Preservation and characteristics of plant remains in Iranian natural bitumens

International Journal o f Coal Geology, 4 (1985) 321--334

321

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

PRESERVATION AND CHARACTERISTICS OF PLANT REMAINS IN IRANIAN NATURAL BITUMENS

FARIBORZ GOODARZI

Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 330333rd Street N.W., Calgary, Alta, T2L 2A7, Canada (Received March 1, 1 9 8 4 ;a c c e p te d for publication September 5, 1984)

ABSTRACT Goodarzi, F., 1985. Preservation and characteristics o f plant remains in Iranian natural bitumens. Int. J. Coal Geol., 4: 321- 334. Well preserved m o n o c o t y l e d o n and pteridophyta remains were found in natural bitumen from Iranian, tar pit and oil seeps, using reflected and fluorescence light microscopy. The plant tissue are impregnated or partially replaced by bitumens, the extent of this replacement is noted. The retention of original botanical structure in natural bitumen indicates that the natural bitumen is an excellent source o f paleoenvironment, and palaeontological information. INTRODUCTION

Oil seeps and tar pits are excellent sources of fossils. It is well known that tar pits act as natural traps for flora and fauna. The trapped materials gradually become impregnated by bitumen and sink to the floor of the tar pit. They become incorporated into a layer of tar and sediment and slowly become bitumenised, preserving their original structure. The Rancho La Brea Tar Pits of California are a good example of this p h e n o m e n o n and preserve an extensive Pleistocene flora and fauna (Woodward, 1971 ; Maloney, 1974). The western Iranian Territory at the foot of Zagros mountains, from Dehluran to Bushehr on the Persian Gulf, is rich in bitumen deposits, tar pits and oil seeps. When the natural bitumen from this area (Fig. 1) was examined using a reflected light and fluorescence microscope it was found that t h e y contained plant remains, preserved in the bitumen matrix. LOCATION AND GEOLOGY

The Asmari limestone is the main oil reservoir in southwest Iran (Fig. 1). Oil seeps and bitumen deposits occur in this heavily faulted area where the Asmari formation is fractured, allowing oils to seep to the surface (Link, 1952). Asphaltic pools and springs are associated with these oil seeps.

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322 Bitumen samples were collected from a group of asphaltic pools near the t o w n o f Dehluran. The bitumens can be classified as Gilsonite (Williams and Goodarzi, 1981), two samples of which were found to contain plant remains. Oil pools at Dehluran are fed by springs and occasionally overflow to cover the surrounding area. Inspissation results in a layer of tar which becomes sticky during the summer months. The bitumen traps anything which comes in contact with it. When the weather cools, the bitumen layer becomes hard and is covered by layers o f dust. Rain washes sediments onto the top of the bitumen and protects it from further weathering. Materials

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323 trapped in the bitumen layers become fossilised in the course of time and slowly a sequence o f tar and alluvial layers is formed. Mamatain bitumen was collected from Mamatain asphaltic deposits, which cover a large area and ranges from 40 cm to 10 m thick. The tar and asphalt deposits near Dehluran oil springs are geologically fairly recent and do n o t extend more than 1.5 m below the surface of the ground. This deposit was estimated to be about 1000 years old, whereas the Mamatain deposits are probably o f Holocene age (N.N. Lay, pers. commun., 1983). The present study attempts to demonstrate the degree of preservation and the extent o f bitumenisation of the plant remaifis in natural bitumen.

Experimental The bitumen samples were prepared as relief polished blocks, and the polished surfaces examined using a Leitz MPVII microscope fitted with water immersion objective (N.A. 0.80 × 25). Reflectance measurements were carried o u t under water immersion at 546 nm wavelength and under standard conditions (I.C.C.P., 1971). The fluorescence spectra {400--700 nm; Figs. 2 and 3) were measured using an UV light excitation filter (BG32 and UG1, barrier filter K430) and with a vertical illuminator using air objective (N.A. 0.35 X 16). The values for the reflectance in oil were calculated from the measured values in water using Jacob's (1975) formula. Photographs of botanical structures were taken using (a) fluorescence from blue light excitation (filter BG32 and BG12, barrier filter K510 and K530), and (b) in the reflected, polarised light, air using water objective × 25 and plane-polarised light. The photographs are mostly composites o f a number of fields o f view. Table 1 includes reflectance, fluorescence and petrographical data for the bitumens. TABLE

