High heat flow effects on a coalbed methane reservoir, East Kalimantan (Borneo), Indonesia

• The CBM reservoir system in East Kalimantan consist of thin coal seams (< 1 to 5 m).
• Gas contents are from < 1 to 13 m3/t.
• Rank varies in both a stratigraphic and geographic context.
• Gas contents are highest close to a geological feature known as the Pinang Dome.

The Balikpapan Formation (Miocene age) in Sangatta, East Kalimantan is thick (> 1500 m) containing abundant coal seams that range in thickness from less than a meter to over 5 m. Coal seams are distributed throughout the section and may represent 5 to 7% of the total formation thickness. Measured gas contents range from < 1 to 13 m3/t (as received basis). The variation is both stratigraphically and geographically controlled. In samples from three drill cores, trends of vitrinite reflectance, calorific value, and moisture content indicate that rank increases down hole. Measured gas content also increases down hole in each core locations. However the rate of change down hole for all of those parameters increases with proximity to the southwestern corner of a geological feature known locally as the Pinang Dome. The Sangatta area has a higher geothermal gradient (50 °C/km) than most other parts of East Kalimantan (25–40 °C/km). It is well documented that the southwest part of the Pinang Dome has elevated organic maturation levels. It is concluded that there is higher heat flow in this area and thus coal beds in proximity have been thermally altered. This is evident not just in the increased rank and measured gas contents but also in the higher CO2 and C2 + gas composition found adjacent to the southwest corner of the Pinang Dome. It is hypothesized that the gas origin in the higher rank area could be thermogenic while gas isotopes from the well furthest from the Pinang Dome, with the lowest rank coals, indicate biogenic origins.

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