One study reports on an interesting mechanism to flush organic carbon from the coast to the deep ocean, and another study offers new data on Cretaceous source rocks from the Orange Basin in offshore South Africa.
Organic-rich rocks in deepwater basins are important for oil and gas exploration, as is understanding the natural source-to-sink pathways of organic carbon on Earth. Why? Because rocks play an important role in carbon sequestration. In this edition, I share two “deep marine carbon” stories—one from New Zealand and one from South Africa.
Rasoul Sorkhabi
Editor, Core Elements
Earthquakes and Deep Marine Organic Matter
Louie Lea/Shutterstock.com
In a study published in Earth and Planetary Science Letters, researchers from New Zealand reported on an interesting mechanism to flush organic carbon from the coast to the deep ocean.
Geologic setting: The study focused on the Hikurangi subduction zone, offshore New Zealand (referred to using the native Māori word Aotearoa in the study), where an oblique subduction and continental collision has occurred between the Australian and Pacific plates for more than 25 million years.
The Hikurangi subduction margin is more than 1,500 kilometers long.
Submarine canyons down-cut the continental shelf and slope and carry sediments to the Hikurangi Channel at a water depth of 4,800 meters and a distance of more than 1,300 kilometers from the coastline.
Kaikoura earthquake:
In 2016, the 7.8-magnitude Kaikōura earthquake and fault ruptures caused almost 30,000 onshore lands and mobilized 0.043 cubic kilometers of rock and soil.
The earthquake triggered sediment transportation from Kaikōura
Canyon and other shelf canyons into the deep ocean.
One noteworthy sedimentary deposit, which resulted from earthquake-triggered submarine canyon flow, is the silica-rich Kaikōura event bed (KEB).
Analyses and results: Researchers collected core samples from four deep marine boreholes in 2012 and 2017. They analyzed the geochemistry of samples, including those from the KEB.
The analyses included bulk composition (weight percentage) of particulate organic carbon and nitrogen, stable carbon-13 and nitrogen-17 isotopes, and parallel ramped pyrolysis oxidation accelerator mass spectrometry and pyrolysis-gas chromatographic-mass spectrometry.
Combined techniques gave data on ages, sources, and partitioning of organic matter within the Kaikōura turbidite deposits.
The results show that 55 percent of organic matter in the KEB is from terrestrial sources.
It is estimated that about 0.09 cubic kilometers of sediments were mobilized in the Kaikōura canyon alone, and about 7 metric tons of organic carbon were flushed.
Why this matters: Turbidity currents may also provide a mechanism for organic enrichment of petroleum source rocks in deep marine environments.
Go deeper: Read the full article here. This study is a follow-up to the researcher’s previous article published in 2024.
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A recent article in AAPG Bulletin offers new data on Cretaceous source rocks from the Orange Basin in offshore South Africa.
Orange Basin:
The basin is located on the passive continental margin of southwest Africa in the South Atlantic Ocean. It formed during the Late Jurassic due to continental breakup between Africa and South America.
With recent important oil discoveries in the Orange Basin in offshore Namibia, exploration companies have extended activities into offshore South Africa.
In 2023, TotalEnergies and Canada-based Africa Oil Corp. began operating in Block 3B/4B in the Orange Basin in offshore South Africa.
Africa Oil Corp. announced important oil finds in the Block 3B/4B but with few details.
This block is estimated to hold 3 billion barrels of oil and 5.5 trillion cubic feet of gas.
Gas chimneys on seismic images also indicate good discovery potential in the basin.
Source rocks: The source rocks in the Orange Basin include shale formations of:
Upper Jurassic-Neocomian lacustrine deposits
Barremian-lower Aptian marine deposits
Cenomanian-Turonian marine deposits
A new study:
Researchers collected 32 source rock samples from Cretaceous horizons from seven wells in offshore South Africa.
They analyzed the aromatic hydrocarbon fractions by gas chromatography-mass spectrometry.
Aromatic hydrocarbons in oil and rock extracts provide information about the thermal maturity of generating rocks.
Results:
The source rocks have a total organic carbon content of 0.76 to 5.91 percent. Their kerogen is dominantly type III.
The rock samples have high concentrations of 1,2,5-trimethylnaphthalene compared with 1,2,7-trimethylnaphthalene, which is a typical characteristic of thermally mature rocks.
The methylnaphthalene ratio and dimethylnaphthalene ratio of the rock samples from the seven wells investigated revealed that Coniacian, Albian, and Aptian source rocks are more thermally mature.
Phenanthrene thermal maturity indices indicate that Turonian source rocks are the least thermally mature.
Why it matters: In deepwater basins, such as the Orange Basin, petroleum source rocks are the last element of a petroleum system to be drilled and studied.
Go deeper: Read the full article in AAPG Bulletin. This paper is a follow-up to previous works done by Adekola and Akinlua.
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