ABSTRACT
The distribution of coal deposits in Sumatra, Indonesia, is controlled by the tectonic setting of collision between the Australian plate and the Eurasian plate. Relatively older, high grade coal is found on both sides of the Barisan Mountains in the foothills. These deposits have potential for bituminous and metallurgical coal. Relatively younger, lower grades of coal occur on both sides of the Barisan Mountains as well. These deposits are located further away from the mountain uplift, in sedimentary basins where stable conditions have resulted in thick accumulations of coal.
Compared to Kalimantan, Sumatra has only a fraction of the coal tenements even though the coal resources are of a similar scale. Until now, coal tenements in Sumatra are clustered in areas where coal is outcropping in easily accessible locations. The future challenge is to locate the next generation of coal deposits that are not so obvious. Huge potential for coal ranging from lignite to sub-anthracite grades remain to be discovered on both sides of the Barisan Mountains.
INTRODUCTION
Conditions suitable for the formation of significant deposits of coal have occurred throughout the island of Sumatra despite the active tectonic setting. In Sumatra plate collision is oblique resulting in significant movement often felt as earthquakes.
Figure 1 regional tectonic setting of Sumatra (Amijaya et al, 2005)
TECTONIC FRAMEWORK OF SUMATRA
The plate boundary of Sumatra is a classic example of an active subduction collision zone. A diagrammatic section through the island shows the main elements of this tectonic setting.
In terms of coal potential the main features include;
Outer Island Arc
This includes the islands of the Mentawai Archipelago. Tectonic movement in response to mega-thrust earthquakes makes this geological setting not suited to the accumulation of significant coal deposits. Lignite grade coal has been found on Nias Island that is relatively thin and often contains high sulphur. Overall the outer island arc is not prospective for the discovery of coal.
Forearc Basin
This area includes the entire west coast of Sumatra. Coal occurrences are known throughout the forearc basin of Sumatra.
These coal occurrences can be classified into two basic groups as follows;
Older coal seams (Middle Miocene)
These coal deposits occur within the foothills of the Barisan Ranges where the oldest sediments of the forearc basin have been uplifted and are now close to the surface. Coal seams greater than 3m thick are common. Deposits have been reported from south of Bengkulu to Sibolga, a strike distance of more than 400km. The coal seam quality is relatively high grade with total moisture contents around 15% and calorific values often in excess of 6500kcal/kg(adb).
These coal deposits are often relatively structurally deformed and limited in extent by structural controls due to close proximity to the uplift of the Barisan Mountains. Examples of these types of deposits are known in Bengkulu, Tapan, Painan and Mandailing. The size of these deposits is relatively small (10-100mt) due to structural controls. Coal quality is most often very good and potential for metallurgical coal is possible due to relatively high heat flows caused by the Barisan uplift over time and contact metamorphism as a result of volcanism.
Younger coal seams (Late Miocene to Pliocene)
These coal deposits generally occur on the coastal plain of western Sumatra. Coal deposits are also relatively thick and laterally extensive. Occurrences occur throughout the west coast of Sumatra from south of Bengkulu to Meulaboh in the north a distance of more than 1000km. The deposits are relatively undisturbed in terms of their structure. Coal quality ranges from 30-50% total moisture and 3000-5500kcal/kg(adb) energy, depending on their age. The age and coal grade of these deposits is generally directly proportional to the distance from the Barisan Mountain range.
Examples of these types of deposits are at Tais, in Seluma regency, south of Bengkulu and Meulaboh in West Aceh. Potential deposit size is relatively large and deposits numerous more than 100 million tonnes are documented.
Figure 3 shows the relative location of coal seams on a diagrammatic section through the forearc basin of Sumatra.
Figure 3 diagrammatic section through the forearc basin of Sumatra
Volcanic Arc (Intramontane Setting)
When the Barisan Mountains formed some coal deposits were caught up in the earth movements and formed outliers within the Barisan mountain range. These deposits are thought to be similar age and origin to those in the forearc basin. They have also been subjected to intensive deformation and are limited in lateral extent by structural controls. The most well-known example of this type of coal is at Ombilin, east of Padang, West Sumatra Province, where coal mining has continued for more than 100 years. There are other examples at Tambang Sawah, in the Lebong regency, and near Gunung Kerinci within the National Park area. Figure 4 shows a diagrammatic section through Barisan Mountains at Ombilin.
Figure 4 section through the intramontain basin at Ombilin, West Sumatra (Verbeek, 1883)
The coal seam quality is relatively high grade with total moisture contents around 12% and calorific values often in excess of 7000kcal/kg(adb). Potential deposits are relatively small and generally between 10-100mt. The potential for metallurgical grade coal is also relatively good as these coals have experienced significant paloethermal upgrading.
