Geochemistry and Pyrolysis of Oil Shales - American Chemical Society

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G e o c h e m i s t r y a n d P y r o l y s i s of Oil S h a l e s

Downloaded by 27.19.2.209 on April 12, 2016 | http://pubs.acs.org Publication Date: August 1, 1983 | doi: 10.1021/bk-1983-0230.ch001

B. P. TISSOT and M. VANDENBROUCKE Institut Français du Pétrole, BP 311, 92506, Rueil-Malmaison, France

Oil shales are defined according to economic criteria : they are rocks yielding commercial amounts of liquid hydrocarbons upon destructive distillation. They contain an organic matter (kerogen) similar to that of petroleum source-rocks. Specific geochemical characteristics of oil shales, as compared to petroleum source-rocks are presented hereunder : low natural thermal evolution, high carbon content, high hydrogen amount of the organic matter. The evolution of some geochemical parameters during pyrolysis is shown for different oil shales. Finally, specific features of the composition of shale oils compared to that of natural petroleums are indicated : presence of unsaturated hydrocarbons, higher amount of heteroatomic compounds. There is no geological or chemical definition of an o i l shale. Any rock yielding oil in commercial amount upon pyrolysis may be considered as an oil shale. The composition of the inorganic fraction may vary from a shale where clay minerals are predominant, such as the Lower Jurassic shales of Western Europe (particularly France and West Germany), to carbonates with subordinate amounts of clay and other minerals, such as the Green River shales of Colorado, Utah and Wyoming. The organic fraction is mainly an insoluble solid material, kerogen, which is entirely comparable to the organic matter present in many petroleum source rocks (1-2). Figure 1 shows the elemental composition of the Green River shales, the Lower Toarcian shales of the Paris Basin and W. Germany and also various o i l shales from different origins. A large number of core samples from the Green River and the Paris Basin shales was taken at various burial depths. They cover the diagenesis, catagenesis and metagenesis stages of thermal evolution (\) (the latter stage was available from the Green River shales only). The diagram shows that these two shales series constitute typical evolution paths of type I and 0097-6156/83/0230-0001S06.00/0 © 1983 American Chemical Society

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


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GEOCHEMISTRY AND CHEMISTRY OF OIL SHALES

0

5

10

15

20 25 30 ATOMIC RATIO 0/Cx100

35

40

45

Figure 1. Van Krevelen diagram showing the elemental composition of oil shale kerogens. The organic constituents of the Green River shales (if) and the Toarcian shales (A) of the Paris Basin are typical kerogens of Types land II, respectively. Other oil shales belong to either Type I (+) or II (M). Key: A, Autun boghead, Permian, France; B, Moscow boghead, Permian, USSR; C, Coorongite, recent, Australia; H, Marahunite, Tertiary, Brazil;!, Iratishales, Permian, Brazil; K and K , Kukersite, Paleozoic, USSR; M, Messel shale, Eocene, W. Germany; R, Kerosene shale, Permian, Australia; S, Tasmanite, Permian, Australia; T torbanite, Carboniferous, Scotland; and T , torbanite, Permian, Australia. The evolution path of humic coals (Type III) (%) is shown for comparison. t

2

r


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Oil Shale Geochemistry & Pyrolysis


1. TISSOT AND VANDENBROUCKE

type I I kerogens according t o the d e f i n i t i o n of T i s s o t et a l ( 3 ) . Furthermore, other o i l shale kerogens belong e i t h e r to type I , such as coorongite and kerosen shales ( A u s t r a l i a ) , t o r b a n i t e (Scot l a n d and S. A f r i c a ) and bogheads ; or t o type I I , such as kukers i t e (USSR), I r a t i ( B r a z i l ) and Messel (W. Germany) shales ; t a s manite ( A u s t r a l i a ) shows an intermediate elemental composition. The e v o l u t i o n path of humic c o a l s i s a l s o shown i n F i g u r e 1 f o r comparison. I t has o b v i o u s l y a lower hydrogen content than any of the o i l shale kerogens, unless they have been deeply a l t e r e d by thermal e v o l u t i o n . I n f r a r e d spectroscopy (Figure 2 and Table I ) of the kerogens (4) from o i l shales shows t h a t a l l of them are r i c h i n a l i p h a t i c bands a t 2900 and 1450 cm r e l a t e d to c h a i n l i k e and c y c l i c s a t u r a t e d m a t e r i a l . However, kerogens of type I , such as Green R i v e r shales and t o r b a n i t e , c o n t a i n a l a r g e r p r o p o r t i o n of long a l i p h a t i c c h a i n s , marked by the a b s o r p t i o n bands at 720 cm"*. Table I : R e l a t i v e importance of a l i p h a t i c bands i n i n f r a r e d spectroscopy of some kerogens from s e l e c t e d o i l shales (arbitrary units).

IR A l i p h a t i c bands (cm~*) K

Type

I

II

Sample

2900 C-H

K

1450 CH +CH 0

K

0

1375 CH«

K

720 (CH ) 0

/Green R i v e r sh. 136.6 (Torbanite 105.3

11.2 10.8

1.5 2.1

1.3 1.1

Toarcian shales • Kukersite Messel shales

10.0 11.5 9.6

2.5 2.8 3.7

0. 0.4 0.2

73.0 72.7 76.0

n>4

The t o t a l o i l y i e l d obtained from the shale upon p y r o l y s i s i s u s u a l l y measured by the standard F i s h e r assay. However, i t i s poss i b l e t o o b t a i n a f a s t and accurate measurement of the o i l y i e l d by u s i n g the Rock E v a l source rock a n a l y z e r ( 5 ) , which operates on small q u a n t i t i e s o f rock, such as 50 or 100 mg. F i g u r e 3 shows the comparison between the value obtained from the Rock E v a l pyrol y s i s and the y i e l d of the F i s h e r assay on the T o a r c i a n shales of the P a r i s B a s i n . A s e r i e s of experiments has been c a r r i e d out t o observe the generation of the d i f f e r e n t c l a s s e s of o i l c o n s t i t u e n t s . A l i q u o t s of two kerogens from the Green R i v e r Shale (type I ) (6) and the Lower Toarcian shales of the P a r i s B a s i n (type I I ) (7) where heated a t a constant h e a t i n g r a t e of 4°C min~l to d i f f e r e n t f i n a l temperatures ranging from 375°C t o 550°C. A humic c o a l from Indon e s i a (type I I I ) was a l s o used f o r comparison ( 8 ) . These v a r i o u s


