Energy & Fuels 1994,8, 1469-1477
1469
Liptinite in Indonesian Tertiary Coals Adrian Hutton,* Bukin Daulay, Herudiyanto, Chairul Nas, Agus Pujobroto, and Hakim Sutarwan Department of Geology, University of Wollongong, Northfields Avenue, Wollongong, N S W , 2522, Australia Received July 6, 1994. Revised Manuscript Received September 6, 1994@
A comparison of the petrographic data for coals from various Indonesian Tertiary basins shows that the coals have similar compositions with vitrinite the dominant maceral group. A feature common t o most of the coals is the abundance of secondary liptinite, especially exsudatinite but also fluorinite. The association of exsudatinite with oil, adjacent to, or within liptinite and vitrinite macerals, suggests that exsudatinite is an indicator of oil generation, but at an early stage. Exsudatinite is probably an intermediate product in the pathway vitrinitehptinite oil. Organic matter referable to exsudatinite andor bitumen is found in coals and clastic rocks from eastern Kalimantan. The petrographic properties of both are the same. It is suggested that, for consistency of terminology, where this material is found in coal it should be termed exsudatinite whereas where it is found in other rocks it should be termed bitumen.
-
Introduction The Indonesian Archipelago formed through the evolution and convergence of the northward-moving Indian-Australian Plate, the westward-moving Pacific Plate and the relatively stationary Eurasian Plate.1-4 Subduction of the Indian-Australian Plate beneath the Eurasian Plate lead to the development of a major magmatic arc system which is divided into two segments, the Sunda Arc in the west, and the Banda Arc in the east. These arcs are associated with a series of subduction zones which migrated, with time, in response to changes in the tectonic setting of the Indonesian region, resulting in the formation of intramontane (Early Tertiary), foreland (Late Tertiary), and interarc (Late Tertiary) basins. Deposition of peat occurred during pretransgressive stages in the intramontane basins and during a late regressive stage in the foreland and interarc basins. The most significant coal deposits, in Sumatera and eastern Kalimantan (Figure l), are a major part of Indonesia’s energy resources. Indonesia is rapidly developing policies that will ensure equitable domestic use of energy resources but a t the same time provide substantial income from export commodities such as coal and petroleum. Utilization of the coal resources, both on the domestic and export markets, depends on a thorough knowledge of the properties of the coal. Indonesia’s petroleum resources are large by AsianPacific standards but many companies are still actively exploring for new resources to augment the existing reserves. For both the coal and petroleum scenarios, organic petrography will become an increasingly useful technique because it is one of the few techniques that quantitatively characterizes organic-rich rocks. Because of the relatively recent exploitation of Indonesian ~~
~
@Abstractpublished in Advance ACS Abstracts, October 1, 1994. (1)Hamilton, W. US.Geol. Sum., Profess. Pap. 1979,1078. (2) Katili, J. A. Tectonophysics 1973,19, 195-212. (3) Katili, J. A. Tectonophysics 1978,45, 2-14. (4) CCOP-IOC. Studies in East Asian Tectonics and Resources (SEATER), UNDPICCOP, Bangkok, 1980.
0887-0624/94/2508-1469$04.50/0
coals on the world markets, the traditional benefits of organic petrography for coal quality determination are yet to be fully utilized. Realization that coal-bearing sequences are source rocks for petroleum generation has placed an added incentive for using organic petrography. Continued exploration for petroleum will utilize organic petrography both for typing source rocks and geothermal modeling, where vitrinite reflectance is the most commonly-used maturation parameter. In this paper, data on the composition of coals and associated organic-bearing clastic sedimentary rocks from eastern Kalimantan are compared with data for coals from Sumatera and several other Indonesian basins. The abundance of secondary liptinite macerals is a common feature of many Indonesian Tertiary coals, and this begs a discussion of the implication of these macerals as indicators of oil generation and/or as intermediates in petroleum generation. It is now generally accepted that coals may serve as source rocks under some circumstances but several problems have remained unresolved. Is there available porosity to allow migration of oil through coal t o the reservoir rocks? Are liptinite macerals, especially exsudatinite, indicators of petroleum generation? If so, are they indicators of limited early oil generation or oil generation on a much larger scale, sufficient to permit migratable amounts of oil. Given the relative abundance of secondary liptinite macerals, comment is made on the suitability of presently-accepted liptinite maceral terminology.
Eastern Kalimantan Coals Economic coal deposits in eastern Kalimantan occur in the Tertiary Tarakan, Kutei, Barito, and Asem Asem Basins (Figure 2) which formed as a result of rifting along, or close to, the eastern edge of the Kalimantan continental block. Barito Basin and Asem Asem Basin coals were deposited in retro-arc settings close to the foreland whereas the Kutei Basin and Tarakan Basin coals formed along the rifted border of eastern Kali-
0 1994 American Chemical Society
1470 Energy & Fuels, Vol. 8, No. 6, 1994
Hutton et al.
