Sandstone Source Rocks with Coal Fragments and Abundant
Leaf Matter in the Kutei Basin
There are different kinds of
sedimentary basin hydrocarbon systems. The type of kerogen generated through
deep burial catagenesis depends on the source of the organic matter. Marine
mudstones are the most common hydrocarbon source rocks, but there are also
occurrences of terrestrial-sourced organic matter. Apparently, this is the case
in the Kutei Basin, offshore Indonesia. This was determined back in 2006 and
explained in an AAPG Bulletin paper led by author Arthur Saller. He noted that
cores and drill cuttings show that the basin’s best source rocks are sandstones
that also carried plant matter that got deposited with the turbiditic deep
water sandstones. Saller noted that the sandstones have significantly higher
TOC values than the shales. It was noted that the organic matter was composed
of:
“…laminar coaly fragments, pieces of wood, resinite, and
other coaly debris. Laminar coaly fragments are dominant, and their size,
shape, and cuticle structure indicate that they were leaf fragments. Fossil
leaf fragments occur in all deep-water sandstone packages from the upper slope
to the basin floor. Fossil leaf fragments were apparently carried into deep
water by turbidity currents during lowstands of sea level.”
It is also noted that source
rocks that contain leaf fragments often result in liquids that are waxy.
A recent article in GeoExPro
by Henk Kombrink explains why the mudstones and shales have much lower TOC
values in what is considered to be a marine basin:
“Well-oxygenated conditions, resulting in rapid
decomposition of the algal material, and strong bottom currents are seen as two
important factors as to why the shales do not have high marine organic
matter contents.”
Kombrink notes that the
organic matter carried by the Miocene-aged sandstones in turbidity currents was
deposited quickly in deeper water, resulting in preservation of the organic
matter. Fluvial-Deltaic systems carried the organic matter to the sea, and
turbidity currents quickly carried it underwater to the slope and basin floor.
Eni’s New Giant Natural Gas and Condensate Discovery – The
Geliga-1 Well
Eni’s new discovery, the
Geliga‑1
exploration well, drilled in the Ganal block in the Kutei Basin, approximately
70 km from the East Kalimantan coast, is estimated to contain in-place
resources of approximately 5 trillion cubic feet (Tcf) of gas and 300 million
barrels of condensate. The well was drilled to a total depth of around 5,100
meters in a water depth of about 2,000 meters. Excellent reservoir conditions
were encountered, and a Drill Stem Test (DST) is planned to confirm production
potential.
The Kutei Basin has a very
successful exploration history, with recent wells in 2023 and 2025 encountering
significant reserves.
According to Eni:
“The Geliga‑1 discovery also follows the recent
Final Investment Decisions (FIDs) for the Gendalo and Gandang gas project
(South Hub), and for the Geng North and Gehem fields (North Hub). The North Hub
project will leverage a newly built FPSO with a handling capacity of 1 bscfd of
gas and 90,000 bpd of condensate, as well as the existing Bontang LNG Plant.”
“Initial assessments indicate that the combined Geliga
and Gula resources have the capacity to produce an additional 1 bscfd of gas
and 80,000 bpd of condensate, opening the possibility—among others—of
establishing, in a fast‑track
mode, a third production hub in the prolific Kutei Basin by mirroring the
development concept of the ongoing North Hub project. Studies are also underway
to evaluate a further rejuvenation of Bontang by resuming additional
liquefaction capacity beyond what is already planned for the North Hub
development, thus further extending the plant’s operational life.”
Eni holds an 82% stake in the
Geliga-1 well, with China’s Sinopec holding the other 18%.
“The Ganal PSC is part of a portfolio of 19 blocks (14
in Indonesia and 5 in Malaysia) that will be contributed to Searah, a jointly
controlled company between Eni and Petronas announced in November 2025. The new
company will integrate assets, technical expertise and financial capabilities
to support growth and strengthen its position in Southeast Asia. Searah’s
business plans include the development of approximately 3 billion barrels of
oil equivalent (boe) of discovered resources and the unlocking of significant
exploration potential. Closing of the transaction is expected within Q2 2026.
The valorization to a third party of a 10% stake in the Eni Indonesia portfolio
withheld from the Searah transaction is underway and expected to be concluded
in 2026. The Geliga discovery adds to the value of this sale.”
Eni has been active in
Indonesia since 2001 and has a net production of about 90,000 barrels of oil
equivalent per day, mainly from the Jangkrik and Merakes fields offshore East
Kalimantan.
In his article, Kombrink
considers the Kutei Basin find as an example of near-field exploration rather
than frontier exploration. He compares near-field exploration wells in the
Kutei Basin to the North Sea near-field wells and concludes:
“…the Kutei Basin still has a lot more running room than
the North Sea.”
In other words, these wells
are bigger than most new North Sea discoveries these days. He also notes via a
LinkedIn post by Dag Helland Hansen of EMGS that a strong increase in
resistivity, similar to the successful Gehem field to the north, was observed
in the Geliga-1 well.
References:
Geliga-1
– the gas discovery sourced by sandstones. Henk Kombrink. GEOExPro. April 20,
2026. Geliga-1
- the gas discovery sourced by sandstones - GeoExpro
Eni’s
major Geliga Gas Discovery Confirms the Strategic Potential of Indonesia’s
Kutei Basin and Unlocks Significant New Volumes for Domestic and International
Markets. Eni. 20 April 2026. Eni’s
major Geliga Gas Discovery Confirms the Strategic Potential of Indonesia’s
Kutei Basin and Unlocks Significant New Volumes for Domestic and International
Markets
Leaves
in turbidite sands: The main source of oil and gas in the deep-water Kutei
Basin, Indonesia. Arthur Saller; Rui Lin; John Dunham. AAPG Bulletin (2006) 90
(10): 1585–1608. Leaves
in turbidite sands: The main source of oil and gas in the deep-water Kutei
Basin, Indonesia | AAPG Bulletin | GeoScienceWorld
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