Assessment of undiscovered conventional oil and gas resources of China, 2020

The U.S. Geological Survey (USGS) quantitatively assessed the potential for undiscovered, technically recoverable conventional oil and gas resources in nine geologic provinces of China (fig. 1). This assessment includes the Tarim Basin, Junggar Basin, Turpan Basin, Qaidam Basin, Sichuan Basin, Ordos Basin, Bohaiwan Basin, Songliao Basin, and the East China Sea Basin Provinces. Within these 9 provinces, 16 geologic assessment units (AUs) were defined, and each AU was assessed for undiscovered conventional oil, gas, and natural-gas liquids. China contains a mosaic of cratonic terranes, remnants of oceanic crust, orogenic belts, suture zones, accretionary complexes, island-arc assemblages, and regional faults that record a complex history of terrane accretion and orogeny along the southern and eastern margins of Eurasia (Liu and others, 2013; Zheng and others, 2013; Zhao and others, 2014; Han and Zhao, 2018; Zhou and others, 2018). Beginning in the Paleozoic, several cratonic blocks separated diachronously from the northern margin of Gondwana and translated north across the Tethys Ocean as oceanic crust was subducted; these terranes eventually collided and accreted, knitting together a collage of tectonic elements. Major cratonic terranes that accreted to Eurasia included the Tarim Basin, Ordos Basin, and Sichuan Basin Provinces. In contrast, the basement of the Junggar, Turpan, Qaidam, and Songliao Basin Provinces are interpreted as fragments of oceanic crust that were not subducted, but rather were incorporated into orogenic belts (Mao and others, 2016; Han and Zhao, 2018). As accretion proceeded, the margins of the cratonic and oceanic fragments became sites of fold and thrust belts, suture zones, faults, and an amalgamation of island-arc and accretionary complexes; several of the terranes developed foreland basins. By the Permian, compressive deformation developed sufficient tectonic topography to isolate several of the basins from marine waters. This topographic relief led to hydraulically closed basins (Garcia-Castellanos, 2006; Marenssi and others, 2020), characterized by the development of extensive, basinwide lacustrine systems. The Junggar and Turpan Basin Provinces developed lacustrine systems by compressive deformation along the margins in the Permian, and lacustrine systems formed following compressional deformation in the Sichuan, Ordos, Tarim, and Qaidam Basin Provinces. In contrast, horst and graben systems along the eastern margin of Eurasia were formed by widespread back-arc extension related to changing motions of the Pacific plate. Extensive lacustrine systems formed within grabens in the Songliao, Bohaiwan, and East China Sea Basin Provinces (Li and others, 2012; Liang and Wang, 2019; Yang and others, 2020). Petroleum source rocks within these nine provinces reflect the long and complex tectonic history (Jiang and others, 2016). As the cratonic blocks separated from Gondwana and traversed the Tethyan realm in the early Paleozoic, organic-rich marine sediments were deposited in basinal positions mainly associated with passive margin carbonate platforms, such as in the Tarim, Sichuan, and Ordos Basin Provinces (Yang and others, 2005). In the late Paleozoic, subsequent terrane collisions led to the development of marginal fold belts and associated foreland basins. Potential source rocks in the upper Paleozoic foreland basins are dominated by marginal marine to nonmarine, coal-bearing gas-prone sequences. Beginning in the Permian, along with the compressional formation of hydraulically closed basins, climatic conditions were appropriate to form extensive lacustrine systems with viable lacustrine source rocks, which are well known in the basins of China (Jiang and others, 2016). The Sichuan Basin Province, which resides in the western part of the South China terrane, provides an example of the evolution of the tectonic control on the development of petroleum source rocks (Shi and others, 2016; Mu and others, 2019). The cratonic terrane of the Sichuan Basin Province was rimmed by passive margins as the terrane