Unconventional Reservoir Potential of the Brown Shale, Central Sumatra Basin, Indonesia From Outcrop Characterization

Abstract

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. 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 220m stratigraphic section of the Brown Shale sitting atop 18+m of mineable coal. An outcrop gamma-ray profile was acquired for the complete section in addition to measured sections and rock sample descriptions. Ten facies (A to J) were described in the Karbindo Coal Mine succession. The Brown Shale section is dominated by calcareous mudstone, calcareous shale and claystone, with some very thin beds of siltstone and sandstone. Mudstones are divided into two main groups: massive and lenticular laminated. TOC values range from 2.51% to 8.56% which are indicative of very good to excellent potential source rock for hydrocarbons. Measured Vitrinite Reflectance in the coal underneath the Brown Shale section is 0.63% which places this section in the early oil window. Tmax, Hydrogen Index and Oxygen Index values are characteristic of Type I (lacustrine - oil prone) kerogen with relatively small input of Type III (terrestrial - gas prone) kerogen. Four depositional stratigraphic cycles (or sequences) were interpreted in the Brown Shale section. Cycle #1 corresponds to Balanced-Fill lake type, Cycles #3 and #4 correspond to Overfilled lake type, and Cycle #2 corresponds to the transition between these two lake types. Each sequence is composed of an early transgressive package and an overlying late regressive package. Based on geological and geochemical data, the Brown Shale Formation is a good prospect for unconventional development (horizontal drilling and fracking) in the subsurface. The recommended target zone is a 20m stratigraphic interval within Cycle #2. This section is represented by the brittle-ductile couplet associated with the transition between the balanced-fill and the overfill lake deposits. The brittle zone of this sequence is associated with the high content of quartz and calcite and low content of clay. The underlying and overlying ductile zones are rich in TOC; especially the overlying zone that contain the highest TOC value in the entire section.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015