Backarc Basin
The backarc basin is the most prolific of coal forming environment in Sumatra. This is because, in these areas, stable tectonic conditions have prevailed, facilitating thick accumulation and preservation of organic materials over long periods of time. Both South and Central Sumatra display evidence for this with thick extensive coal deposits throughout many areas. Figure 5 shows a diagrammatic section through the backarc basin. The backarc coal deposits range in age from Early Miocene near the uplifted mountain range to peat swamps on the east coast of Sumatra where coal is forming today. Again, as in the forearc basin, the coal deposits can be divided into two broad groups as follows;
Older coal seams (Early Miocene)
These are coal seams that occur close to the Barisan and other uplifted mountain areas. These deposits have similar characteristics to the older coal seams in the forearc and intramontane basins. They also have been subjected to deformation (although generally less than the forearc and intramontane deposits) and as such are less limited in lateral extent by structural controls. Examples of this type of coal are found at in Riau Province, in the north, Jambi Province, South Sumatra Province and Lampung Province, a distance of more than 500km.
The coal seam quality is relatively high grade with total moisture contents around 16% and calorific values often in excess of 6300kcal/kg(adb). Potential resources are relatively large between 100-1000mt.
Younger coal seams (Late Miocene to Quaternary and Pliocene)
Lampung, South Sumatra, Jambi and Riau all have coal resources with relatively thick seams (greater than 10m thick is common) extending over large areas. Total moisture contents range from 25-60% and calorific values range from 5500-3000kcal/kg(adb) depending on the age of the coal, which directly related to distance from the uplift of the Barisan Mountains.
Examples of this type of deposit are in South Sumatra and Jambi. Literally hundreds of locations are known to contain thick coal deposits near the surface. Deposit sizes are of massive scale with billions of tonnes of resources in gently dipping coal seams over vast areas. Relatively little structural disturbance has occurred here.
Figure 5 shows a diagrammatic section through the backarc basin and the relative distribution of coal resources.
COAL RESOURCES
The distribution of coal resources in Sumatra is shown in the Figure 6.
Figure 6 the distribution of coal and peat resources in western Indonesia, brown colour represents coal aged 65-23 million years old, yellow colour is coal 23-2.5 million years old and green represents coal 2.5 million years old until present time
As might be expected the resource of coal in the backarc basin areas far exceeds any of the other geological settings. More than 60 billion tonnes of coal is estimated in this area. Most (59 billion tonnes) of this coal is in the South Sumatra Basin. Most of this resource is in the younger age coal formations with total moisture contents around 50%.
The apparent disproportionate concentration of coal in South Sumatra may reflect the level and history of exploration in the area. Much of South Sumatra is more developed, accessible by roads and infrastructure, and as a result, has experienced more exploration and coal discovery. With this in mind it seems likely that Jambi and Riau provinces have excellent potential for significantly more coal to be discovered.
For older coal seams in the backarc basin (closer to the Barisan Mountains and other uplifted areas) resources seem relatively small. Deposits such as Muara Bungo in Jambi Province, and Indrapura in Riau, show that large resources are possible and additional exploration in these areas is warranted.
In the forearc basin, younger coal seams of lignite grade make up most of the currently known coal resource. Meulaboh in West Aceh Province, has coal resources of more than 500 million tonnes. More recent exploration work suggests more coal is possible and will be discovered as more work is completed. Further south in the northern part of the Bengkulu Block, significant resources of lignitic grade coal are currently being explored and resources in this area is also likely to increase dramatically as more work is done. Significant resources of coal, with grade equivalent to that at Bukit Asam, are also being exploited from North Bengkulu regency. Exploration work is steadily increasing coal resources in this area as well.
Older coal seams in the forearc basin may also be under explored. Evidence for high grade coal in outcrops throughout the forearc basin for hundreds of kilometers suggests that the current resources may be under estimated. Although reported resources are small production from these deposits near Bengkulu, has continued for more than 25 years while the remaining resources have remained constant.
In the intramontane setting only one significant deposit has been exploited to date. This is at Ombilin. It seems very unlikely the Ombilin is the only intramontane basin throughout the entire Barisan Mountains. Evidence for coal in outcrops is known in other places but no significant, systematic exploration in these areas has taken place. Protected Forests and National parks may cover many of these areas as the location coincides with the Barisan Mountains nature conservation zone.
CONCLUSIONS
The easiest and most accessible coal deposits in Sumatra have already been found and are being exploited. Most of these locations were discovered many years ago where coal was recognized outcropping on the surface. At this time only 18% of all the Contract of Work areas for coal in Indonesia are in Sumatra. Large parts of the island are free of mining licenses despite good coal potential in many areas. There is huge potential for the discovery of new coal areas where little or no previous exploration work has been carried out.
Future challenge for exploration will be to locate the next generation of coal deposits that are not so obvious and may not be expressed as outcrops at the surface. The regional distribution of the main coal formations of Sumatra suggests that the opportunity for discoveries of high grade coal with metallurgical potential occurs on both sides of the Barisan Mountains. Similarly, further discovery of younger, lower grade coal is also likely also on both sides of the Barisan Mountains where no significant exploration work has been carried out. The potential is massive and the field is wide open.
About the author:
Daniel Madre has been exploring for coal in Indonesia for more than 25 years. Through his coal exploration contracting company, PT Danmar Explorindo, he has worked on many diverse coal projects. This experience has provided a unique perspective of the geology and distribution of coal deposits throughout the country.