n as t o w h e t h e r t h e r m o g e n i c p r o c e s s e s o r s o u r c e r e l a t i o n s h i p s are r e s p o n s i b l e f o r the t y p e and d i s t r i b u t i o n of n a t u r a l gas i s s t i l l open t o debate. Many b e l i e v e t h a t t h e r e a r e r e g i o n a l v a r i a t i o n s i n the composition of the organic matter i n the Upper Devonian shales and attempts have been made to q u a n t i f y and map the d i s t r i b u t i o n of the t e r r e s t r i a l and a q u a t i c components (8,9). As n o t e d above, s t a b l e c a r b o n i s o t o p e s have not p r o v i d e d a r e l i a b l e q u a n t i t a t i v e d i s t i n c t i o n between t e r r e s t r i a l and aquatic carbon. Neither have elemental compositions, because such measurements are not s e n s i t i v e t o s u b t l e changes i n kerogen type and maturat i o n e f f e c t s o f t e n cannot be s e g r e g a t e d f r o m s o u r c e changes. M i c r o s c o p i c examinations have revealed that a major f r a c t i o n o f the o r g a n i c m a t t e r i s amorphous (j9). However, a t t e m p t s t o c l a s s i f y t h i s m a t e r i a l as aquatic should be viewed w i t h s k e p t i cism because t e r r e s t r i a l m a t e r i a l can a l s o e x i s t i n an amorphous s t a t e (6.). M i c r o s c o p i c e x a m i n a t i o n s by Z i e l i n s k i (9) r e v e a l e d the p r e s e n c e o f i n c r e a s i n g amounts o f woody o r c o a l y p l a n t fragments i n Upper Devonian shales as one approaches the eastern part o f the b a s i n . This suggests that the kerogen becomes more coaly towards the eastern p a l e o s h o r e l i n e . Recently, s o l i d - s t a t e jjMR (using cross p o l a r i z a t i o n w i t h magic-angle spinning) has been suggested as a method of d e t e r m i ning the type of kerogen i n shales (10). The degree of a r o m a t i c i t y of Holocene humic substances has been proposed as a method to determine the d i s t a l / p r o x i m a l r e l a t i o n s h i p s to modern shorel i n e s (11). I n t h i s instance the amount o f aromatic carbon was d i r e c t l y r e l a t e d t o the c o n t r i b u t i o n of v a s c u l a r p l a n t s w h i c h s u p p l y l i g n i n t o the s e d i m e n t s . Humic s u b s t a n c e s i n m a r i n e sediments having no c o n t r i b u t i o n s from v a s c u l a r p l a n t s show very low a r o m a t i c i t i e s ( 10%) i n t h e i r NMR s p e c t r a (1_1)· A l t h o u g h c o a l i f i c a t i o n o r m a t u r a t i o n p r o c e s s e s a l t e r humic substances, the b a s i c c h e m i c a l f r a m e w o r k i s e x p e c t e d t o s u r v i v e . Thus, woody d e b r i s i s c o a l i f i e d to a substance whose NMR spectrum i s mostly aromatic, whereas a l g a l kerogen y i e l d s a substance whose NMR spectrum i s mostly a l i p h a t i c (12). Consequently, one should be able to q u a n t i f y the v a s c u l a r and nonvascular p l a n t c o n t r i b u t i o n s to kerogen simply on the b a s i s of a r o m a t i c i t y . T h i s r e l a t i o n s h i p , however, i s not as s t r a i g h t f o r w a r d as one might e n v i s i o n . Maturation or metamorphic a l t e r a t i o n induced