Figure 1. Coal basins in Indonesia.
mantan. The coal measures sequences of Eocene and Miocene age were deposited in environments ranging from fluvial to deltaic. The thickness of the coal seams varies from a few centimeters to 40 m with dips ranging between 5" and 25" near the surface. Typically the Miocene coals are thicker than the Eocene coals. Variations in thickness are associated with splitting (particularly in Eocene coals), wash-outs and wedge-outs. Splitting was probably caused by channel activity at the time of peat accumulation.
Organic Petrography In hand specimen, coals from eastern Kalimantan are composed dominantly of clarain and vitrain lithotypes. Inertinite-rich dull layers are very rare but are more common in Miocene coals, particularly those from Mahakam and Sangatta, than in the Eocene coals. The vitrinite-rich bright layers were derived from peat that accumulated under water, in more reducing conditions than were present for the inertinite-rich, dull layers which were probably derived from peat that was exposed to an oxidizing atmosphere above the water table. Maceral terminology used in this paper is that of the Australian Standard for Maceral A n a l y ~ i s . ~ Vitrinite Reflectance. The rank of coals from eastern Kalimantan generally spans the range of 0.3 to 0.6% vitrinite reflectance (Table 11, that is, from soft brown coal to high-volatile bituminous ranks, with thermally-altered coals from Sangatta reaching semianthracite rank (up to 2.03% vitrinite R,max). Four groups are recognized: (1)Miocene, soft brown to subbituminous coals subjected to regional coalification in areas with geothermal gradients normal for the Indonesian islands; mean maximum reflectance (R,max) values of 0.30-0.55%; (2) Miocene, subbituminous to low-volatile bituminous coals subjected to regional coali(5) Standards Association of Australia, Standard, AS 2856, 1986.
fication in areas (characterized by strongly folded strata) where geothermal gradients were above those normally expected for the Indonesian islands; Rvmax values are 0.48-0.71%; these coals are restricted to the Sangatta area where there is a relatively high geothermal gradient, related to intrusions, that has not previously been reported; (3) Miocene, semianthracitic coals affected by contact thermal metamorphism; Rvmaxvalues of 1.602.03%; and (4)Eocene, brown to low-volatile bituminous coals subjected to regional coalification in areas with geothermal gradients normal for the Indonesian islands; RvmaX values of 0.43-0.66%; these coals were buried to greater depths than the Miocene coals. Although vitrinite reflectance of coals increases with depth in deep drill holes, no significant general trend was found within any single coal seam, except in the Berau and Senakin coals where vitrinite reflectance exhibits an increase from the top to the bottom of the seam. These changes are assumed to be related t o differences in vitrinite type. Maceral Composition. Vitrite and clarite are the dominant microlithotypes, with subordinate vitrinertite (both vitrinite- and inertinite-rich microlithotypes), duroclarite, and inertite. In some of the Mahakam, Tanjung, and Sangatta coals, vitrite and vitrinertite are dominant. Vitrinite. Petrographically, vitrinite is the dominant maceral, both in Miocene and Eocene coals, with the vitrinite content of Miocene coals (range of 63.5-98.0%, average of 82.9%; Table 2) slightly higher than for Eocene coals (range of 61.9-93.9%, average of 79.4%). Vitrinite consists predominantly of telovitrinite and detrovitrinite with gelovitrinite content invariably low. Telovitrinite, ranging from 0.04 to 0.20 mm in thickness, consists predominantly of textinite, texto-ulminite, eu-ulminite, and lesser telocollinite. Thin layers of telovitrinite are generally surrounded by a thick detro-
Energy & Fuels, Vol. 8, No. 6, 1994 1471
Liptinite in Indonesian Tertiary Coals
’-*
SANGATTA
\
T v)
v)
*
T
200KM
LEGEND PLIOCENE COAL
MIOCENE COAL
EOCENE COAL
Figure 2. Coal basins in eastern Kalimantan with resources.
vitrinite groundmass but some telovitrinite bands are interbedded with detrovitrinite. Attrinite and densinite are the most common detrovitrinite macerals with desmocollinite a minor component. Sparse to abundant gelovitrinite is disseminated throughout the telovitrinite and detrovitrinite with porigelinite occurring as thin bands within telovitrinite.
Znertinite. Inertinite content is generally very low and is more abundant in Miocene coals (average of 4.2%) compared to Eocene coals (average of 2.2%). Dominant macerals are semifusinite, sclerotinite, and inertodetrinite with minor fusinite, micrinite, and macrinite. Semifusinite commonly occurs as layers (up to 1.0 mm in length), lenses, or isolated fragments, generally
1472 Energy & Fuels, Vol. 8, No. 6, 1994
Hutton et al.
