The Late Miocene Coalbed Methane System in the South Sumatra Basin of Indonesia

The purpose of this paper is to provide a view on the South Sumatra basin CBM development potential. The basin ranks as one of Indonesia's most prospective coalbed methane (CBM) basins, but well testing is still in the earliest stages. In S. Sumatra basin there are three distinct identifiable sub-basin evolution periods. First, a period of horst-graben development controlled by major localized extensional faults that initiated the basin architecture. This episode was followed by rapid to moderate sedimentation controlled by normal faults resulting in deposition of Oligocene Talang Akar alluvial, fluvial and deltaic deposits. Secondly, a period of decreased fault activity along with regional subsidence coinciding with the deposition of the post-rifting Miocene Air Benekat and Muara Enim Formations. The third period is marked by regional uplift resulting in wide spread basin inversion over the entire region. Coal seams in the Muara Enim Formation are thickest and most numerous in the SW half of the South Sumatra basin representing key target for CBM development. This is the axial region of the basin where the subsidence has been most pronounced and the formation is thickest. While the coals are quite thick and laterally extensive in the broad synclines, most of the Miocene Muara Enim Formation has been eroded out along the anticlines. The Lematang Depression and the Central Palembang sub-basin are considered the most prospective CBM areas on the basis of coal thickness, depth, and gas kicks on mud logs. The Muara Enim formation comprises more than 3500 ft of paralic sandstones and mudstones, with thick intercalated coal seams. Typically some 10 to 15 individual coal seams are present.

The coals are of sub-bituminous rank and characterized by low vitrinite (huminite) reflectance (VR_r = 0.35 – 0.46%). These low rank coals are dominated by huminite (34.6 – 94.6 vol. %). Less abundant are liptinite (4.0 – 22.5 vol. %) and inertinite (0.2 – 43.9 vol. %). Minerals are found only in small amounts (0 – 5 vol. %); mostly as iron sulfide. Kaolinite occurs as cleat fillings at some places. The coals are characterized by high moisture content (4 – 21 %) and volatile matter content (> 40 wt.%, daf), and less than 80 wt. % (daf) carbon cotent.

CO₂ and CH₄ sorption isotherms on six different coal samples were carried out at reservoir temperature. Given the uncertainty around the gas content measurements, the coals tend to suffer from some serious under saturation. The sorption capacities of the coals tend to decrease with increasing depth. This behavior can either be related to the increasing moisture content of the coal with depth or with the significant variation of the vitrinite content of the deeper seams. The high CO₂ adsorption capacity and the low rank tend to make these coals ideal targets for CO₂ sequestration.