Undiscovered Conventional Oil and Gas of China: USGS Assessment, 2020

Using a geology-based assessment method, the U.S. Geological Survey (USGS) estimated mean undiscovered, technically recoverable conventional resources of 13.4 billion barrels of oil and 244.4 trillion cubic feet of gas in nine geologic provinces of China.

The nine provinces

The assessment covered the Tarim, Junggar, Turpan, Qaidam, Sichuan, Ordos, Bohaiwan, Songliao and East China Sea Basin Provinces. Within them the USGS defined 16 assessment units (AUs), and assessed each for undiscovered conventional oil, gas and natural gas liquids.

Figure 1. The nine geologic provinces of China assessed in this study. Base map from the U.S. Department of the Interior, National Park Service. Credit: U.S. Geological Survey.

Geologic setting

China is a mosaic of cratonic terranes, remnants of oceanic crust, orogenic belts, suture zones, accretionary complexes, island-arc assemblages and regional faults. Together they record a complex history of terrane accretion and mountain building along the southern and eastern margins of Eurasia.

From the Paleozoic onward, several cratonic blocks broke away from the northern margin of Gondwana at different times and moved north across the Tethys Ocean as oceanic crust was subducted beneath them. The terranes eventually collided and accreted, stitching together a collage of tectonic elements.

• Cratonic terranes that accreted to Eurasia include the Tarim, Ordos and Sichuan Basin Provinces.
• Oceanic fragments. The basement of the Junggar, Turpan, Qaidam and Songliao Basin Provinces is interpreted as pieces of oceanic crust that escaped subduction and were built into orogenic belts.

As accretion went on, the edges of these cratonic and oceanic pieces became fold and thrust belts, suture zones, faults and a jumble of island-arc and accretionary complexes; several terranes developed foreland basins. By the Permian, compression had raised enough tectonic topography to cut several basins off from the sea. That relief produced hydraulically closed basins, marked by extensive lacustrine systems spanning whole basins:

• In the Junggar and Turpan Basin Provinces, lacustrine systems developed through compressive deformation along the basin margins in the Permian.
• In the Sichuan, Ordos, Tarim and Qaidam Basin Provinces, lacustrine systems formed after compressional deformation.
• Along Eurasia's eastern margin, widespread back-arc extension linked to the changing motion of the Pacific plate formed horst and graben systems, and extensive lacustrine systems formed within grabens in the Songliao, Bohaiwan and East China Sea Basin Provinces.

Source rocks

The petroleum source rocks of the nine provinces reflect this long history.

• Early Paleozoic marine source rocks. As the cratonic blocks left Gondwana and crossed the Tethyan realm, organic-rich marine sediments were laid down in basinal settings, mostly beside passive-margin carbonate platforms — as in the Tarim, Sichuan and Ordos Basin Provinces.
• Late Paleozoic foreland basins. Later terrane collisions produced marginal fold belts and foreland basins, where potential source rocks are mainly marginal-marine to nonmarine, coal-bearing sequences prone to gas.
• Lacustrine source rocks. From the Permian, as compression closed off basins, the climate allowed extensive lake systems with viable lacustrine source rocks, which are well known in China's basins.

Three examples

• Sichuan. The Sichuan Basin Province, in the western South China terrane, shows how tectonics controlled source-rock development. Passive margins rimmed the cratonic terrane as it moved north from Gondwana, and widespread organic-rich marine source rocks were deposited. As the South China terrane pressed against the Eurasian margin, a foreland basin formed and received marine, marginal-marine and coal-bearing sequences. In the Late Triassic, the South China terrane collided with the North China and Songpan Ganzi terranes; the compression raised topography, closed the basin hydraulically and produced an extensive lake system across the basin. A similar succession from marine to nonmarine source rocks is interpreted for the Tarim Basin Province.
• Junggar. Because the Junggar Basin Province is floored by upper Paleozoic oceanic crust, it has no lower Paleozoic section. Permian deformation cut the basin off from the sea, and its main source rocks developed in a lacustrine system. The Turpan and Qaidam Basin Provinces followed a similar tectonic path to lacustrine source rocks.
• Bohaiwan. Here lacustrine conditions developed in an extensional system, driven by complex motion during subduction of the Pacific plate. Extension produced a regional system of horsts and grabens isolated from the sea, and many grabens developed lake systems. The same process formed lacustrine systems in the Songliao and East China Sea Basin Provinces.

The key input data for the assessment units are in Table 1.

Table 1. Key input data for the 16 conventional assessment units in China. Shading means not applicable. AU, assessment unit; MMBO, million barrels of oil; BCFG, billion cubic feet of gas. Credit: U.S. Geological Survey.

Results

The fully risked mean totals for the nine provinces are:

F95 is a 95-percent chance of at least the amount shown; the other fractiles are defined the same way.

Mean results for each assessment unit (Table 2 gives the full range):

"—" means not applicable. Results for the Pearl River Mouth, Beibuwan, Qiongdongnan and Taixinan Basin Provinces were published earlier (Schenk and others, 2020).

Table 2. Results for the 16 conventional assessment units in China. Results are fully risked estimates; shading means not applicable. MMBO, million barrels of oil; BCFG, billion cubic feet of gas; NGL, natural gas liquids; MMBNGL, million barrels of natural gas liquids. Credit: U.S. Geological Survey.

Sources

• Schenk, C.J., Mercier, T.J., Woodall, C.A., Ellis, G.S., Finn, T.M., Le, P.A., Marra, K.R., Leathers-Miller, H.M., and Drake, R.M., II, 2021, Assessment of undiscovered conventional oil and gas resources of China, 2020: U.S. Geological Survey Fact Sheet 2021–3051. https://doi.org/10.3133/fs20213051
• The map, both tables and the per-unit results are taken from the fact sheet's PDF; the page imported from its HTML lacked them. The fact sheet cites the geological literature behind each statement; see its reference list.
• Rewritten in hubnx's own words.