1

B i t u m e n s a m p l e s - reflectance, fluorescence and petrographic data Sample

Locality

Appearance

DH1

Dehluran asphaltic spring No. 1

Black, heterogeneous, sticky solid

DH3

Dehluran asphaltic spring No. 3

M1

Softening point, oC

%R w

%Roi 1

Solubility

70

0.95

0.11

S o l u b l e in i m m e r s i o n oil

Black, brittle, solid

210

0.45

0.11

S o l u b l e in i m m e r s i o n oil

Mamatain asphaltic deposit

Black, brittle, solid

N.D.

1.70

0.24

S o l u b l e in i m m e r s i o n oil

M3

Mamatain asphaltic deposit

Black, brittle, solid

N.D.

1.01

0.14

S o l u b l e in i m m e r s i o n oil

M5

Mamatain asphaltic deposit

Black, brittle, solid

N.D.

1.01

0.14

S o l u b l e in i m m e r s i o n oil

324 RESULTS AND DISCUSSION

The plant remains found in the bitumens consist of parts of members of the monocotyledon (e.g. Maize) and pteridophytal {e.g. Fern) families (e.g. stem [Plates 1 and 5a] , stem and leaf [Plate 2], Fern rachis and rachilla [Plates 3, 5b and d], and rootlet [Plates 4 and 5c]) and are impregnated by the bitumen matrix to varying degrees. The extent of this impregnation is

PLATE 1

Monocotyledon stem in bitumen (B), cuticle (C), epidermis (EP), hypodermis (H), cortex (C), vascular bundles (V) and pitch cavity (P). Reflected light, water immersion, plane polarised.

325

estimated by the changes in fluorescence colour, the properties of fluorescence spectra, and reflectance value. The bitumen matrices examined in this study were soluble in immersion oil and had oil reflectance values (% Ro) of 0.13--0.24, which classes them as Gilsonites (Jacob, 1975; Khavari-Khorasani, 1975; Williams and Goodarzi, 1981). The extent of bitumen impregnation for a monocotyledon stem can be assessed in Plates 1, 2 and 5a. Details of morphology are given in captions to the plates.

PLATE 2 Monocotyledon stem with sheathing leaf in bitumen showing leaf sheath (L), leaf mid rib (LR), cuticle (C), epidermis (EP), scleronchyma (S), vascular bundle (V) and pitch cavity (P). Reflected light, water immersion plane polarised.

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327 The changes in fluorescence, from dark brown in the Gilsonite to light brown for the stem tissue adjacent to the Gilsonite matrix (the cuticle, hypodermis, and part of the cortex), and to yellow for the inner tissue o f the stem cortex, (Fig. 2) probably reflects the degree of penetration o f bitumen into the plant remains. The reflectance also decreases from 0.36% for the stem tissue adjacent to Gilsonite matrix (a) to 0.15% for the inner tissue of the stem (b). This probably indicates that a change in properties of stem tissue has taken place. The variations between the fluorescence spectra for Gilsonite (Fig. 2c), adjacent stem tissue (Fig. 2b), and the inner tissue of the stem (Fig. 2a), reflect the change in properties of the tissue resulting from the invasion of the bitumen. Another possibility for the variation in the fluorescence colour is the filtration of lower molecular weight hydrocarbons through the stem tissue into the pitch cavity of the stem, causing a shift o f the fluorescence to a colour and properties of shorter wavelength, e.g. from brown to yellow (Table 2). The sections of rachis and rachilla (Plates 3, 5b and d) of a fern also show the extent of bitumenisation of plant tissue. The plant tissues are largely bitumenised, but the morphological identity of plant tissues are still evident, e.g. cuticle (C), epidermis (Ep), parenchyma (P) cells containing fibre bundle (FB), endodermis (En) and xylem (X) (Plate 3). The fluorescence microscopy also shows similar morphology for another section of rachis (Plate 5b), b u t it is also evident that some bitumenisation of tissue has taken place. The bitumenised cuticle is only faintly present, b u t the endodermis (En) and xylem (X) still show their original identity. The Gilsonite TABLE 2 Comparison of fluorescence properties of natural bitumen matrix and impregnated plant tissues Sample

