Assessment of Paleozoic Shale-Oil and Shale-Gas Resources in the Tarim Basin of China, 2018

The U.S. Geological Survey (USGS) quantitatively assessed the potential for unconventional (continuous) oil and gas resources within two Paleozoic organic-rich shales in the Tarim Basin of China (figs. 1 and 2): Lower Cambrian Yuertusi Formation and shales in the Middle Ordovician Series. These strata are the principal source rocks for conventional oil and gas fields in the interior of the Tarim Basin (Li and others 2018; Zhu, Chen, and others, 2018). The Tarim Basin, the largest petroleum basin in China, encompasses 563,000 square kilometers (km), and its Phanerozoic strata are as much as 16 km thick (Qiu and others, 2012). Although numerous oil and gas fields have been developed along its northern margin and in several uplifts in the basin’s interior, large parts of this remote basin remain unexplored. The USGS previously assessed the Tarim Basin’s conventional oil and gas resources (Charpentier and others, 2012). Paleozoic marine formations are currently preserved at depth and locally exposed around the basin’s periphery. They were deposited on the passive margin of the Tarim craton, a continental fragment proximal to the Gondwana margin. Since the late Paleozoic assembly of Central Asia, the Tarim Basin has been a vast nonmarine basin bounded on the south by the Tibetan Plateau and on the north by the Tien Shan. From the Carboniferous to the present, the basin’s margins have been strongly influenced by contractional deformation, including the ongoing Himalayan orogeny. This tectonic history has resulted in a relatively cool geothermal setting throughout the basin’s history; the current geothermal gradient is in the range of 20–23 degrees Celsius per km (Zhang, Huang, and others, 2015). Geologic Background