Peat deposits cover 48,000 km² on the lowlands of Riau Province, Sumatra, Indonesia. Two areas containing typical dome-shaped peat deposits were selected for study. These peat deposits are topographically highest in the geographic interior of the deposit and are drained radially outward by blackwater streams. The source of the water in the peat is precipitation, which exceeds evapotranspiration throughout the year. In cross section, the peat deposits are biconvex; they rest on a nearly level surface, which is within a few meters of sea level. The peat accumulated in the past 5,000 years after stabilization of sea level following the rapid sea-level rise during glacial retreat. In the interior area of the peat deposits, the initial peat accumulation rate was rapid (4–5 mm/yr) for approximately 1,000 years; the rate decreased to less than 2 mm/yr for the past 3,500–4,000 years.

These peat deposits have a fibric to hemic texture with slight to moderate humification, a low ash yield, and a low sulfur content; and they contain acid water. A thin layer at the bottom of the deposits tends to be more sapric in texture, more humified, higher in ash yield, higher in sulfur content, and less acid than the overlying peat. Proximate and ultimate analyses of a suite of samples from the interior of each peat deposit show no significant differences in peat quality between the Siak Kanan and Bengkalis Island peat deposits.

A primary goal of this study was to evaluate peat resources. The 6.6 × 10⁹ m³ of peat in the Siak Kanan peat deposit and 3.0 × 10⁹ m³ of peat in the Bengkalis Island peat deposit constitute a significant fuel resource. This resource study has contributed a three-dimensional framework and peat quality data that can provide insight into the earliest stage of certain types of coal formation.