This paper was first published in February 2014, authored by Daniel Madre and Tobias Maya.
ABSTRACT
Coal resources in Sumatra are the largest in Indonesia. At this time almost all of these coal resources are concentrated in South Sumatra Province. This may not be because South Sumatra has more coal than other provinces in Sumatra but more likely a consequence of the relative lack of exploration in other areas.
Coal in Sumatra has been observed from Lampung in the south, to Aceh in the north, a distance of more than 2000km. The distribution of coal deposits in Sumatra is controlled by the tectonic setting of collision between the Australian plate and the Eurasian plate. Relatively high grade coal is found in the foothills on both sides of the Barisan Mountains. These deposits have been subjected to significant tectonic movement and as a result are often structurally disturbed. Relatively younger, lower grades of coal occur on both sides of the Barisan Mountain Range as well. These deposits are further away from the Barisan Mountains where more stable conditions have allowed thick coal to accumulate.
Even though Sumatra has the largest coal resource in Indonesia it still accounts for less than 10% of the total annual coal production. This is because, unlike Kalimantan, most of the coal deposits are not close to deep rivers or ports where coal ships can be loaded. The future of coal production from Sumatra will rely on significant investment in transportation infrastructure.
INTRODUCTION
Conditions suitable for the formation of significant deposits of coal have occurred throughout the island of Sumatra despite the active tectonic setting. Sumatra, as we know it today, is formed by the collision between the Australian plate and the Eurasian plate. In Sumatra this collision is oblique resulting in significant and movement which is often felt throughout the region as earthquakes and their associated tsunamis. Figure 1 shows a tectonic map of Indonesia.
Figure 1 tectonic map of Indonesia (McCaffery, 2009)
TECTONIC FRAMEWORK OF SUMATRA
The plate collision offshore, to the west of the island, has caused Sumatra to align itself in a north, northwest direction parallel to the collision forces. As a result huge breaks in the earth’s crust occur along the length of the island that have divided Sumatra into four major structural blocks that control the distribution of coal deposits. Figure 2 shows a map of the major faults that run along the length of Sumatra.
Figure 2 major faults in Sumatra (McCaffery, 2009)
A diagrammatic section, through the island, shows these main structural blocks which are known as tectonic settings (see figure 3).
Figure 3 diagrammatic cross-section through the island of Sumatra
These tectonic settings that divide coal deposits into several distinct types are;
outer island arc
forearc basin
volcanic arc
backarc basin
The coal potential of each tectonic setting is summarized below;
Outer Island Arc
This includes the islands of the Mentawai Archipelago. Tectonic movement and in particular vertical changes in elevation in response to mega-thrust earthquakes makes this tectonic 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. The overall the outer island arc is not prospective for coal.
Forearc Basin
This area includes the entire west coast of Sumatra. Coal occurrences are known throughout the forearc basin of Sumatra.
Figure 4 forearc basin coal deposits modified after Horkel, 1989
These coal occurrences can be classified into two basic groups as follows;
• Old 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 to the surface. These coal measures and the associated coal seams are relatively thick with seams > 4m commonly observed. The deposits have been observed from south of Bengkulu to north of Padang a strike distance of more than 400km. The coal seam quality is relatively high grade with
total moisture contents approximately 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 Sibolga. The size of these deposits are likely to be relatively small (10-100mt) due to structural controls. Coal quality is likely to be very good and potential for metallurgical coal is possible due to relatively high heat flows caused by the Barisan uplift over time.
• 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 25-60% total moisture and 3000-5500kcal/kg(adb) energy depending on the age. The coal grade in these deposits is generally directly proportional to the distance from the Barisan Mountain range.
Examples of these types of deposits are at Lais and Ketaun in North Bengkulu and Meulaboh in West Aceh. Potential deposit size is relatively large and deposits around 100mt or more are documented.
Figure 5 is a diagrammatic section through the forearc basin showing the relative position of the older coal seams in the foothills of the mountains and the younger coal seams on coastal plain.
