Carbon dioxide sequestration is an important measure against global greenhouse warming. This week, we will look at carbon storage studies in Lower Paleozoic deep reservoirs and recent soils; these two are based on entirely different sequestration methods.
Rasoul Sorkhabi
Editor, Core Elements
Carbon Sequestration in Paleozoic Oklahoma
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Anna Turnini and Matthew Pranter of The University of Oklahoma have published two sequel articles in AAPG Bulletin that discuss the potential for carbon dioxide storage in Lower Paleozoic formations in Oklahoma. Let’s take a look.
Why carbon sequestration? The researchers point out that:
Carbon dioxide emissions from fossil fuels have increased from 3.2 gigatons in 1922 to 35.6 gigatons in 2021.
Earth’s surface temperature has risen by 1.3 degrees Celsius since the 19th century.
In 2020, about 4,715 million tons of carbon dioxide were emitted in the United States alone.
The need for detailed assessment: In 2012, the U.S. Geological Survey released a national assessment of carbon dioxide storage in 36 of the country’s sedimentary basins. However, each formation in each basin requires its own detailed investigation.
What they did:
The researchers used information from 80,000 wells for structural modeling and 1,126 well logs for porosity, lithology, and other rock property characterization.
To determine the carbon dioxide storage, the researchers adopted the U.S. Department of Energy’s methodology, which takes into account the reservoir rock volume and porosity, saline storage efficiency, and carbon dioxide density.
Where they looked: Oklahoma during early Paleozoic times was a region of rifting within the Laurentia supercontinent.
The researchers studied the following formations in Oklahoma:
Timbered Hills Group (Late Cambrian age)
Arbuckle Group (Late Cambrian to Early Ordovician)
Simpson Group (Early-Middle Ordovician)
Viola Limestone (Late Ordovician)
Hunton Group (Early Silurian)
What they found:Researchers estimated the amount of carbon dioxide resources for:
The Arbuckle Zone (consisting of Timbered Hills and Arbuckle groups):
51 to 118 million metric tons over 116 thousand square kilometers, of which 30 percent of the storage lies at depths of less than 10,000 feet
Ordovician Simpson-Viola Groups:
6,501 to 20,877 million metric tons, of which 35 percent is at depths of less than 10,000 feet
Silurian Hunton Group:
1,331 to 3,094 million metric tons, of which 17 percent lies at depths of less than 10,000 feet
AI/ML tools integrate with our interpretation software and upstream data, supporting consistent, iterative workflows from data preparation through analysis.
Soil Carbon Sequestration Studies in China and Canada
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Now, let’s look at three studies covering soil sequestration. Soil organic carbon sequestration (SOCS) has drawn much attention in recent years as a way of reducing atmospheric carbon dioxide and global warming.
Why SOCS matters:
Next to the oceans, the world’s soils hold most of carbon in earth’s ecosystems.
However, soil carbon is concentrated in topsoil, which is vulnerable to erosion.
Measures to mitigate soil erosion also help SOCS or “negative emissions.”
Study #1: Lian Liu and colleagues discuss how soil conservation measures enhanced SOCS in China.
What they did: The researchers constructed 846 datasets from 55 published studies, covering nine regions across China. They considered:
No or low-tillage measures
Engineering measures, mainly check dams
Biological or crop management measures
What they found:
The study found that soil conservation measures enhanced SOCS 30 percent more (or 3.17 grams of carbon per kilogram of soil) than areas without soil conservation.
Researchers ranked these factors in terms of importance:
Researchers used soil data collected from 1979 to 1984 as well as climate zoning (humid, semi-humid, arid, and semi-arid regions).
They then created digital soil maps to produce spatial patterns of density of SOCS potential at five soil depths: 0–5 centimeters, 5–15 centimeters, 15–30 centimeters, 30–60 centimeters, and 60–100 centimeters.
What they found:
SOCS was highest in northwestern, northern, and eastern China and lowest in southeastern Tibet and northeastern China.
In terms of topsoil, climate and vegetation were dominant factors in arid regions, while vegetation and land use were most important in semi-arid regions.
For subsoil layers, climate and land use were dominant factors in both arid and semi-arid regions.
What’s next: Similar country-wide studies could help better quantify SOCS worldwide and promote these practices efficiently.
Study #3:
A group of Canadian scientists investigated SOCS potential of different soil-sized fractions under perennial crops of miscanthus (silvergrass), willow, and switchgrass.
The study was conducted at the University of Guelph Agroforestry Research Station at an altitude of 346 meters and with a mean average annual temperature of 6.5 degrees Celsius and annual precipitation of 923 millimeters.
What they did:
Soil samples for whole soil and different soil-sized fractions were taken at depths of 10–10, 10–20, and 20–30 centimeters for elemental analysis.
Carbon-13 stable isotope analysis was made to quantify the carbon contribution of the perennial crops at different soil-sized fractions.
What they found: After 12 years of perennial cultivation, the researchers found:
SOCS content increased by 3.1 percent in miscanthus and 2.5 percent in willow, but it decreased by 3.7 percent in switchgrass compared to baseline SOC content.
Fertilization plays an important role in enhancing SOCS even in perennial crop plantation.
SOCS depends on crop species. Of the tree crop species studied, miscanthus had higher SOCS contents even at 20–30-centimeter depths.
Why it matters: Perennial crops have longer root systems and store carbon deeper in the soil.
Go deeper: You can read this research in full in GCB Bioenergy.
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