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GEOCHEMISTRY AND CHEMISTRY OF OIL SHALES


Aliphotic CyH i

Mostly aromatic C;C I

Figure 2. Typical IR spectrum of the kerogen isolatedfrom lower Toarcian shales of the Paris Basin.

©

l100-

o

0

20

40 60 80 • FISCHER ASSAY (kg oil / ton of rock)

Figure 3. Correlation between oil content obtained by the Fischer assay and by the Rock-Eval pyrolysis.



1.

TISSOT AND VANDENBROUCKE

Oil Shale Geochemistry & Pyrolysis

samples have experienced a comparable thermal h i s t o r y i n g e o l o g i c a l c o n d i t i o n s : they belong t o the f i n a l stage of d i a g e n e s i s (1) ( v i t r i n i t e r e f l e c t a n c e between 0.4 and 0.5%). The mass balance of the o r g a n i c f r a c t i o n i s shown i n F i g u r e 4 as a f u n c t i o n of the f i n a l temperature. At 375°C, most of the o r g a n i c m a t e r i a l i s s t i l l made of kerogen. With i n c r e a s i n g f i n a l temperature an i n c r e a s i n g l y l a r g e f r a c t i o n i s converted t o o i l which condensates i n a c o l d t r a p , l e a v i n g a s o l i d r e s i d u e or char. The non-recovered f r a c t i o n i s assumed t o be mainly carbon d i o x i d e , water and l i g h t hydrocarbons non condensable i n the c o l d t r a p . A somewhat d i f f e r e n t behaviour i s observed a c c o r d i n g t o the t y pe of kerogen : the Green R i v e r shale (type I) r e q u i r e s h i g h e r temperatures, as the maximum r a t e of c o n v e r s i o n occurs c a . 475°, versus 425-450°C f o r the P a r i s B a s i n shale (type I I ) and the humic c o a l (type I I I ) . Furthermore the conversion r a t i o and the compos i t i o n o f the products are d i f f e r e n t : the t o t a l c o n v e r s i o n r a t i o (condensate p l u s non-recovered products) decreases from over 80% f o r type I , t o 55% f o r type I I and 35% o n l y f o r c o a l . The amount of o i l (condensate) generated i s r e l a t i v e l y h i g h i n kerogens from o i l shales : 62% f o r type I and 37% f o r type I I , whereas i t i s low (less than 12%) f o r c o a l . This i s partly due t o an important generation of carbon d i o x i d e and water from humic c o a l s . The r e l a t i v e p r o p o r t i o n of hydrocarbons ( s a t u r a t e d , u n s a t u r a t e d , a r o matics) compared t o N,S,0 - compounds a l s o decreases from type I to type I I I . The t o t a l amount and composition of the hydrocarbons generated i s shown i n F i g u r e 5. The Green R i v e r o i l shale (type I) produces mainly l i n e a r or branched hydrocarbons, whereas the P a r i s B a s i n shale (type I I ) generates mainly c y c l i c - p a r t i c u l a r l y aromatic hydrocarbons. The percentage of aromatics i s a l s o important i n c o a l p y r o l y s i s , but the a b s o l u t e amount i s much s m a l l e r . The bottom p a r t of F i g u r e 6 shows the d i s t r i b u t i o n of n-alkanes i n shale o i l s : i t i s r e g u l a r l y decreasing from C17 t o C30 i n the o i l d e r i v e d from type I I kerogen, which i s a f l u i d s y n t h e t i c o i l ; i t i s r e l a t i v e l y f l a t up t o C30 (type I) o r even i n c r e a s i n g towards a C25-C29 maximum (type I I I ) i n the two other s y n t h e t i c o i l s which have a waxy c h a r a c t e r . Furthermore, a s l i g h t predominance of the odd-numbered molecules (C25, 2 7 ' 29^ ^ i °il d e r i v e d from humic c o a l p o i n t s t o a c o n t r i b u t i o n of n a t u r a l waxes from higher p l a n t s t o the o r g a n i c m a t e r i a l . A d i r e c t p y r o l y s i s - g a s chromatography of the kerogens was a l s o performed and i s presented i n F i g u r e 7 (9). The chromatograms taken a t p y r o l y s i s temperature of 475°C show the t o t a l d i s t r i b u t i o n of hydrocarbons, w i t h the r e l a t i v e importance of l o n g - c h a i n molecules up t o C30 i n types I and I I I . I t a l s o shows the importance of l o w - b o i l i n g aromatics (B : benzene ; T : toluene ; X : xylenes) generated from humic c o a l (type I I I ) as compared t o those generated from o i l shales (types I and I I ) . Composition of the s o l i d o r g a n i c r e s i d u e of p y r o l y s i s was a l s o analyzed i n order t o f o l l o w the p r o g r e s s i v e change from the immature kerogen t o the f i n a l char. F i g u r e 8 presents the elemenc

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Geochemistry and Pyrolysis of Oil Shales - American Chemical Society

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