184


by t i m e and t e m p e r a t u r e d u r i n g b u r i a l can l e a d t o changes i n kerogen s t r u c t u r e . These s t r u c t u r a l changes c o u l d e f f e c t i v e l y i n v o l v e c r a c k i n g o f a l i p h a t i c c a r b o n - c a r b o n bonds, p r o d u c i n g mostly l i q u i d or gaseous hydrocarbons and l e a v i n g the r e s i d u a l k e r o g e n r e l a t i v e l y d e p l e t e d of a l i p h a t i c c a r b o n . Thus i n t h e c o u r s e of m a t u r a t i o n , the k e r o g e n c o u l d become p r o g r e s s i v e l y more aromatic. D i f f e r e n t i a t i o n of whether a r o m a t i c i t y i s i n t r o duced by mixing of a l g a l and v a s c u l a r p l a n t "end members" or by m a t u r a t i o n c o u l d be d i f f i c u l t w i t h o u t a d d i t i o n a l s u p p o r t i n g data. At the present time we are u n c e r t a i n about the magnitude of the e f f e c t maturation has on kerogen a r o m a t i c i t y although a g e n e r a l l y i n c r e a s i n g trend i n a r o m a t i c i t y i s observed f o r c o a l as a f u n c t i o n of rank (13). The purpose of t h i s paper i s t o r e p o r t the p r e l i m i n a r y r e s u l t s of a s t u d y of Upper D e v o n i a n s h a l e s by C NMR i n an attempt to understand r e g i o n a l v a r i a t i o n s i n kerogen composition as r e l a t e d to the e v o l u t i o n of o i l and gas. Laboratory and F i e l d Methods Sample s e l e c t i o n . Samples to be analyzed were chosen from among a s u i t e of samples taken from the Huron Member of the Ohio Shale and the R h i n e s t r e e t S h a l e Member of the West F a l l s F o r m a t i o n . These s t r a t i g r a p h i e u n i t s were sampled from cores c o l l e c t e d by the Department of Energy's Morgantown Energy Technology Center under the a u s p i c e s of the E a s t e r n Gas S h a l e P r o j e c t . They were s e l e c t e d f o r study because they are a r e a l l y e x t e n s i v e throughout the Appalachian b a s i n (14,15). The cores were sampled at 2-8 m i n t e r v a l s , depending on f o r m a t i o n thicknesses. One sample from each d r i l l h o l e ( T a b l e I and F i g u r e 1) was s e l e c t e d f o r processing. Sample préparât i o n . Core s a m p l e s were washed w i t h c o l d t a p w a t e r and a s t i f f w i r e b r u s h , t h e n a i r - d r i e d . C o a l i f i e d wood fragments were scraped from bedding-plane surfaces. Elemental a n a l y s e s and C NMR s p e c t r a f o r t h e s e c o a l samples were obtained without f u r t h e r p r e p a r a t i o n . A p p r o x i m a t e l y 100-200 grams of the s h a l e samples were c r u s h e d i n a s t e e l jaw c r u s h e r and t h e n ground t o -200 mesh i n an agate s h a t t e r box. S e v e r a l grams of the ground s h a l e were r e t a i n e d f o r carbon and ash d e t e r m i n a t i o n s . A f t e r g r i n d i n g , the p u l v e r i z e d shale samples were e x t r a c t e d f o r 4-7 days i n a l a r g e S o x h l e t a p p a r a t u s , u s i n g c e l l u l o s e e x t r a c t i o n t h i m b l e s and a m i x t u r e of benzene and m e t h a n o l ( v : v / l : l ) as the e x t r a c t i o n s o l v e n t . S h a l e r e s i d u e s were a i r - d r i e d and a s e r i e s of a c i d e x t r a c t i o n s was then used to i s o l a t e and concentrate the organic matter (kerogen) from the m i n e r a l m a t r i x . The bitumen-extracted s h a l e s were f i r s t t r e a t e d w i t h 0.1 N HC1 t o remove c a r b o n a t e s . A f t e r a d i g e s t i o n p e r i o d of 3-5 days, the HC1 was d e c a n t e d and the s h a l e s were washed w i t h d e i o n i z e d w a t e r . The s h a l e s were


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185

T a b l e I. L o c a t i o n d e s c r i p t i o n s , mean l o s s e s on i g n i t i o n , and mean t o t a l carbon contents of the zones sampled i n t h i s study


Core designation Ohio OH-3 0H-4 0H-5 0H-6-1 OH-6-2 OH-7 OH-8 OH-9

County

Knox Ashtabula Lorain Gallia Gallia Trumbull Noble Meigs

West V i r g i n i a WV-1 Jackson WV-3 Lincoln Mason WV-5 Kentucky KY-1 KY-4 KY-5

Perry Johnson Rowan

Pennsylvania PA-5

Lawrence

Depth of sample(m)

329 366 331 762 697 656 1059 916

Mean l o s s on i g n i t i o n *

13.8 18.4 13.8 12.3 15.1 19.4 15.6 11.5

9.86

1042 1040 925

23.2

931 432 191

15.5 16.6

1147

-

Mean t o t a l carbon content

6.8 8.1 6.7 5.2 6.7 11 8.1 4.8

3.8 7.1 16

-

5.1 8.4 7.9

6.49

0.9]

