By the USGS Sumatra Basins Assessment Team: Christopher J. Schenk, Timothy R. Klett, Marilyn E. Tennyson, Tracey J. Mercier, Michael E. Brownfield, Janet K. Pitman, Stephanie B. Gaswirth and Thomas M. Finn. USGS Fact Sheet 2016–3089, December 2016.
Using a geology-based method, the U.S. Geological Survey estimated a mean of 20 trillion cubic feet of undiscovered, technically recoverable gas in the coal beds of the Central and South Sumatra Basin Provinces, Indonesia.
The two basin provinces and the two assessment units. Base: National Park Service. Map: USGS.
What was assessed
For each basin the USGS defined a Neogene Coal Total Petroleum System and one assessment unit (AU): the Central Sumatra and South Sumatra Neogene Coalbed Gas AUs. Minable low-rank Neogene coal has been mapped in both basins, and there have been recent attempts to produce coalbed gas in South Sumatra. Coal thickness, composition, gas content and maturity there are well studied.
Three things shaped the assessment:
- Low rank: the coals are immature, under 0.5 percent vitrinite reflectance.
- Biogenic gas: the gas was made by bacteria, not by heat.
- Undersaturated: the coals hold less gas than they could.
Thermally generated coalbed gas was not assessed.
The geology
Both basins are backarc basins, formed behind the subduction zone along Sumatra's margin. In the late Paleogene, the Australian plate pushed obliquely against the Sumatran plate, forming the right-lateral Sumatran Fault and lifting the Barisan Mountains. Sediment shed from those mountains built a clastic wedge eastward across both basins, and the coals formed within it.
Buried only shallowly, the coals stayed thermally immature (except near intrusions). Bacteria made gas within them. Liptinite gives some coals hydrogen indices as high as 300 milligrams hydrocarbon per gram organic carbon, so they might yield liquids — but given their low rank, liquids were not assessed.
The inputs
Drainage areas, success ratios and recoveries per well come from U.S. analogs, especially the biogenic coalbed gas of Wyoming's Powder River Basin:
| Input | Central Sumatra | South Sumatra |
|---|---|---|
| Potential production area (acres): min / mode / max | 1,000 / 2,569,000 / 12,844,000 | 1,000 / 4,076,000 / 16,302,000 |
| Production area, calculated mean (acres) | 5,137,933 | 6,792,833 |
| Well drainage area (acres): min / mode / max | 40 / 80 / 120 | 40 / 80 / 120 |
| Success ratio (%): min / mode / max (mean) | 50 / 60 / 70 (60) | 50 / 70 / 80 (66.7) |
| Recovery per well, billion cubic feet: min / median / max (mean) | 0.08 / 0.2 / 0.35 (0.206) | 0.08 / 0.2 / 0.35 (0.206) |
| AU probability | 1.0 | 1.0 |
The results
Fully risked estimates of undiscovered gas, in billion cubic feet (BCFG). F95 means a 95 percent chance of at least that amount; F5, a 5 percent chance.
| Assessment unit | F95 | F50 | F5 | Mean |
|---|---|---|---|---|
| Central Sumatra | 1,880 | 7,216 | 17,160 | 8,119 |
| South Sumatra | 2,989 | 10,677 | 24,829 | 11,921 |
| Total | 4,869 | 17,893 | 41,989 | 20,040 |
- In all: a mean of 20,040 BCFG (about 20 trillion cubic feet), with a range of 4,869 to 41,989 BCFG.
- Liquids: none — natural gas liquids were estimated at 0.
More assessments: the USGS Energy Resources Program.
Sources
Based on Assessment of Coalbed Gas Resources of the Central and South Sumatra Basin Provinces, Indonesia, 2016, USGS Fact Sheet 2016–3089, U.S. Geological Survey; a work of the United States government in the public domain. The map and both tables are taken from the fact sheet's PDF; the import had lost them.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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