Yusong Yuan

Western Hubei Eastern Chongqing area is a major exploration block of marine‐facies petroleum as well as a currently popular area of shale gas exploration. It has suffered long term uplift and erosion since the Late Yanshan‐Himalayan period. Erosion thickness is an indispensable basic parameter for study of thermal evolution of source rocks, reservoir formation and evolution, and basin modeling to calculate the amount of resources. Considering the inherent limitations and application conditions of the existing methods of erosion thickness reconstruction, firstly we used the paleogeothermal gradient inversion method to reconstruct the erosion thickness of boreholes after analyzing the vitrinite reflectance (Ro) profiles. Then, we studied the relationship between the erosion thickness of boreholes and the upmost sequences of the boreholes drilled. After that, a contour map was compiled based on the geological map. The results show that the vitrinite reflectance (Ro) profiles of boreholes in the western Hubei Eastern Chongqing area do not show obvious “dislocations” or “jumps”, recorded maximum palaeotemperatures of sequences in the maximum burial depth. There exists a clear correlation between the erosion thickness and the upmost sequence of the wells: the greater age of the upmost sequence, the greater erosion thickness of the well, and vice versa. Generally, the late Yanshan‐Himalayan erosion amount gradually decreases from east to west in the Western Hubei Eastern Chongqing area. From the Lichuan area to Shizhu and to the west of the Fangdoushan, the erosion thickness decreases from 2000∼4000 m to 1500∼2500 m and to 1000∼1500 m.

The onset of uplifting of the Mid‐upper Yangtze area is constrained by the combination of analysis of apatite fission track (AFT) age and modeling of fission track length distribution. The results show that the general trend of onset of uplifting is characterized by decreasing westward in geological time for the different tectonic units in the Mid‐upper Yangtze area. Two episodes of uplifting in Jianghan basin occurred within 157~97 Ma and 10~0 Ma; Xiang'exi‐Wuling area experienced an uplifting in 137~0 Ma; E'xi Yudong area, Chuandong folds, Qianzhong Hights and Dian‐Qian‐Gui areas in 97~0 Ma; Northern Sichuan basin and Chuanzhong areas in 56~0 Ma; and Chuanxi depression and Dianxi region in 23~0 Ma. The major faults of Qiyueshan, Huanyinshan and Longquanshan controlled the boundaries of the onsets of uplifting. The onsets of uplifting for the areas east of Qiyueshan fault, in between Qiyueshan and Huayinshan faults, in between Huayinshan and Longquanshan faults, west of Longquanshan fault are late Jurassic‐early Cretaceous (J₃‐K₁), late Cretaceous (K₂), Paleogene (E) and Neogene (N), respectively.

Both the structural evolution and thermal history of the Middle ‐ Late Permian Sichuan Basin are closely related to activity of the Emeishan Supper Mantle Plume (ESMP). The ESMP played an important role in the thermal evolution of the pre‐Middle Permian hydrocarbon source rocks in the southwestern part of the Sichuan basin. On the basis of reconstructed thermal history of the Sichuan basin, based on vitrinite reflectance (R_o) from boreholes in the basin, using a paleo‐heat flow method, the effect of the ESMP on the evolution of the pre‐Middle Permian source rocks is studied. The results show that in the southwestern part of the Sichuan Basin (the intermediate zone of the ESMP) the organic matter of the Paleozoic source rocks became mature in middle Late Permian. This indicates that the thermal evolution of the Paleozoic source rocks was strongly affected by the ESMP. In more detail, in the region close to the ESMP, the source rocks of lower Permian were heated to peak maturity (maximum R_o > 3%) at around 260 Ma and later there was no secondary hydrocarbon generation. However, in the regions far away from the ESMP, secondary Mesozoic hydrocarbon generation of the source rocks existed. The thermal evolution of the Mesozoic source rocks was related with Mesozoic foreland basin evolutionburial history and subsequent regional denudation of the basin, rather than the ESMP. Therefore, except in the regions far away from the ESMP, the prospect of oil and gas from Paleozoic source rocks is unfavorable.