It isn’t a trick question.
The following recent discoveries definitely have something in common.
In April, Occidental reported it found oil with its Bandit prospect well in the Green Canyon area of the Gulf of Mexico (America). The high-pressure, ultra-deepwater well was drilled about 125 miles south of the Louisiana coast. Bandit “encountered high-quality, full-to-base, oil-bearing Miocene sands,” the company announced. Occidental is operator, with partners Chevron USA and Woodside Energy.
Eni identified a giant natural gas accumulation in the Kutei Basin offshore Indonesia with the Geliga-1 exploration well. It reported preliminary estimates of about 5 trillion cubic feet of gas and 300 million barrels of condensate in place in the targeted Miocene interval.
Geliga follows the Geng North giant discovery and the 2025 Konta-1 gas find, also in the Kutei Basin. Sinopec has an 18-percent interest in the Geliga project.
Azule Energy, equally owned by Eni and BP, made an oil discovery with an estimated 500 million barrels of crude in place in Block 15/06 offshore Angola, Eni announced earlier this year. Sonangol and joint venture SSI (Sonangol Sinopec International) also hold interests.
The Algaita-01 discovery well was drilled in about 2,200 feet of water and encountered oil-bearing sands in multiple Upper Miocene intervals, Eni reported.
These new discoveries all involve Miocene reservoirs.
“The Miocene delivered a significant amount of sand with excellent reservoir characteristics around the world,” noted Erik Scott, exploration geologist and consulting geologist/sedimentologist. He’s also an adjunct professor in the Department of Earth, Environmental and Planetary Sciences at Rice University.
“Why it’s so good is that it has delivered really good reservoir conditions,” he said.
Why the Miocene Dominates
Miocene reservoirs have been workhorses in the U.S. deepwater Gulf, where they now account for more than 40 percent of the established hydrocarbon reserves, according to the U.S. Bureau of Ocean Energy Management. Last year, BP reported another Miocene discovery with its Far South exploration well, also in the Green Canyon area.
BOEM’s 2026 National Assessment of Undiscovered Oil and Gas Resources of the U.S. Outer Continental Shelf identified undiscovered technically recoverable Miocene resources of more than 9 billion barrels of oil and 19 Tcf of gas.
“In the Gulf region, when you’re past the Cretaceous and into the Cenozoic, you’re turning from a majority carbonate environment to where sand dominates,” Scott said.
“In the Gulf during the Miocene, you’ve got a pulse of sand coming into the offshore setting. It is a mature sand, mostly medium to fine grain, generally well sorted, and that makes for a really good hydrocarbon reservoir,” he added.
Scott described the Miocene epoch as having plate tectonics similar to today and generally higher sea levels, with fluctuations. Crucially, for North America and other areas it was a post-mountain building period with heavy weathering.
“The Miocene climactic conditions produced a lot of weathering, to bring sediments down to the coast. You have a well-known increase in sediment supply during that period,” he said.
He has a sedimentologist’s view of reservoir building, which is entirely appropriate for the Miocene. Scott said the Gulf reservoirs tend to show no fine grains or silt filling up pores – no “clogging.”
“The more the sand grain size is sorted due to transportations lengths and sedimentation processes, the better your porosity and permeability will most likely be. That’s a large part of the development of the good reservoir properties in the offshore Miocene world,” he said.
Also, this period of Cenozoic reservoir creation followed the Mesozoic Era, when the world is often described as teeming with life. Abundant organic content was deposited during the Cretaceous and Jurassic, around the world.
“Generally, hydrocarbons are coming from deeper, older rock – the Cretaceous, the Jurassic. By the time these source rocks get to the (hydrocarbon) generation window, the Miocene reservoirs are in place,” Scott said.
He characterized the Miocene sands as “in the right place at the right time to receive the generated hydrocarbons from the deep source rock.”
Finally, mud and shale and salt created sealing opportunity for the Miocene sands, “some salt but mostly mud and shale,” Scott said. Generally, the Miocene deposits are surrounded by fine-grained muds that produced sealing potential, he noted.
“The increase in sediment supply during the Miocene also brought in muds that provide the sealing lithology of the reservoir sands. While this increase was helpful, the seals of the Miocene are due more to deepwater sedimentation processes than to the Miocene itself,” Scott said.
In hydrocarbon exploration, Miocene reservoirs benefit from what Scott called “benign tectonics.” They’re relatively well placed for the drill bit in offshore exploration, often near-offshore and high up in the Cenozoic.
“Most of the Miocene reservoirs around the world are in passive margin settings, so are generally shallower in the sedimentary section and within easier drilling depths,” Scott said.
That’s “relatively” well placed in terms of deepwater drilling. Eni’s Geliga discovery was drilled about 44 miles off the East Kalimantan coast to a total depth of around 5,100 meters. The company plans another well in the basin this year and two more in 2027.
Seismic Offers New Advantages
Advances in geophysics and a better understanding of Miocene reservoir characteristics have helped open up multiple new plays and drilling areas. Those range from the Gulf to offshore India to the Nile Delta and the Eastern Mediterranean.
“Our seismic acquisition and imaging has dramatically improved, allowing us to explore in new areas as well as next door to past discoveries and fields” Scott observed.
“While there is always a structural component involved, there’s also a lot of stratigraphic compartmentalization and trapping with these offshore reservoirs,” he said.
Miocene reservoirs now attract attention from offshore explorers because of the chance for sizeable discoveries with reserves that produce for 30 years or more. Prolific production from Miocene sands can help justify the substantial risk and cost of today’s deepwater drilling.
“The Miocene, because of its better reservoir quality, can provide the production rates to support the drilling out there (in the deepwater offshore),” Scott said.
Scott now focuses his own efforts on onshore Miocene and expects lots of activity to come, onshore and offshore.
“There’s still a lot of places to go. There’s still a lot of running room,” he said. “Just when you think you’ve done everything and drilled everything, there’s more to do.”