Table 1. Reflectance Data for Eastern Kalimantan Coals gelovitrinitel detrovitrinite R,max range
telovitrinite R,max range Miocene Berau Sangatta a Mahakam Asem Asem Tanjung Eocene Tanjung Pasir Satui Senakin
mean R,max
0.45 0.64 1.88 0.48 0.36 0.61
0.39-0.55 0.49-0.72 1.61-2.03 0.40-0.55 0.30-0.41 0.57-0.65
0.44 0.62 1.81 0.46 0.35 0.39
0.37-0.54 0.47-0.69 1.55-2.06 0.37-0.54 0.29-0.41 0.33-0.47
0.45 0.63 1.87 0.47 0.36 0.40
0.61 0.63 0.51 0.57
0.57-0.65 0.58-0.67 0.44-0.54 0.54-0.64
0.59 0.61 0.50 0.56
0.54-0.63 0.57-0.65 0.42-0.53 0.50-0.63
0.60 0.62 0.50 0.56
Thermally altered coals.
Table 2. Petrographic Data for Eastern Kalimantan Coalsa vitrinite Miocene Beru Sangatta Mahakam Tanjung Asem Asem Eocene Pasir Tanjung Satui Senakin a
inertinite
liptinite
range
mean
range
mean
range
73.2-95.1 63.7-95.8 64.3-98.0 77.2-87.4 63.5-94.2
82.0 85.9 82.1 82.5 82.0
0.6-13.7 0.2-12.1 0.6-31.3 2.8-6.4 0.2-9.9
3.6 1.7-18.4 4.8 0.2-11.2 9.1 0.2-25.9 4.5 6.2-13.2 3.4 0.8-30.9
10.2 5.6 9.1 9.3 10.7
75.3-86.1 79.3-85.9 61.9-90.7 69.6-93.9
80.8 78.3 77.3 81.3
0.6-3.7 0.9-4.2 0.5-5.7 0.2-6.1
2.0 2.5 2.3 2.1
9.4 13.2 15.5 8.3
3.3-15.5 5.6-19.3 4.8-33.3 1.4-18.0
mean
Mineral matter not included in the table
Table 3. Petrographic Data for Other Indonesian Coals (Data from Various References Cited in Text) maceral composition vitrinite inertinite liptinite Sumatera Perapnap West Aceh Pliocene Miocene Oligocene Meulaboh Neogene Paleogene Ombilin Banko Barat Bukit Assam Java Bayah Bojongmanik
reflectance (range)
90-92
1-2
6-7
44-94 60-98 64-92
0-11 0-22 0-24
7-50 2-29 6-26
50-95 70-90 83-94 80-90 70-95
0-7 1-5 0-4 1-5 0-7
10-50 8-20 1-14 3-20 2-15
0.20-0.40 0.45-0.70 0.70-0.80 0.30-0.55 0.30-0.50
71-93 81-91
0-3 0-3
2-15 2-18
0.53-0.83 0.30-0.40
associated with vitrinite (mainly telovitrinite); in some cases, cell lumen of semifusinite are filled with either resinite, fluorinite, or mineral matter. Some of the Miocene Mahakam coals contain anomalously high percentages of inertinite (31.3 and 18.3%, respectively). These coals probably formed in areas with more oxidizing conditions, possibly caused by a lowering of the water table during peat formation, resulting in more frequent exposure to the atmosphere. Inertodetrinite is commonly associated with vitrinite and semifusinite. Sclerotinite, consisting of unilocular and bilocular teleutospores and sclerotia, is generally scattered throughout the samples. Lzptinzte. Liptinite is abundant in all coals (Figure 3) with the exception of thermally-affected coals from
Sangatta. (In the Sangatta coals, liptinite is not easy to recognize because of the high rank.) Liptinite contents average 11.6% which is typically higher than for the Miocene coals where the average of 9.0%. These differences are thought to represent differences in the floral assemblages at the time of peat formation of the respective coals. Resinite, suberinite, cutinite, sporinite, and liptodetrinite are the most abundant liptinite macerals, both in Eocene and Miocene coals, constituting 7 0 4 0 % of all liptinite in most samples. Resinite has bright greenish-yellow to dull orange fluorescence. It occurs as discrete bodies and lenses with some occurring as diffuse cell fillings in telovitrinite. Suberinite commonly occurs as distinct layers (0.050.40 mm thick) with greenish-yellow to orange fluorescence, although in some of the Sangatta coals the fluorescence is very weak brown or absent. Cell walls of weakly fluorescing suberinite are thinner than the more strongly fluorescing suberinite. Suberinite commonly occurs in association with corpogelinite, rarely with resinite and exsudatinite, and is more abundant in Miocene coals, particularly in coals with lower vitrinite reflectance. Liptodetrinite is rare to abundant in most samples and mainly occurs in clarite where it has greenishyellow to orange fluorescence. Large fragments of liptinitic material (typically > 7 ,um diameter) in some of the Berau and Asem Asem coals are included as liptodetrinite maceral because they cannot be assigned to any other maceral. Rare t o