Figure 5 diagrammatic section of the forearc basin showing location of coal deposits
• Volcanic Arc (Intramontain Setting)
When the Barisan Mountains formed some coal deposits became caught up in the earth movements and formed outliers within the mountain range. These deposits have similar characteristics to the old coal seams in the forearc basin. They are of similar age and origin. They also have been subjected to intensive deformation (even more than those in the forearc basin) 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. There are other examples at Tambang Sawah and near Gunung Kerinci within the National Park area. Figure 6 shows the location of some intramontain coal deposits.
Figure 6 location map of some intramontain coal deposits relative to the major faults
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
Back arc basins are the most prolific of coal forming environments in Sumatra. This is because in these areas stable tectonic conditions 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. The location of backarc coal measures are shown in Figure 7.
Figure 7 backarc basin coal measures location map (modified after Horkel, 1989)
Figure 8 shows a diagrammatic section through the backarc basin. The back arc coal deposits range in age from Early Miocene near the uplifted mountain range to peat swamps forming coal now on the east coast of Sumatra.
Figure 8 diagrammatic section through the backarc basin with coal deposit locations
Again as in the forearc basin the coal deposits can be divided into two broad groups as follows;
• Old 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 old coal seams in the forearc and intramontain basins. They are of similar age and origin. They also have been subjected to deformation (although generally less than the forearc and intramontain deposits) and as such are less limited in lateral extent by structural controls. Examples of this type of coal are found at Riau in the north, Muara Bungo in Jambi Province and in South Sumatra and Lampung a distance of more than 500km.
The coal seam quality is relatively high grade with total moisture contents approximately 16% and calorific values often in excess of 6300kcal/kg(adb). Potential deposit size is relatively large between 100-1000mt.
• Younger coal seams (Late Miocene to Quaternary and Pliocene)
Lampung, South Sumatra, Jambi and Riau all have coal deposit with relatively thick seams (>10m thick is common) extended 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 caused by the volcanic arc.
Examples of this type of deposit are at Muara Enim in South Sumatra and Cerenti in Jambi. Literally hundreds of locations are known to contain exploitable coal deposits. Deposit sizes are of massive scale with billions of tonnes of resources in gently dipping coal seams over vast areas. Little structural disturbance has occurred here other than gentle undulating folds.
COAL RESOURCES
The distribution of coal resources in Sumatra is shown in the resources map (see figure 9).
Figure 9 coal resources of western Indonesia location map
As might be expected the resource of coal in the backarc basin areas far exceeds any of the other tectonic 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. Almost all of this resource is in the younger age coal formations with are in grades ranging from Bukit Asam quality to lignites with total moisture contents of more the 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 allot 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 Bungo, and Indrapura 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 a published resource of 450 million tonnes. More recent exploration work suggests allot more coal is possible and will be discovered as more work is completed. Further south in the northern part of the Bengkulu Basin, 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. 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 or even slightly increased.
In the intramontain setting only 1 significant deposit has been exploited to date. This is at Ombilin. It seems very unlikely the Ombilin is the only intramontain basin throughout the entire Barisan Mountains. Evidence for coal on outcrops is known in other places but no significant exploration in these areas has taken place. Many of these areas are within Protected Forests and National Parks. Figure 10 shows the coal resources of Indonesia.
Figure 10 coal resources of Indonesia
Figure 11 shows the distribution of coal resources by province. South Sumatra has the largest coal resources in the country at this time.
Figure 11 coal resources by province
TRANSPORTATION LOGISTICS
Again the tectonic setting is in control of this aspect of Sumatra coal mining potential. Figure 12 shows a coal port infrastructure map of Sumatra. As might be expected, the forearc basin, where geological structure and uplift occur relatively close to the sea shore, is relatively good for potential harbor and shipping prospects. The shoreline in many areas has deep natural harbours where ships can be loaded. Some of these have been recognized by the Indonesian Government and multi user coal loading terminals have been constructed. The best known of these is at Teluk Bayur near Padang in West Sumatra. This area is connected by rail to Ombilin coal field. The port and associated transport infrastructure make it a loading point for coal for producers up to several hundred kilometers away. Another example is Pulau Baai at Bengkulu. Recently dredged to 6.5m depth, small vessels (to 25000t) can be loaded directly and larger vessels loaded by offshore trans-shipment at Pulau Tikus. Coal transport gravitates to Pulau Baai from deposits more than 100km away.
