A large number of research articles on Chinese basins are published in petroleum geology journals. While few international oil companies operate in these basins, the published articles may offer informative field studies and analogs for similar basins in other parts of the world. This week, we will take a look at a few new studies from China.
Rasoul Sorkhabi
Editor, Core Elements
A Basin Stacked with Three Unconventional Plays
China's beautiful Yucha Canyon lies in the Qinshui Basin. Image credit: Maple90/Shutterstock.com
A study in the AAPG Bulletin describes an interesting study of coal bed methane, shale gas, and tight sandstone gas—all coexisting in a single basin in China, the Qinshui Basin.
About the basin:
The Qinshui Basin in northern China has an area of 23,500 square kilometers.
It is a large synclinorium bounded by mountain uplifts on all four sides.
The basin is filled with Paleozoic and Mesozoic sediments.
The target sediments are from the Taiyuan Formation (Pennsylvanian age and 61–150 meters thick) and the Shanxi Formation (Permian age and 35–60 meters thick).
What they did: The researchers measured for pyrolysis, total organic carbon (TOC), vitrinite reflectance (Ro), gas contents, porosity, and permeability, using core samples and well logs.
What they found:
Coal bed methane (CBM):
Twenty coal seems have been identified in the basin, but two units—No. 3 and No. 15—are the best candidates for CBM exploration.
No. 3 coal in the Shanxi Formation is 2–8 meters thick with Ro values of 3.3-4.0 percent.
No. 15 coal in the Taiyuan Formation is 1–5 meters thick with Ro values of 3.3–4 percent.
Shale gas:
Dark shale beds occur in the Taiyuan and Shanxi formations with bed thickness from 0.2 to 13.5 meters.
TOCs range from 0.2 to 46.4 percent, with an average value of 2.6 percent.
Kerogen is predominantly type III.
The shales in both formations have an average porosity of 3 percent and permeability of 0.5 millidarcy.
Tight sandstone gas:
Sandstone beds are more frequent and thicker in the Taiyuan Formation than in the Shanxi Formation.
Sandstone beds in the Taiyuan are up to 5 meters thick with an average porosity of 2.9 percent and permeability of 0.96 millidarcy.
Sandstone beds in the Shanxi are up to 2 meters thick with an average porosity of 1.3 percent and permeability of 1.03 millidarcy.
Basin history:
The basin sediments underwent rapid burial to depths of 4,000 meters and temperatures of 200°C during Carboniferous-Triassic times (Hercynian/Indochina Orogeny).
Sediments also experienced progressive uplift during the Jurassic-Cretaceous times (Yanshanian Orogeny) and Cenozoic times (Himalayan Orogeny).
Uplift events created fractures facilitating gas migration into sandstone reservoirs.
Hydrocarbon generation occurred during Triassic and Cretaceous periods.
Why it matters: The study describes the co-existence of double source rocks and triple reservoirs rocks in the same basin. The methodology can be applied to other basins.
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Chinese cratonic basins Photo credit: AAPG Bulletin
Recently, PetroChina Co. launched a project to drill wells greater than 10,000 meters in selected Chinese basins. The company has drilled two wells: the Take-1 well in Tarim Basin and the Chuanke-1 well in Sichuan Basin.
Driving the news: An article in AAPG Bulletin describes these deep drills.
Stratigraphic horizons: Thedeepest wells also enter the oldest hydrocarbon plays in the following stratigraphic periods:
What they did: Researchers examined tectonic evolution, paleogeography, stratigraphy, inorganic chemistry, pyrolysis, TOC, and Ro measurements of Cryogenian, Ediacaran, and Cambrian source rocks in the Sichuan and Tarim basins.
What they found:
Basement and basin tectonics:
The Sichuan and Tarim basins sit atop Precambrian cratons that went through intracontinental rifting and passive continental margin during 820–540 Ma. This was related to the breakup of the Rodinia Supercontinent.
Although details differ, both basins were filled with Neoproterozoic-Cambrian sediments.
Cryogenian source rocks:
In the South China Craton basins, these source rocks are represented by the Datangpo Formation with thicknesses of 8–70 meters, TOCs up to 4 percent (average 1.5 percent), and kerogen type I. The source rocks were deposited under anoxic conditions (evidenced from iron and pyrite ratios data).
In the Tarim Basin, these source rocks occur in the lower part of the Tereeken Formation with TOCs up to 5 percent (1.5 percent average) and Ro values of 1.3–1.6 percent.
Ediacaran source rocks:
In the South China Craton basins, these source rocks are two black shales (Dou-2 and Dou-4 members) interbedded with carbonate rocks in the Doushantuo Formation. TOC values range from 0.6 to 17.1 percent (3.4 percent average) and layer thicknesses are 10–120 meters with a total sedimentary thickness of 200 meters. TOC values vary from 0.3 to 14 percent.
In the Tarim Basin, the Upper Ediacaran Sugetbrak Formation consists of black shales, 300 meters thick, with TOC values of 0.3–1.2 percent (0.57 percent average) and Ro values of 1.6–2.5 percent.
Cambrian Source Rocks:
The Lower Cambrian Qiongzhusi Formation in the Sichuan Basin consists of black shales more than 200 meters thick with TOC values of 1.7–3.5 percent (2.7 percent average).
The Lower Cambrian Yuertusi Formation in the Tarim Basin is less than 30 meters thick with TOC values higher than 5 percent.
Natural gas potential: The researchers estimated there are 6,551 trillion cubic meters of natural gas in the South China Craton basins and 2,332 trillion cubic meters in the Tarian cratonic basins.
Why it matters: This study contributes to how Neoproterozoic-Cambrian strata in Chinese basins not only offer new prospects for petroleum exploration. It also provides little-known records of major tectonic, biotic, and atmospheric changes during Neoproterozoic and Cambrian times associated with the breakup of the Rodinia Supercontinent.
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