abundant cutinite commonly occurs in association with vitrinite and resinite but in some cases it is associated with suberinite and exsudatinite. It generally has greenish-yellow to orange fluorescence, although some has very weak brown or no fluorescence, particularly in the Sangatta coals. Sporinite (including crassispores, pollen, and sporangia) has greenish-yellow to orange fluorescence and is less abundant in Miocene coals than in Eocene coals. It commonly occurs in association with detrovitrinite, resinite, and suberinite. The distinction between pieces of thick suberinite and sporinite within a single sample is difficult in some cases although the sporinite generally has yellow to orange fluorescence whereas suberinite fluoresces greenish-yellow to yellow. Exsudatinite, the secondary liptinite maceral that is derived from other liptinite and vitrinite and which infills fractures and pores in coal, is abundant in many samples and constitutes up to 10% of some samples. It occurs in most coals and commonly has bright greenishyellow to orange fluorescence. It has various shapes and occurrences including infillings in fractures, bedding plane cavities, and cell lumens. Fluorinite and Botryococcus-related telalginite are minor components and rarely exceed 1%of the bulk rock. Fluorinite is rare to abundant in some coals and typically occurs as isolated bodies and lenses with bright green to greenish-yellow fluorescence of very strong intensity. Botryococcus-related telalginite with bright yellow to orange fluorescence occurs in Miocene Satui, Senakin, and Tanjung coals and in a few samples of the Berau coal. Telalginite is commonly disseminated throughout the samples although some concentrations are present. Maximum percentages of the Botryococcus-
Liptinite in Indonesian Tertiary Coals
Energy & Fuels, Vol. 8, No. 6, 1994 1473
Figure 3. Fluorescence mode except where stated; field width = 0.34 mm. (1) Exsudatinite, infilling fracture, and sclerotinite in coal composed of vitrinite (black), cutinite, and minor sporinite and liptodetrinite. (2) Same field as (l), reflected white light. Nonfluorescing macerals are vitrinite and unilocular sclerotinite. Oil smear on vitrinite (right of field) escaping from fracture. (3) Exsudatinite infilling fracture in vitrinite enclosed in cutinite; resin bodies also present; close association of exsudatinite with cutinite suggests exsudatinite is derived from cutinite. (4)Exsudatinite infilling fracture between two layers of suberinite; clearly exsudatinite is sourced from the suberinite. (5) Exsudatinite in vitrinite, with oil flowing from the exsudatinite, suggesting close association between oil and exsudatinite. (6) Exsudatinite infilling fractures adjacent to resinite; exsudatinite unequivocally formed from resinite.
related telalginite is 0.4%. Telalginite is also reported in Eocene coals from Ombilin6y7and coals from Melawi and Ketungau Basins8 and North Sumatera Basin.g Mineral matter (mainly clay minerals, quartz, pyrite, and carbonate) is sparse to common. (6) Daulay, B. Petrology of Some Indonesian and Australian Tertiary Coals. M.Sc. (Hons) Thesis, The University of Wollongong, Wollongong, 1985 (unpublished). (7) Daulay, B.; Cook, A. C. J. Southeast Asian Earth Sei. 1988, 5 , 45-64. (8) Sutjipto, R. H. Sedimentology of the Melawi and Ketungau Basins, West Kalimantan, Indonesia. Ph.D. Thesis, The University of Wollongong, Wollongong, 1991 (unpublished).
Oil and Oil-Related Substances. Coal is a sedimentary rock comprising organic matter, originally deposited as plant fragments, which was converted by biogenic and physicochemical alteration. This implies that coal is solid in the same manner as other sedimentary rocks. However, coal bed methane desorption experiments and organic petrography show that coal contains components not regarded as macerals, inchd(9) Hadiyanto. Organic petrology and geochemistry of the Tertiary formations at Meulaboh area, West Aceh Basin, Sumatera, Indonesia. Ph.D. Thesis, The University of Wollongong, Wollongong, 1992 (unpublished).
