Researchers report a recently discovered gas field in an offshore crystalline basement of Archean age, and we take a deep dive into the geology of the deepest core dug.
The first month of 2025 is nearly at its end. We’ll wrap up January with a look at a newly discovered gas field in an offshore crystalline basement of Archean age and a deep dive into the geology of the deepest mantle core ever dug. Let’s go.
Rasoul Sorkhabi
Editor, Core Elements
Natural Gas in an Archean Basement Reservoir in the Bohai Bay Basin
Bohai Bay in China/Igor Grochev/ Shutterstock.com
Crystalline (igneous or metamorphic) basement rocks are not common as oil and gas reserves. Petroleum accumulation in Precambrian rocks is not common, either. Tao Ye in AAPG Bulletin has reported a very interesting case: A recently discovered gas field in an offshore crystalline basement of Archean age.
The Bozhong 19-6 Gas Field:
The Bozhong 19-6 Gas Field is about 100 kilometers offshore in the Bohai Bay in eastern China, in the so-called Bozhong Sag of South China Sea, at a water depth of 23 meters.
The Bohai Bay Basin is China’s second largest oil producing basin.
CNOOC discovered Bozhong 19-6 in 2007 and began production in November 2023.
The operation in the Bozhong 19-6 field consists of offshore platforms, a treatment plant, and two undersea pipelines.
Petroleum system:
The Archean basement reservoir is granitic gneiss covering about 300 square kilometers with a gas column height of 1,000 meters.
The rock matrix has a porosity of less than 8 percent and permeability of less than 1 millidarcy.
The petroleum was fed by Upper Eocene (Shahejie Formation) and Oligocene (Dongying Formation) shale.
Currently, 18 wells are producing approximately 37,500 barrels of oil equivalent per day.
The Bozhong 19-6 field contains proven reserves of 200 billion cubic meters of natural gas and 200 million cubic meters of condensate oil.
Fractured reservoir: Crystalline rocks have very low porosity and permeability. They require significant fracturing to enhance permeability. Researchers describe the following geologic history for fracturing of the Bozhong reservoir:
Tectonic compression during the Triassic Indosinian orogeny induced east-west trending fractures.
The highest density of fractures occurs in the hinge zone of the thrust folds.
The basement rock also experienced exposure and weathering in the Cretaceous (during the Yanshan orogeny).
The Cenozoic extensional tectonics reopened or enhanced the natural fractures.
Brittle mineralogy played an important role in tectonic fracturing. More than 70 percent of the minerals in the basement rock are felsic (feldspar and silica).
Join Wood Mackenzie’s webinar on the 11th February to listen their experts explore the outlook for the subsurface sector in 2025 and, ask your key questions directly to them.
The Geology of the Longest Mantle Core Ever Drilled
Lesley Anderson/ IODP
To drill into Earth’s mantle is a geologist’s dream. This is what IODP JOIDES-Resolution drillship accomplished in 2023 during the IODP expedition 399. My colleague Sarah Compton covered the discovery and its technology in AAPG’s technology newsletter Enspired, but I wanted to take an even closer look at the geology of the core here.
Drill site:
The IODP expedition drilled atop the Mid-Atlantic ridge, 30-degrees North, and the size of Mount Rainier.
The area is called Atlantis Massif, a domal oceanic rock complex capped by a normal fault that was active between 0.4 and 2.0 million years ago.
Here, at water depths of 850 meters, the mantle material is in contact with Earth’s oceanic surface.
Core description: The recovered core is 1,268 meters long—the longest mantle core ever drilled.
The core consists of:
68 percent serpentinized peridotite including harzburgite (82 percent), orthopyroxene-bearing dunite (13 percent) and dunite (5percent)
31 percent gabbroic intrusions, located at the bottom of the core
The rock is depleted in pyroxene due to dissolution as the rock began to rise toward the ocean floor. It has also gone through serpentinization as it reacted with hot water, produced serpentine minerals, magnesium-iron rich minerals, and released hydrogen and methane.
Why it matters:
Detailed analysis of the core reveals dynamic processes involved in the interface of mantle-ocean floor regions including melt flow, element cycling, crust formation, serpentinization (a major source of geologic hydrogen), and terrestrial magnetism.
The drill site is only 800 meters away from the Lost City—a famous hydrothermal vent, which is interesting to microbiologists trying to understand the origin of life during Earth’s earliest history.
Preliminary results show that chemical alterations in the core are consistent with hydrothermal fluid-rock interactions observed in the Lost City.
Last week’s quiz was: What is the currently used scale for earthquake magnitude? How does it differ from Richter’s scale?
Layton Payne and Ramiro Bracamonte sent correct responses summarized below:
The primary scale used today for measuring earthquake magnitude is the Moment Magnitude Scale (Mw), which was developed in 1979 and measures the total energy released by an earthquake based on its seismic moment. This measurement is a product of the fault slip, the area of the fault that slipped, and the rigidity of the rocks that broke. This scale is a more accurate representation of an earthquake's size. The Richter magnitude scale (ML) designed in 1935 is measured from the amplitude of seismic waves recorded by seismographs and is suited for local and moderate-sized earthquakes, but it is not accurate for earthquakes larger than magnitude 7.
Instead of a new quiz question this week, I would like to ask: What topics would you like to see covered in future articles and quiz questions? All ideas are welcome! What would you like to learn more about?
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