Several deepwater gas reservoirs in the offshore Mahakam Delta in Indonesia have been discovered: Geliga, Gehem, Konta, Geng North, and Rangga.
These gas discoveries are largely Miocene turbidite plays in the Kutei Basin—particularly middle to upper Miocene channel, fan, and basin-floor sand systems. These targets stand out for their strong reservoir quality, reliable gas charge, and the potential for large, laterally connected accumulations within combination traps.
Common Characteristics of Miocene Turbidite Plays
- Sand-rich fan and channel reservoirs: Slope channels, leveed channels, lobes, and basin-floor fans with good lateral continuity—supporting large, connected gas accumulations.
- Reservoir quality retained at depth: Quartz-rich sands can preserve workable porosity and permeability despite deep burial.
- Strong gas charge: Predominantly sourced from Miocene coals and carbonaceous shales, yielding dry gas and condensate.
- Clear seismic indicators: Bright spots, flat spots, AVO anomalies, and gas chimneys help de-risk prospects.
- Combination traps: Stratigraphic pinchouts commonly occur with subtle folding and faulting.
What Stands Out in the Recent Discoveries
- Larger, deeper middle Miocene fan systems: Typically support thicker gas columns and higher GIIP than younger slope-channel plays.
- Ultra-deepwater setting: Commonly ~3,000–6,000 ft water depth, driving subsea and development complexity.
- Overpressure: May help preserve porosity and seal integrity, but increases drilling risk and execution complexity.
- Gas-dominant hydrocarbons: More gas/condensate than oil, consistent with higher thermal maturity and more distal source inputs.
- Stacked pay potential: Vertically repeated turbidite sands can improve recoverable volumes and development flexibility.
- Possible local charge contributions: Organic-rich interbeds may add hydrocarbons in some areas.
One-Line Summary
Mahakam deepwater success reflects the combination of high-quality Miocene turbidite reservoirs, a strong coaly gas charge, subtle but large structural–stratigraphic traps, and modern seismic imaging that reliably highlights hydrocarbons.
Main Exploration Sweet Spot
Sweet spot: middle Miocene basin-floor fan sands, charged from mature coaly source kitchens and trapped within mixed structural–stratigraphic closures.
This post is adapted from the content written by Eddy Ong, a retired geologist with 42 years of E&P experience.
Thanks for the update and technical background. It is super great to see latest exploration technologies pat off for Indonesia.
Night vs day relative to my days in central Sumatra chasing oil in the early 1970s!
What company did you do the work for in Central Sumatra in the early 1970s?
PT Caltex Pacific Indonesia.
The construction project was to bring on stream the Bangkok, Balam, South Mengala and Kotabatak oil fields. It also involved a new parallel crude pipeline from Minas to Duri, then on Dumai with a new export wharf and infrastructure. I recall total added production was 246,000 bpcd, and at the end, the exports did spurt to 1,000,000 bpcd during a maximum production trial.
Great memories! One of which was that the Indonesia workers had a great knack at developing mechanical skills. The CPI concession had ~348 pieces of Caterpillar equipment alone. I was introduced to operating bull dozers, cranes, earthmovers and such. We all – a work force of near 3,500 – completed a then world class project in 18 months.
Beautiful country and people. I had a 135cc motorcycle and was able to visit a lot of remote regional communities, including as far as Bukittinggi. Saw wild animals from elephants to orangutan to tigers and giant monitor lizards. And intermingled with the Sumatran indigenous Sakai people. All are my fondest memories of a 35 year career in the oil industry.
I am very pleased indeed, that Indonesia is “taking-off” economically. Good luck and God speed the nation and its people forward.
Thank you, George Barnett, for sharing your amazing experience working for Caltex in Central Sumatra.