1474 Energy & Fuels, Vol. 8, No. 6, 1994 ing gases such as methane and carbon dioxide and liquid components referred to by various names such as oil, oil droplets, oil hazes, and oil smears. These “nonsolid components may have been formed from the coal or have migrated into the coal. Definitions of these and other organic matter are as follows. Hydrocarbons: used in a chemical sense in that the material is composed of predominantly carbon and hydrogen; oil and methane are examples. Petroleum: naturally-formed liquid and gaseous hydrocarbons. 0il:liquid hydrocarbon derived from components of the rock in which it occurs o r that has migrated into one rock from another source rock; oil infills cavities in coal as well other rocks; under the microscope oil may occur as an (a) oil haze: fluorescing cloud emanating from oil and dissolving in the immersion oil where this is used; (b) oil smear, oil stain: fluorescing or nonfluorescing stain on the surface of the sample; commonly brown in reflected white light; (c) oil drop, oil droplet, free oil: oil occurring in fractures and cell cavities or as drops on the surface or edges of grains. Bitumen: solid hydrocarbon residues occupying fractures and other cavities; as will be discussed in detail later, bitumen is equivalent to the maceral exsudatinite and should be regarded as a secondary maceral; the term bitumen is used in a petrographic sense not a chemical sense. Comparison with Other Coals. The maceral compositions and rank of coals from other Indonesian basins are similar to those from eastern Kalimantan. Many Indonesian coals, apart from being vitrinite-rich, have low ash contents. These features of coals have been interpreted as indicating a high moor origin.lOJ1 Coal type, or the petrographic composition of coal, is related to paleoclimate, geological age, and tectonic setting. The tectonic setting also plays an important role in any subsequent burial metamorphism. As a result of these factors, spatial and temporal variations in paleoclimate, geological age, and tectonic setting can cause variations in coal type or coal type provincialism.12 The range of plant components preserved in the peat and the extent of alteration t o these components during the diagenesis of the peat, and subsequent coalification, determine coal type variations.13J4 Coals from eastern Kalimantan are largely derived from ombrogenous peat mires15J6which contained peats which were analogs of the ombrotrophic peats described by C0u1ter.l~The vegetation precursors of this type of peat is typically tropical rainforest species dominated by angiosperms (many of which were herbaceous), ferns and mosses that developed in lowlands. Given the Indonesian coals are (10) Smith, G. C; Cook, A.C. Fuel 1980,59,41-646. (11)Titheridge, D. G. The geological and depositional setting of the Brunner coal measures, New Zealand, and the influence of these factors on seam thickness and petrological characteristics of Brunner coals. Ph.D Thesis, The University of Wollongong, Wollongong, 1988 (unpublished). (121 Cook, A. C. In Australian black coal - its occurrence, mining, preparation and use; Cook, A. C., Ed.; Australasian Institute of Mining and Metallurgy: Illawarra Branch, Australia 1975; pp 66-83. (13) White, D. Bull. Acad. Sei. 1915,5,189-212. (141 Smith, A. H. V. In Coal and coal-bearing strata; Murchison, D. G., Westoll, T. S., Eds.; Oliver and Boyd: London, 1968; pp 31-40. (151 Tennison-Woods, J. E. Nature 1885, 42, 113-116. il6)Anderson, J. A. R. J . Trop. Geogr. 1964,18, 7-16. (17)Coulter, J. K. Malay. Agric. J . 1957,40, 36-41.
Hutton et al. all vitrinite-rich suggests that there is little coal type provincialism in Indonesian coals.
Exsudatinite and Oil Generation Is Coal a Source Rock? The role of coal as a source rock for hydrocarbons has received increasing recognition over the past two decades. Numerous reservoirs of significant size are associated with coal-bearing sequences and in many instances, very few, if any, clastic rocks with a marine origin are associated with these sequences. The possibility of a marine source rock for these sequences is unlikely unless the oil migrated great distances. These types of reservoirs are found in Australia, China, and Southeast Asia and this is now taken t o be overwhelming evidence that hydrocarbons are sourced from terrestrial matter in coal and that the oil is expelled from the coal to reservoir rocks. Notwithstanding this, “The dispute such as it still exists centers upon the question of expulsion, Le., whether the oil, once formed, can escape from the coal into the surrounding strata”.18 Hunt19stated that the high-wax, low-sulfur coals with C29 steranes dominant and pristane to phytane ratios usually above 5 indicated that the oils of the Gippsland Basin of southeast Australia were derived from organic matter deposited with terrigenous sediments. Hunt argued that coal and terrestrial kerogen with either WC ratios above 0.9, Rock-Eva1 hydrogen indices above approximately 200 or liptinite contents of 15% or more, have the potential to generate and release oil as well as gas. Powell et a1.20confirmed that Australian coals and terrestrial organic matter ranging in age from Permian