Abstract

The forearc basins of northwestern Sumatra have several biogenic gas discoveries but are often overlooked and have been under-explored for many years. In the shallow water offshore western Aceh, biogenic gas was found within Upper Miocene Carbonate buildups, for examples in Meulaboh-1, Keudapasi-1 and Singkil-1. Similar gas is also present in the Pliocene sandstones of Palambak-1 well near Banyak Islands, and within the Miocene sandstones of Merah-1 well near Nias Island. The carbonate play usually attracts the most attention due to the proven discoveries and potentially large undrilled closures. Clastic plays have also been targeted, however most of the wells are situated closer to the eastern edge of the outer-arc high, where structural closures are present within backthrust–controlled anticlines.

Interpretation of 2008–2010 regional 2D seismic lines and well data indicate a Middle Miocene Clastic play sourced from Bukit Barisan potentially also exists in the deep-water area. On seismic, this sedimentary package shows stacked channel features that occur on a slope off paleo-shelf break where some of them were delivered to the basin through valleys. The Middle Miocene clastic play has been identified as submarine fans deposited on a slope to basinal setting and it is likely to be analogous to Baong Sandstones in the North Sumatra Basin. Estimated porosity range of this potential sandstone reservoir is 15% to 30%. Sedimentation rate and temperature model of the area indicate that the reservoir packages are still within biogenic gas generation window. Reprocessed seismic lines show amplitude brightening in several spots and AVO screening indicate the possibility of having Class 3 AVO, suggesting gas is present in the system. The Middle Miocene Clastic play can contain up to multi-TCF gas potential. In addition to the proven shallow-water Late Miocene carbonate play, the Middle Miocene Clastic play should be considered as the target for future forearc exploration in the offshore Western Sumatra.