The Brown Shale Formation: Lacustrine Deposit in the Central Sumatra Basin (Indonesia)

Abstract

The Brown Shale, part of the Pematang Group, is an Eocene-Oligocene sequence of lacustrine mudstones deposited in the Central Sumatra basin in Indonesia. It is considered a world-class source rock, and the most important one in Central Sumatra Basin Petroleum System. This work shows the results of the characterization of one of the few available outcrops of this formation in Indonesia: The Karbindo Coal Mine section. This study consists of field and lab data: measured section, outcrop gamma-ray profile, facies description, RockEval and vitrinite reflectance (%Ro) measurements, X-Ray Diffraction (XRD) mineralogy, thin section and SEM imaging, porosity, permeability, palynology, and X-Ray Fluorescence (XRF) elemental profiles.

The Karbindo Coal Mine section is a 220m (720 ft) stratigraphic exposure of the Brown Shale. Dominant facies are massive calcareous mudstones, laminated calcareous shales, claystone, and thin beds of siltstone and sandstone. Total Organic Content ranges from very good (2.5%) to excellent (8.5%) source rock potential. This section is categorized within the early oil window from %Ro data measured in the directly underlaying coal interval. Kerogen type is interpreted to be Type I (lacustrine, oil-prone) from RockEval analysis, and supported by biomarkers and palynology. Main mineralogical components are calcite and clays, with few proportion of quartz. Four depositional stratigraphic sequences were interpreted: Sequence 1 corresponds to Balanced-Fill lake type, Sequence 3 and 4 correspond to Overfilled lake type, and Sequence 2 corresponds to the transition between these two lake types; all in a tectonic-controlled lake basin type progression. A comparison to the stratigraphic framework of the Green River Formation is provided in this work.

Previous work was focused in study the source rock potential of this formation, however in this study we make emphasis in the stratigraphic component and vertical variability of facies, as well as the main characteristics that contribute to develop this formation as an unconventional shale resource, like for example brittleness index, organic richness, and possible target zones for horizontal drilling. This section can be used as a stratigraphic analog to the Brown Shale in the subsurface during the assessment and development of this unconventional play.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018