to Tertiary contain aliphatic structures capable of producing paraffinic oils. These liptinite-poor coals ( < l o % liptinite) produced oil but of a type that has a lower wax yield. Of the three maceral groups, liptinite is considered to have the greatest potential to produce hydrocarbons, especially crude oi1.10~21~22 This concept was also suggested for a specific study on rocks thought to be the source for the oils of the Ardjuna Basin, northwest Java. In a study of the high-wax oils from that basin, Horsfield et alSz3stated that the potential precursors were long chain waxy paraffins in the coals of the Talang Akar Formation. It was stated that the resinite and “related macerals might play an especially important role in petroleum expulsion”. Large amounts of vitrinite, exsudatinite, and oil drops and oil hazes in coal or carbonaceous shale are thought to be indicators of hydrocarbon generation in these rocks.24 If this hypothesis is accepted, Tertiary coals from Indonesia are excellent source rocks providing the oil is expelled to reservoirs. Exsudatinite in Indonesian Coals. Exsudatinite content of eastern Kalimantan coals ranges from (0.1 (18) Levine, J. R. A m . Assoc. Pet. Geolog. Stud. Geol. Ser. 1993,3, 39-77. (19) Hunt, J. Org. Geochem. 1991,17, 673-680. (20) Powell, T. G.; Boreham, C. J.; Smyth, M.; Russell, N.; Cook, A. C . Org. Geochem. 1991,17, 373-394. (21) Snowdon, L. R.; Powell, T. G. Bull. Am. Assoc. Pet. Geol. 1982, 66,775-788. (22)Tissot, B. P.; Welte D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1984. (23)Horsfield, B; Yordy, K. L; Crelling, J. C. Org. Geochem. 1987, 13, 121-129. (24)Teichmuller, M.; Durand, B. Int. J . Coal Geol. 1983,2, 197230.
Liptinite in Indonesian Tertiary Coals
to 9.9% with an average of 0.8%;Miocene coals contain relatively higher exsudatinite contents than Eocene coals. In coals of both ages, exsudatinite normally occurs adjacent to and within fractures in vitrinite and liptinite (particularly resinite, cutinite, and suberinite, Figure 3, parts 1-4, 6); by inference most, if not all, of the exsudatinite originates from these macerals. Some exsudatinite shows oil smearing and oil stains (Figure 3, part 5). Exsudatinite in thermally-altered coal, referred to as meta-exsudatinite to distinguish it from fluorescing exsudatinite in lower rank coals, is present in some of the Sangatta semianthracite which formed adjacent t o an intrusion. In reflected white light, this metaexsudatinite has a higher reflectance than associated macerals, including vitrinite, and does not fluoresce which is assumed to indicate chemical alteration, specifically the loss of hydrogen. During coalification, the loss of hydrogen is associated with the loss of volatile hydrocarbons and water. Reflectance of meta-exsudatinite is 2.70% whereas that of the associated vitrinite is 1.74%,that of inertinite 1.58%, and that of liptinite 2.11%. Meta-exsudatinite has also been recognized in anthracite found in Bukit Asam, South Sumatera. In the anthracite, the reflectance of exsudatinite is also higher than that of the associated vitrinite and inertinite. Meta-exsudatinite in Bukit Asam is also formed during the thermal alteration of high volatile bituminous brown coals heated by intrusions. In Indonesian coals, exsudatinite typically infills veins, cell lumens, bedding planes and wedge-shaped fractures, features also noted by M u r c h i ~ o nand ~ ~ Stach et a1.26 It is also a binding agent for gelovitrinite in some coals. From a review of early studies, Stach et a1.Z6 noted that exsudatinite is mainly found in liptiniterich coals of subbituminous to high-volatile bituminous rank. MurchisonZ5and ShibaokaZ7suggested that veinfilling secondary macerals (including exsudatinite) are found in bituminous coals because they form as expulsions from other macerals, and subsequently migrate, during the subbituminous stage. However, more recent studies extended that range and exsudatinite now is known to occur in coals of varying rank, ranging from soft brown coal t o bituminous rank. Earlier research shows exsudatinite may be directly related to the formation of ~ i l . In~ low-rank ~ , ~ ~eastern Kalimantan coals (for example, Asem Asem and Berau coals) oil, oil hazes, and oil smears occur in many samples; in some of these samples the oil is closely associated with exsudatinite (Figure 3, part 5 ) suggesting that the oil is formed either from the exsudatinite or, alternatively, the exsudatinite and oil formed at the same time from the same or different precursors. In clastic rocks associated with the coals, oil droplets and oil hazes occur but these same rocks also contain the same maceral assemblages as the coals, including bitumen which has similar optical properties to the exsudatinite in the coals. Thus the oil in these rocks ( 2 5 ) Murchison, D. G. Fuel 1976,55,79-83. (26) Stach, E.; Mackowsky, M.-Th.; Teichmuller, M.; Taylor, G. H.; Chandra, G.; Teichmuller, R. Stach’s Textbook of Coal Petrology; Gebruder Borntraeger: Berlin, 1982. (27) Shibaoka, M. Fuel 1978,57, 73-77. (28) Cook, A. C.; Struckmeyer, H. 2nd WA Oil Explor. Symp. 1415 Nov 1985, Melbourne 1986,419-432. (29) Teichmuller, M. Int. J . Coal Geol. 1989,12, 1-87.