In other areas on the west coast open ocean swell makes ship loading dangerous and harbours difficult to construct.
In north Bengkulu construction of a coal loading facility has been an engineering challenge for its owners.
At Meulaboh currently port construction will be completed in June 2014. An unprotected coast line, consistent open ocean swell and relatively shallow bathymetry is making shipping a challenge.
Access to the ports on the west coast is predominantly via public roads.
In the back arc basin the gentle, shallow dipping tectonic environment makes for shallow, meandering rivers and a coastline that is generally unsuitable for loading coal to vessels. Again the Indonesian Government has made some efforts to establish infrastructure for the coal industry. Muara Enim is connected to Tarahan port at Lampung via a railroad that has been operating for more than 25 years. The rail road is currently being upgraded to increase capacity. The Tarahan port has the capacity to load large vessels but the distance from the South Sumatra coal fields is very far. Rail links used by PT BA have also been built to Palembang from Muara Enim where barge loading of coal takes place. Other producers are using public roads to get coal to the Musi River where barges of various sizes, according to river depth, are loaded. The main loading ports at Palembang are at Muara Banyasin and Tanjung Api Api. Similarly in Jambi coal producers are utilizing public roads for coal hauling and loading barges of various sizes, depending on river levels, to get the coal to mother vessels. In general the haul distances are relatively far and barging coal on shallow rivers makes coal transportation inefficient and expensive. Coal trans-shipment generally occurs at Muara
Sabak. Some Jambi producers have been hauling coal to Teluk Bayur a distance of more than 200km to reach a port where coal loading to mother vessel can occur. Riau province has a similar situation. Coal is being loaded onto barges at Mumpa for trans- shipment offshore.
The future of the coal industry in the Sumatra will depend on the development of infrastructure. Some companies have recognized this opportunity and plan to build toll roads dedicated for coal haulage. In south Sumatra publicly listed toll road operator PT CIpta Marga Nusaphala Persada plans to build 100km of coal haul road from Lahat to Palembang and the Servo Group plan to build a haul road for 230km to connect Lahat to Tanjung Lago harbour. A similar initiative is being pursued in Jambi province to construct a 230km road from Sarolungun Ujung Jabung. Government plans also to dredge the Batanghari River as an alternative route for coal transportation.
Figure 12 coal ports in Sumatra
COAL PRODUCTION
Despite the fact that coal resources in Sumatra are larger than those in Kalimantan the production of coal from Sumatra is less than 10% of Indonesia’s total production of coal. This is partly a consequence of the tectonic setting of the island. The biggest deposits in Sumatra are in the east of the island where navigable rivers and a shallow coastline restrict coal transportation. On the west coast more possibilities for harbours exist but current coal resources tend to be more limited in size making investment in infrastructure difficult to justify.
At this time almost all of these coal resources are concentrated in South Sumatra Province. This may not be because South Sumatra has more coal than other provinces in Sumatra but more likely a consequence of the relative lack of exploration in other areas. Figure 13 shows the location of the coal lease COW and IUP production areas of Sumatra.
Figure 13 location of coal leases (in light green) of Sumatra
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. The coal lease map in Figure 13 represents the distribution of these most obvious coal deposits that were outcropping or near the surface.
Future challenge for exploration will be to locate the next generation of coal deposits that are not so obvious and lie concealed beneath the earth. The character and distribution of the main coal formations of Sumatra is well documented and the next wave of coal discovery will be made by those willing to interpret the known geology, predict where coal deposits should occur and test these theories in the field. The potential is massive and the field is wide open.
REFERENCES
Horkel A, 1989, On the Plate Tectonic Setting of Coal Deposits in Indonesia and the Philippines
Indonesian Directoctorate of Mineral Resources, 1990, Distribution of Coal and Peat Resources in Indonesia
McCaffery R, 2009, Tectonic Framework of the Sumatran Subduction Zone Ministry of Energy and Mineral Resources, 2011, Indonesian Coal Resources Petromindo 2010/11 & 2012/13 Indonesian Coal Book