Shale Oil in Indonesia's Central Sumatra Basin: USGS Assessment, 2015

Summary

Using a geology-based method, the U.S. Geological Survey estimated the undiscovered, technically recoverable shale oil and gas in the Central Sumatra Basin, Indonesia. Mean estimates:

• 459 million barrels of shale oil;
• 275 billion cubic feet of associated gas;
• 23 million barrels of natural gas liquids.

The Central Sumatra Basin and its Brown Shale assessment unit. USGS; base map from the National Park Service.

The basin

Exploration since the 1940s has found more than 200 oil and gas fields here, mostly oil. Conventional resources were assessed in 2010; this study looks at shale oil — oil still held in its source rock.

• How it formed. The basin is one of a chain of back-arc basins tied to subduction along the Sumatra margin. Rifting in the Eocene to early Oligocene opened grabens, which filled with river, delta and lake sediments, including lake shales.
• The source rock. The Oligocene Brown Shale of the Pemantang Group, an organic-rich lake shale, sourced most of the basin's oil and gas.
• Maturing. After rifting stopped, the crust cooled and sagged, burying the shale deeper and making its organic matter more thermally mature.
• Squeezing. Compression from the middle Miocene to the Holocene built many structures that trap conventional oil. It may also have driven oil out of the Brown Shale — reducing what remains in it.

The assessment

To count as a continuous (shale) oil system, a rock must:

1. hold more than 2 weight percent total organic carbon;
2. be in the thermal maturity window for oil;
3. contain more than 15 meters of organic-rich shale;
4. hold Type I or Type II organic matter.

On those criteria the USGS defined the Brown Shale–Sihapas Total Petroleum System and a Brown Shale Continuous Oil Assessment Unit. The range in its possible area reflects uncertainty about how far the source rock extends in the grabens, how mature it is, and how much oil stayed after compression. Well drainage areas, success ratios and recoveries per well come from analogs, including the lake-deposited Green River Formation of Utah's Uinta Basin.

Key inputs

Results

F95 means a 95 percent chance of at least that amount; F5, a 5 percent chance.

Using the mean area, drainage area and success ratio, recovering the oil would take about 5,200 hydraulically fractured wells.

Sources

Based on Christopher J. Schenk and others, "Assessment of Shale-Oil Resources of the Central Sumatra Basin, Indonesia, 2015," U.S. Geological Survey fact sheet (2015); a work of the United States government in the public domain. The map and both tables are taken from the fact sheet's PDF.