Energy & Fuels, Vol. 8, No. 6, 1994 1475 was probably generated in the rocks as is the oil generated in the coal. The presence of the oil is not indicative of migration of hydrocarbons through the rocks. Exsudatinite is found in eastern Kalimantan coals of soft brown coal rank and is therefore presumed it can be generated during the soft brown coal stage (approximately 0.35% R,max), that is, in the very early stages of coalification. The repeated intimate occurrences of exsudatinite with resinite, suberinite, cutinite, and vitrinite macerals indicate the exsudatinite is derived from these macerals. Given the close association of liptinitehitrinite, exsudatinite, and oil, two probable pathways for the involvement of exsudatinite in oil generation are suggested: primary liptinite vitrinite
-
-
exsudatinite
exsudatinite
-
oil
-.oil
Terminology In previous literature a number of terms have been used for organic matter referable to bitumen. The most acceptable is that of Jacob,30who provided a classification of bitumen which showed that all types of bitumen were derived from immature oils. The classification was a tripartite classification as three types of oils could be the starting material for bitumen formation-asphaltenerich, paraffin-rich, and naphthene-rich. The composition of the immature oil determined the chemistry of the intermediate products and the end bitumen. Optically, bitumen falls into two main groups: (i) Nonfluorescing, vitrinite-like bitumen (spherical thucholites are probably related to this form of bitumen); this bitumen represents coalified (or mature), heavy fractions of “petroleum” derived from liptinite and/or vitrinite during the normal oil generation processes. (ii) Fluorescing bitumen: liptinite-like bitumen that occurs as pods and cavity-fillings and in the groundmass between clastic grains; this form is common in Green River oil shale (Figure 4, part 4). More recently, the term migrabitumen was introduced by the ICCP, partly to indicate that bitumen is of secondary origin rather than a primary maceral. Stach et a1.26and ICCP (1990 Annual Meeting) defined migrabitumen as natural solid bitumen occurring in sedimentary rocks, particularly in carbonates where it infills intergranular porosity and fractures. Alpern et ~ 1 discussed . ~ the ~ optical morphology of hydrocarbons and oil progenitors in sedimentary rocks and divided migrabitumen into three types using reflectance as the discriminant, although the adjectives for the types were colors. The plates given to illustrate the types of migrabitumen, clearly showed all examples of bitumen were in noncoal rocks. With reference to other literature, many authors, including S t r ~ c k m e y e r ,Panggabean,33 ~~ and Sutrisman,34 described migrabitumen in dispersed organic (30) Jacob, H. Int. J . Coal Geol. 1989,11, 65-79. (31)Alpern, B.; Lemos de Sousa, M. J.; Pinheiro, H. J.;Zhu, X. Publi. Museu Laboratorio Mineral. Geol. Faculd. Ciencas Porto 1992,3,53. (32) Struckmeyer, H. I. M. Source rock and maturation characteristics ofthe sedimentary sequence of the Otway Basin, Australia. Ph.D. Thesis, The University of Wollongong, Wollongong, 1988 (unpublished).
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1476 Energy & Fuels, Vol. 8, No. 6, 1994
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Figure 4. Fluorescence mode unless otherwise stated; field width = 0.34 mn,. ,1)Bitumen in claystone underlying a coal seam; bright yellow fluorescing organic matter mostly bitumen, orange-yellow fluorescing organic matter is bitumen mixed with mineral matter. (2) Same field as 1; reflected white light. (3) Bitumen impregnating pods of mineral matter in a coal. (4) Green River (USA) oil shale composed of alginite-rich layers (top and bottom) enclosing a pod of mineral matter impregnated with bitumen. (5)Bitumen infilling cavity, containing pyrite, formed by intact ostracode shell; Stuart (Australia) oil shale. Bitumen formed by thermal alteration (contact metamorphism) of oil shale when intruded by a dyke. (6) Large pod of bitumen with pyrite in Irati (Brazil) oil shale. Note the two phases of bitumen as indicated by the different fluorescence colors.
matter (DOM) but stated that this organic matter is referable to exsudatinite. As all bitumen is of secondary origin and all bitumen is likely to have migrated from the source, in some cases this may be a great distance whereas, in other cases, the migration may be out of fractures or porosity in the (33) Panggabean, H. Tertiary source rocks, coals and reservoir potential in the Asem Asem and Barito Basins, Southeastern Kalimantan, Indonesia. Ph.D. Thesis, The University of Wollongong, Wollongong, 1991 (unpublished). (34)Sutrisman, A. Source rock distribution and evaluation in the Talang Akar Formation, Onshore Northwest Jawa Basin, Indonesia. MSc. Thesis, The University of Wollongong, Wollongong, 1991 (unpublished).
source, the prefix “migra” is redundant and adds little t o the name or understanding of bitumen. “Migra” implies migration; it is likely that at least some bitumen is formed in situ. It is difficult to justify the continued use of the term migrabitumen. In clastic rocks associated with Indonesian coals, exsudatinite-like material is found. However, it is not found as large-grained dispersed organic matter (DOM) except where present in very large vitrinite phytoclasts; it is mostly small interstitial organic matter referable to bitumen and most of this material is probably better termed bitumen.
