The Geological Setting of Indonesian Coal Deposits_Friedrich_1999-1

The geological setting of Indonesian coal deposits By M C Friederich1, Mem Member ber,, R P Langfor Langford d2 and T A Moore3 ABSTRACT Over the last 15 year Over yearss Indo Indones nesia ia has exp experi erience enced d a sur surge ge in coa coall exploration, which has led to the development of a major export industry. This coal exploration exploration has improv improved ed the underst understanding anding of the geology of  the Indonesian coal deposits. The main economic coal deposits are of  Eocene and Miocene to Pliocene age, which mainly occur in Kalimantan and Sumatra Sumatra and were form formed ed from peat deposits deposits in an equa equator torial ial paleoclimate paleoc limate similar similar to that prev prevailing ailing today. today. Some of these peats were domed peats, which grew above the normal water tables, under a climate of yea year-r r-round ound rainfall; rainfall; these pea peats ts gre grew w abo above ve the level at whi which ch waterborne mineral matter can enter the system, resulting in low ash and sulphur, and locally very thick coal. It is believed that such peats have formed the unusually thick, low ash, and low sulphur Miocene coals of  Indonesia. Coal deposits of Eocene age are typically characterised by thinner seams, with relatively higher contents of ash and sulphur. The Mioc Mi ocen enee co coal alss an and d Eo Eoce cene ne co coal alss bot both h ap appe pear ar to ha have ve fo forme rmed d in lacustrine, coastal plain or deltaic depositional environments, similar to the modern peat-forming environments of eastern Sumatra and parts of  Kalimantan. Kalim antan. The Eocene coals formed mainly in extensional tectonic tectonic settings. setti ngs. Miocene - Pliocene coal deposits formed in a range of tectonic settings.

INTRODUCTION Recent coal exploration in Indonesia was initiated in the 1970s by several groups. However the level of activity was boosted in the early-1980s, early-1980s, after the entry of sev several eral foreign companies under a new phase of foreign investment agreements for coal explor exp lorati ation on and min mining ing.. Exp Explor lorati ation on in the ear earlyly-198 1980s, 0s, in particular, was immensely successful, and resulted in the start-up of several major coal mines and the creation of an important new export industry. Coal production has risen from less than one millio mil lion n ton tonnes nes in 1982 1982,, to ov over er 59 mil millio lion n ton tonnes nes in 199 1998. 8. 1.

PT Austind Austindo o Nusantar Nusantaraa Energi, Energi, Graha Irama, 3rd Floor, Floor, Jalan HR Rasuna Said Kav, 1-2, Jakarta, Indonesia.

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11 Toms Cres Crescen cent, t, Ain Ainsli sliee AC ACT T 2602 2602..

3.

Coall Re Coa Rese sear arch ch Limit Limited ed,, PO Box 29-415, 29-415, Chris Christc tchu hurch rch,, Ne New w Zealand.

100°

Indonesia has grown quickly to become the world’s third largest exporter of thermal coal, mainly used for electricity generation. The state-owned company, PT Tambang Batubara Bukit Asam (PTBA) operates coal mines at Ombilin (West Sumatra) and near Tanjung Enim (South Sumatra). The main private sector coal mines include those in Eastern Kalimantan of PT Kaltim Prima Coal (50 per cent RTZ, 50 per cent BP), PT Arutmin Indonesia (80 per cent BHP, 20 per cent Bakrie and Brothers), and PT Adaro Indonesia. Future developments will supply coal for the next generation of Indones Indonesian ian coal-f coal-fired ired power plants plants,, althou although gh this has been slowed by the current economic situation. Coal exploration, associated research, and on-going work by the petroleum companies has led to an improved understanding of the geological controls of the better known coal deposits of  Indonesia. These deposits are mainly in Kalimantan and Sumatra, although reference, in this paper, will also be made to some lesser-known deposits in Java and Sulawesi (Figure 1). The coal deposits that are currently mined in Indonesia are restricted to Eocene Eoc ene and Mio Miocen cenee age seq sequen uences ces.. Hig High h rel relati ative ve sea lev levels els during the Oligocene resulted in deposition of mainly marine sediments throughout Indonesia. Pliocene age coals and lignites occur, but are of low rank due to the young age, and are not being mined. EOCENE COAL DEPOSITS Structural setting Coal-b Coal -bea eari ring ng ri rift ft ba basi sins ns we were re in init itia iate ted d in Su Suma matr traa an and d Kalimantan during the Early Tertiary (Paleogene). Much of the margin of Sundaland, from western Sulawesi, through eastern Kalimantan, the Java Sea, and Sumatra, was the site of Eocene extension. Deposition probably began in the middle Eocene, as these are the oldest known sediments. This early Tertiary rifting on the margin of Sundaland was in a back-arc setting, which was influenced by the subduction of the Indian Ocean plate (Cole and Crittenden, Critt enden, 1997). The oldest known sediments with reliabl reliablee dates dat es are of mid middle dle Eocene age, alt althou hough gh it is pos possib sible le tha thatt deposition may have begun earlier than this (Hutchison, 1996).

120°

140°

Medan KALIMANTAN 

SUMATRA

0°

Pontianak

Padang

Palembang Banjamasin JAKARTA

0

N

500km

100°

0°

Samarinda

. Balikpapan

Jayapura

SULAWESI  IRIAN 

      -

JAYA

Ujang Pandang

JAVA

120°

 - Coal-bearing sequences

140°

FIG 1 - Distribution of coal-bearing sequences in Indonesia.