Fluorescence colour

M5 (Plate 1) (a) Bitumen matrix dark brown (b) Plant tissue impregnated by bitumen brown (c) Plant tissue, inner zone yellow ML (Plate 4) (a) bitumen matrix brown (b) plant tissue impregnated by bitumen light brown (c) plant tissue, inner zone greenish yellow

kmax

Red/Green quotient

697

2.09

642

1.87

583

1.13

696

2.11

638

1.58

529

0.77

C

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333 matrix shows a light brown fluorescence, but the endodermis and phloem show a yellowish-green fluorescence, which probably indicates that these tissues are moderately affected by the process of bitumenisation. The rootlet in Plates 4 and 5c shows root vascular bundles (V), xylem cells (X) and parenchyma (P). The similarity of the appearance of the parenchyma (P) to bitumen matrix (B) under reflected light is evident (Plate 4), b u t it is also apparent that the root vascular bundle (V) and xylem cells (X) are not affected greatly by bitumen matrix. However, the various parts become more evident under fluorescent light where the bitumen matrix (B) fluoresces light brown, the parenchyma (P) fluoresce dark brown, and the xylem cells (X) and vascular bundle (V) fluoresce greenish yellow (Plate 5c). The cortical cells (parenchyma) are probably bitumenised by the bitumen, whereas the xylem cells and vascular bundle retained some of their original chemical structure. The process of bitumenisation varies between the different plant organs. The bitumenisation was greater for the fern rachis and rootlet, probably because they consist mainly of cellulose, whereas the m o n o c o t y l e d o n stem (Plate 5a) was more resistant to bitumen impregnation and probably acted as a filter through which the lower molecular weight hydrocarbon (yellow fluorescence} passed into the central cavity. The present results indicate that the natural bitumen may contain organic fragments, trapped from the environment. These organic fragments are preserved by the process of bitumenisation. Study of these organic fragments may help to determine the paleobotany and paleoenvironment (e.g. dry-wet climate, t y p e of plant etc.} at the time the natural bitumen was formed. CONCLUSIONS The following conclusions can be drawn from this study: (1) The plant remains found in natural bitumen retain their original botanical identity, b u t become bitumenised depending on the original tissues. (2) The extent of the bitumenisation of plant tissue by the bitumen can be assessed using fluorescence microscopy. ACKNOWLEDGMENTS The author would like to thank Dr. D.T. Blackburn, Department of Botany, University of Adelaide, South Australia for his help in identification o f plant remains in bitumen. The manuscript benefited from critical review by Drs. A. Cameron and L.R. Snowdon of the Institute of Sedimentary and Petroleum Geology, Calgary.

334 REFERENCES International Committee for Coal Petrology, 1971. International Handbook of Coal Petrology. 2nd edition 1963, supplement to 2nd edition 1971; Centre National de le Recherche Scientifique, Paris. Jacob, 1975. Mikroskopphotometrische Analyse Natiirlicher Fester Erd61bitumina. In: B. Alpern, P~trographie Organique et Potentiel P~trolier. Centre National Recherche Scientifique, pp. 103--113. Khavari-Khorasani, G., 1975. The properties and structural ordering of some fossil bitumens. Ph.D. thesis, Newcastle-Upon-Tyne, England. Link, W.K., 1952. Significance of oil and gas seeps in world oil exploration. Am. Assoc. Pet. Geol., Bulletin, 36 (8): 1505--1540. Maloney, N.J., 1974. Probable origin of fossil deposits in pit 91, Rancho La Brea Tar pits, California. Abstracts of Cordilleran section; 70th Annual Meeting, Las Vegas. Geological Society of America, 6 (3) : 212--213. Williams, P.V.F. and Goodarzi, F., 1981. Iranian bitumens;late-stage alteration products of crude oils. In: J. Brooks (Editor), Organic Maturation Studies of Fossil Fuel Exploration. Academic Press, London, pp. 319--336. Woodward, G.D., 1971. Late Pleistocene stratigraphy, Rancho La Brea Fossil Deposits. Abstracts of Cordilleran section, 67th Annual Meeting, University of California; The Geological Society of America, 3 (2): 218--219.