Liptinite in Indonesian Tertiary Coals The connotation of migration cannot be the reason for assigning the term bitumen to organic matter. If this was the case, some exsudatinite in Indonesian coals would have t o be called bitumen. Some of the exsudatinite has strong fluorescence and this is probably a function of its greater mobility relative to other types of exsudatinite. A mobile origin is inferred as this type of exsudatinite is found in cell lumens of semifusinite and sclerotinite, macerals that could not themselves generate or expel large amounts of secondary liptinite. Thus some of the exsudatinite is not adjacent t o the probable sources. In carbonaceous shale or coal with pods of mineral matter, exsudatinite-like organic matter is found both as a cavity/fracture filling and in the mineral-rich pods as well (Figure 4, parts 1 and 2). The fluorescence properties are the same for both occurrences. Exsudatinite in the mineral-rich zones is identical to bitumen in other rocks. The two are derived from the same sources, primary liptinite, and by the same processes. In most cases, if not all, exsudatinite is essentially equivalent to bitumen. However, which is the best term for it? Much of the material reported to be bitumen or migrabitumen is difficult to distinguish optically from exsudatinite as both have similar properties (Figure 4, parts 3, 4,and 6). Commonly it is only the association with macerals or mineral matter that allows distinction between bitumen and exsudatinite. In contact metamorphic aureoles associated with intrusions in at least two Tertiary oil shale in Australia, Rundle-Stuart, and Nagoorin, mobile organic matter formed by the pyrolysis of the alginite in the oil shales migrated away from the source and condensed in pores and cavities such as in osctracode shells (Figure 4,part 5). This mobile organic matter is optically similar to bitumen andor exsudatinite, depending on the use of the latter terms. Bitumen in Green River oil shale (Figure 4, part 5) and bitumen in the Irati oil shale (Brazil; Figure 4, part 6) also have properties the same as exsudatinite. Given the properties of both and the origin of both, it would appear that the terms bitumen and exsudatinite are interchangeable and in fact have been used variably in the literature. Clearly there is a problem with terminology-bitumen in one rock is exsudatinite in another. Thus, as a means of simplification, the following terminology is suggested; the distinction between the two is based on association rather than origin or optical properties.
Energy & Fuels, Vol. 8,No. 6, 1994 1477 1. Secondary, fluorescing liptinite found in fractures, pores, and other cavities in coal, irrespective of optical properties, should be assigned to the maceral term exsudatinite. 2. Secondary, fluorescing liptinite found in clastic rocks such as sandstone and shale, irrespective of optical properties, should be assigned the name bitumen. (Some of this bitumen may have been derived from a source some distance from where it is observed.)
Conclusions 1. Indonesian Tertiary coals are vitrinite rich with varying amounts of liptinite (0-25 vol %). Inertinite is a minor component with sclerotinite the most abundant inertinite maceral. 2. Most coals are within the brown coal to highvolatile bituminous rank except those that are closely associated with intrusions; the rank of these coals commonly approaches semianthracite to anthracite close to the intrusion. 3. An interesting feature of the Indonesian Tertiary coals is the relative abundance of secondary liptinite, especially exsudatinite and, to a lesser extent, fluorinite. It is suggested that (i) the association of primary liptinite with exsudatinite indicates exsudatinite is derived from primary liptinite, particularly resinite, suberinite, and cutinite. There is some evidence, although not convincing at this stage, that exsudatinite is also derived from vitrinite. (ii) Indonesian Tertiary coals with vitrinite reflectance 0.30-0.35% (which is below the oil generation window of 0.5% vitrinite reflectance postulated for Australian Tertiary terrestrial source rocks) are probably at the low end of the oil generation window. 4. Exsudatinite and bitumen (which has several synonyms) are petrographically the same organic matter. For consistency of terminology, (a) secondary, fluorescing liptinite found in fractures, pores, and other cavities in coal, irrespective of optical properties, should be called exsudatinite; (b) secondary, fluorescing liptinite found in clastic rocks such as sandstone and shale, irrespective of optical properties, should be called bitumen. 5. Probable pathways for the formation of some of the oil derived from Indonesian coals are primary liptinite vitrinite
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