The AusIMM Proceedings

No 2 1999

23

M C FRIEDERICH, R P LANGFORD and T A MOORE

The sedimentary sequences in these Paleogene rifts are described by Sudarmono, Suherman and Eza (1997), who have shown the similarities in lithologies between these Paleogene rifts, and relate the stratigraphic sequence to successive stages in their evolution. Coal measures of Eocene age are known from the following basins: Pasir and Asam Asam (South and East Kalimantan provinces); Barito (South Kalimantan); Upper Kutai (East and Central Kalimantan); Melawi and Ketungau (West Kalimantan); Tarakan (East Kalimantan); Ombilin (West Sumatra); Central Sumatra Basin (Riau Province). Eocene coal measures, but with generally thin seams, also occur in rift sequences in southwestern Sulawesi and southwest Java. Eocene coal is currently being mined in SE Kalimantan (PT Arutmin Indonesia; PT Kendilo Coal) and in Sumatra at Ombilin, West Sumatra Province. The quality and recent production levels of the main Eocene producing mines are shown in Table 1. The initial deposition of the early-rift phase in these Paleogene rifts was freshwater, mainly fluviatile, alluvial fan and shallow lacustrine deposits. In SE Kalimantan, these were probably deposited in the Middle to Late Eocene, but they may have been younger in Sumatra, from Late Eocene to Early Oligocene. In the Central Sumatran rifts, the initial fluvial phase was overlain by sediments deposited in freshwater lakes (Cole and Crittenden, 1997). In Southeast Kalimantan, however, the basal alluvial fan and fluvial deposits were overlain by laterally continuous coal seams formed in a coastal plain setting, which was then overlain by sediments that have been interpreted as Late Eocene transgressive marginal marine deposits (Friederich et al, 1995).

Coal mines

1 Kaltim Prima Indominco Mandiri Tanito Harum Multi Harapan Private Companies Senakin

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Petangis Satui  Adaro Private Companies Kideco Sebuku Berau

Tarakan

Ketungau Mandai 0°

Melawi

0°

Kutai Barito   s    t  u   a   e  r    M

N 0

250km

115°

Pasir 

 Asam Asam

FIG 2 - Tertiary basins and coal mines in Kalimantan.

Kalimantan basins

upper coal member. T3 is mainly fossiliferous marine marl, mudstone with interbedded clayey sandstone, and minor thin limestone towards the top. The T2 above the basal coal at least partly formed in large shallow embayments, with access to marine conditions, similar to the embayments now found on the present day coast of SE Kalimantan. These were depocentres for mainly fine grained clastics, and were transitional to the more fully marine conditions of the T3 member. Economically important coal is being mined at Satui and Senakin (by PT Arutmin Indonesia) and Petangis (by PT Kendilo Coal). It occurs near the base of the T2 member. The overlying portion of the T2 member, above the basal coal, is a presumed marine-paralic dominated unit, 70 to 100 metres thick, comprising a regular alternation of claystone, thin sandstone beds, and thin siderite beds. The basal coal unit is up to 9 m thick, but is more typically 4 to 6 m. The seam is typically laterally continuous, without sudden changes in seam thickness. (Friederich et al, 1995). Some 20 km north of Petangis, unambiguous field evidence is found for a marine transgression where a marine fossiliferous sandstone unit directly overlies the basal coal seam which contains very high sulphur contents. Several kilometres further north, the fossiliferous sandstone is replaced by a limestone.

The initial deposition is believed to be Middle Eocene. It is interpreted as a syn-rift sequence and, as noted by Van de Weerd and Armin (1992) and Moss et al (1997), is remarkably similar throughout Kalimantan. The stratigraphic setting of the important known coal occurrences is described in more detail below.

Pasir and Asam Asam Basins, Southeast Kalimantan Eocene age coal deposits occur within the Pasir and Asam Asam Basins of SE Kalimantan (Figure 2). Stratigraphic nomenclature varies between the basins, although this paper refers to units defined in the Asam Asam Basin. The basal Eocene sequence, the Tanjung Formation, unconformably overlies the Mesozoic basement. Milligan and Shatwell (1982) subdivided the Tanjung Formation into three members, T1, T2 and T3. The formation was deposited within a transgressive depositional system. Deposition began with basal conglomerate and overlying quartz-lithic sandstone, siltstone and claystone of the T1 member, with a thickness from several metres to over 150 metres. The thickness is quite variable, probably related to infilling of  grabens. The overlying T2 unit contains a thick basal coal member, overlain by clastics, minor carbonate, and, locally, an

TABLE 1  Average coal quality, selected Eocene deposits. Mine

Basin

Company

Total moisture % (ar)

Inherent moisture % (ad)

Ash % (ad)

Volatiles % (ad)

Sulphur % (ad)

Heating value (kcal/kg)(ad)

Satui

Asam Asam

PT Arutmin

10.0

7.0

8.0

41.5

0.80

6800

Senakin

Pasir

PT Arutmin

9.0

4.0

15.0

39.5

0.70

6400

Petangis

Pasir

BHP Kendilo

11.0

4.4

12.0

40.5

0.80

6700

Ombilin

Ombilin

PTBA

12.0

6.5