Outcrop Characterization and Sequence Stratigraphic Framework of the Brown Shale, Central Sumatra Basin, Indonesia: Implications as an Unconventional Resource Shale

Brito, Richard J.; Slatt, Roger M.; Kusuma, Didit P.; Singh, Baldeo

The Brown Shale (Pematang Group) is an Eocene-Oligocene sequence of lacustrine mudstones deposited in the Central Sumatra basin. It is the major and aerially extensive hydrocarbon oil and gas source rock for shallower reservoirs in Central Sumatra (Williams et al. 1985). The objective of this research was to characterize the Brown Shale from an unconventional resource perspective; to do this, an outcrop located inside the Karbindo Coal Mine in Central Sumatra Island was studied.

The outcrop is a 200+m stratigraphic section of the Brown Shale sitting atop 18+m of mineable coal. An outcrop gamma-ray profile was acquired in the complete section including the coal. The sampling interval of the geological description and gamma-ray acquisition was one (1) meter.

The strata are subdivided into three intervals. The lower interval (0-60m) of organic-rich shale with high gamma ray count. The top of this interval is a dark gray - black mudstone with high content of macrofossils. The middle interval (60-110m) is composed of silty shales, dark black mudstones and fossilifeous shales. The upper interval (110 to 200m) contains the most competent rocks in the section with low gamma ray values and more siliceous content. On the top of the section there are highly weathered, fissile shales interbedded with massive mudstones. The lower interval is mainly ductile while the middle and upper intervals are more brittle. Ro indicates this section is in the early oil to peak oil window and TOC ranges from 2% in the upper interval to 6.2% in the lower interval. A sequence stratigraphic framework has been interpreted indicating the Brown Shale was deposited in three transgression-regression-transgression cycles.

The stratigraphic characterization of this outcrop is the first step toward better understanding this potential reservoir rock in the Central Sumatra basin and the opportunity to develop the Brown Shale as an unconventional resource.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013