Ketungau and Melawi Basins, West Kalimantan

Ketungau and Melawi Basins, West Kalimantan

The Ketungau and Melawi Basins are located in the West Kalimantan region, adjacent to the Ma­laysian border. The Melawi Basin, in the south is separated from the Ketungau Basin by the Semitau High. Tectonically, the Ketungau and Me­lawi Basins can be classified as intramontane basins. The Ketungau and Melawi basins are separated from each other by a belt of deep-water rocks and a belt of melange. 

Regional geological pattern of Kalimantan

Preliminary exploration and assessment of the Ketungau and Melawi Basins were carried out during 1980s and 1990s by several oil companies. The most recent assessment work is conducted by Lemigas Team, and resulted in the Kayan Play Model for Melawi Basin exploration. In 2009-2010, the Geological Agency carried out field work activities in the Ketungau and Melawi Basins, which was intended to collect field data, outcrop samples, and sedimentologic and stratigraphy data, as reported by Santy et al. (2009), Gumilar et al. (2009), Heryanto et al. (2009), Santy et al. (2010), and Gumilar et al. (2010). Observation had been done in the Ketungau Formation, outcrop­ping in Ketungau and Sekalau River, and Silat For­mation, outcropping in Silat Rivers and its tributaries in the Melawi Basin. The aim of the study is to assess the probability of petroleum source rock potential for hydrocarbon play in the Ketungau and Melawi Basins. Several samples had been selected for an organic geochemistry analysis.

General Geological Setting

Tectonic activities in this region were controlled by the movement of Eurasian Plate to the southeast during Cretaceous - Early Tertiary. Pre-Tertiary tectonic activities created uplifting of Semitau Complex and Boyan Melange Complex separat­ing the Ketungau and Melawi Basin. Nevertheless, Halls and Nichols (2002) suggest that the Ketungau and Melawi Basins are not conventional foreland basin formed by loading of thrust sheets, indicated by the absence of thin skinned thrusting in the highly eroded areas.

The pattern of Ketungau-Melawi Basin bound­aries follows the direction pattern of NW-SE strike slip zone developing during Eocene-Oligocene (± 30 Ma) at the Sundaland Margin in Kalimantan. A 45o counter clockwise rotation during Late Oligocene to Early Miocene (± 20-10 Ma) resulted in a basin configuration as observed today. The next Neogene tectonic activities caused an E-W trending thrust system, sediment fold­ing, and created Ketungau, Silat, and Melawi syn­clines, as well as Sintang anticline.

 The base of Ketungau and Melawi Basins is not exposed, though there is a thick succession of lithic arenite sandstone sequence consisting of sandstone, silt, and mudstone. The thick sediment succession is a result of basin subsidence as the response of sedi¬ment infill in the boundary between a linear zone of granite and schist in the northern part (Semitau High), and the base of continental plate in the south¬ern part (Schwaner Mountain Zone).

Sediment infill in the Ketungau-Melawi basin was dominantly from the eroded rocks of older orogen in the Kalimantan Island. Small part of sedi¬ment supply probably also comes from Indochina land (Halls and Nichols, 2002). High rate of clastic detrital sediment supply in this basin had suppressed the productive development of carbonate benthic, therefore no well developed carbonate sediments exist.

Sedimentary phase of the Ketungau Basin oc¬curred during Eocene until Oligocene, with the deposition of fluvial conglomerate unit gradationally change to be lacustrine and shallow marine sediment unit of Kantu Formation. The Kantu For¬mation is conformably overlain by a fluvial clastic unit of the Tutoop Formation and a fluvio-marine de¬posit of the Ketungau Formation. Stratigraphic succession in the early develop¬ment of the Melawi Basin has a similar character¬istic and lithologic distribution with the Ketungau Basin. Those formations were deposited above the Pre-Tertiary basal sediments of Selangkai Formation. The Haloq For¬mation, the oldest sediments deposited in the basin, is regarded as an equivalence of Lower Ketungau. This formation consists of fluvial quartz sandstone and conglomeratic unit, deposited during Upper Eo¬cene. The Ingar Formation unconformably overlying the Haloq Formation, consists of alternating mud¬stone, silt, and fine sandstone of lacustrine deposit. The Dangkan Formation, which is considered to be equivalent to the Tutoop sandstone, was deposited unconformably over the Ingar Formation. It is fol¬lowed by the Silat Shale, regarded to be equivalent to the Ketungau Formation, that was deposited dur¬ing Oligocene. When the sediment deposition in the Ketungau Basin had terminated, the deposition in the Melawi Basin still occurred where fluvial units of the Payak, Tebidah, and Sekayam Formations were deposited.

Geological Map of Ketungau-Melawai Basins

Description of Ketungau and Silat Formations

As a whole packet, the Ketungau Formation is 900 m thick, consisting of claystone, shale, silt, fine sandstone, and thin bedded coal in the upper part. Claystone layers usually contain silt or fine sand concretions and mollusk fossils of Gastropods and Bivalves. Ichnofossils of Planolites, Thalassinoides, and Ophiomorpha were sometimes found in some layers. Sandstone is usu­ally micaceous and contains framboidal pyrite as an indication of marine influence. Shale layers are flaky, rich in organic matters, and contain mollusk fossils of Gastropods and Bivalves, of which some of them are in juvenile forms. The depositional environment of this forma­tion is fluvio-marine, with the interval of shallow marine sediments appearing periodically.

Stratigraphic comparison between Ketungau Basin and Melawi Basin, and Lupar-Serawak Valley

The Silat Formation consists of 1000 thick sedi­ments, dominated by black carbonaceous mudstone, shale, slaty shale, minor dark coloured siltstone, fine- to medium-grained sandstone, and occasionally thin layer of coal. In several spots, there are also rich layers of Gastropod, Pelecypod, and plant remains. The depositional environment of Silat shale is fluvio-marine to open.

References:

• Hall, R., 1996. Reconstructing Cenozoic SE Asia: In: Hall R. and Blundell D., (eds.) Tectonic evolution of Southeast Asia. Geological Society of London, p. 153-184
• Hall, R. and Nichols, G., 2002. Cenozoic Sedimentation and Tectonics in Borneo : Climatic Influences on Orogenesis. In: Jones, S.J. and Frostick, L. (eds.), 2002 Sedimen Flux to Basins : Causes, Controls, and Consequences, The Geological Society of London, Special Publication.
• Heryanto, R. Williams, P.R., Harahap, B.H., and Pieters, P.E., 1993a. Peta Geologi Lembar Putussibau, Kalimantan, Skala 1: 250.000, Pusat Penelitian dan Pengembangan Geologi, Bandung.
• Heryanto, R., Williams P.R., Harahap B.H., Pieters P.E., 1993b. Peta Geologi Lembar Sintang, Kalimantan skala 1 : 250.00. Pusat Penelitian dan Pengembangan Geologi, Bandung.
• L. D. Santy and H. Panggabean, 2013, The Potential of Ketungau and Silat Shales in Ketungau and Melawi Basins, West Kalimantan: For Oil Shale and Shale Gas Exploration, Indonesian Journal of Geology, Vol. 8 No. 1
• Pieters, P.E., D.S. Trails, and Supriatna S., 1987. Correlation of Early Tertiary Rocks Across Kalimantan. Proceedings of Sixteenth Annual Convention of Indonesian Petroleum Association,16, p.291-306.
• Williams, P. R., Supriatna, S., Trail, DS., and Heryanto, R., 1984. Tertiary Basin of West Kalimantan, Associated Igneous Activity and Structural Setting. Indonesian Petroleum Association 13th Annual Convention Proceed­ings, p.151-160.