* l o s s on i g n i t i o n at 750°C

t h e n t r e a t e d w i t h a m i x t u r e o f 48% HF and 36% HC1 ( v : v / l : l ) t o remove s i l i c a t e s . F o l l o w i n g d i g e s t i o n on a steam bath, the a c i d was decanted, and r e p l a c e d by f r e s h a c i d mixture. T h i s procedure was r e p e a t e d a p p r o x i m a t e l y 10 t i m e s , a f t e r w h i c h t h e kerogen was r i n s e d w i t h 6N HC1, thoroughly washed w i t h d e i o n i z e d water, f r e e z e - d r i e d , and weighed. E l e m e n t a l a n a l y s e s . E l e m e n t a l (C,H,N, and 0) a n a l y s e s o f t h e k e r o g e n i s o l a t e s , and a n a l y s e s o f t h e t o t a l c a r b o n c o n t e n t o f the whole s h a l e were o b t a i n e d , d i r e c t l y , u s i n g a C a r l o E r b a Model 1106 E l e m e n t a l A n a l y z e r ( a n y use o f t r a d e names i n t h i s p u b l i c a t i o n a r e f o r d e s c r i p t i v e purposes o n l y and does n o t c o n s t i t u t e indorsement by the U. S. G e o l o g i c a l Survey. Element a l analyses (C,H, and N) of the c o a l samples were accomplished using a Perkin-Elmer Model 240 Elemental Analyzer, w i t h oxygen contents c a l c u l a t e d by d i f f e r e n c e . The ash content of the whole s h a l e was d e t e r m i n e d by c o m b u s t i o n o f 10-20 mg a l i q u o t s i n a m u f f l e f u r n a c e a t 750°C. A l l samples ( c o a l and k e r o g e n ) were



186


#

Huron Member of Ohio Shale

A

Rhinestreet Shale Member of West Falls Formation

— — — approximate boundary of the Appalachian basin

Figure 1. Locations of drill cores sampled in this study.


10.


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187

c o r r e c t e d f o r moisture content by heating i n an oven at 60°C f o r 2-3 hours p r i o r to a n a l y s i s .

ιο


A J

NMR analyses. C NMR s p e c t r a of the c o a l and kerogen samples were obtained on a spectrometer at C o l o r a d o S t a t e U n i v e r s i t y u s i n g the c r o s s p o l a r i z a t i o n m a g i c - a n g l e s p i n n i n g (CPMAS) t e c h n i q u e d e s c r i b e d by B a r t u s k a et a l . (16). A l l s p e c t r a were a c q u i r e d at a f i e l d s t r e n g t h of 2.5 T e s l a . S o l i d , powdered samples, packed i n t o a b u l l e t - t y p e r o t o r , were spun at frequen c i e s of 3.9-4.1 kHz. The pulse r e p e t i t i o n time was 1 second and the c o n t a c t t i m e was 1 m i l l i s e c o n d . The number of p u l s e s r e q u i r e d t o o b t a i n s p e c t r a h a v i n g good s i g n a l - t o - n o i s e r a t i o s ranged from 3000 to 40,000, w i t h an average of 4000-5000 pulses b e i n g s u f f i c i e n t f o r most samples. Carbon a r o m a t i c i t i e s were c a l c u l a t e d by i n t e g r a t i n g peaks f o r a r o m a t i c carbons (100170ppm) and d i v i d i n g the r e s u l t i n g a r e a s by t h o s e of the combined aromatic and a l i p h a t i c (0-70ppm) peaks. E r r o r s i n the measurement of a r o m a t i c i t i e s are estimated to be +0.01 (5) at an a r o m a t i c i t y value of 0.50. Results and D i s c u s s i o n Locations, d e s c r i p t i o n s , and mean t o t a l carbon contents of the zones sampled i n t h i s study are given i n Table I. The mean t o t a l carbon contents of the whole shale samples g e n e r a l l y range f r o m a p p r o x i m a t e l y 4-16 p e r c e n t . These v a l u e s a r e s i m i l a r t o the t o t a l organic carbon content of most o r g a n i c - r i c h Devonian black shales i n the Appalachian b a s i n (17). D r i l l core l o c a t i o n s a r e shown i n F i g u r e 1. We a t t e m p t e d to o b t a i n an adequate sampling of the e n t i r e Appalachian b a s i n , but our coverage was l i m i t e d to samples from d r i l l c o r e s c o l l e c t e d m o s t l y i n the western part of the b a s i n .

Elemental compositions. C o a l i f i e d wood fragments, which i n d i s p u t a b l y r e p r e s e n t t e r r e s t r i a l m a t e r i a l , have the f o l l o w i n g moisture-and-ash-free average elemental compositions: Carbon

83.6

+. 2.3

percent

Hydrogen

6.6

+_ 0.5

percent

Nitrogen

2.3 .+0.2

percent

Oxygen

8.2

percent

+_ 2.7

These v a l u e s a r e t y p i c a l of h i g h v o l a t i l e b i t u m i n o u s c o a l s , although the n i t r o g e n content i s s l i g h t l y elevated. The data, when p l o t t e d on a Van K r e v e l e n d i a g r a m ( a t o m i c H/C v s . a t o m i c 0/C r a t i o s , F i g u r e 2), c l u s t e r i n a r e g i o n t h a t i s s l i g h t l y above the development t r e n d f o r v i t r a i n i n most humic c o a l s . G e n e r a l l y , the p o i n t s l i e i n the range e x p e c t e d f o r h i g h -


188



1.2

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Upper Devonian Black Shales of the Eastern ... - ACS Publications

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