Abstract:

The Sichuan Basin locates at the transition zone between PaleoPacific Tectonic Domain and TethysHimalayan Tectonic Domain, of which the marine and marinetoterrestrial black shale has experienced substantial burial (high thermal maturation), significant uplift and erosion, and strong deformation from Mesozoic to Cenozoic. Apatite fissiontrack data, geophysics, sedimentary thickness and vitrinite reflectance etc., are used to decipher the geologic features of the burial and superimposition of the Lower Paleozoic shale gas across the Sichuan Basin and its peripheries in this paper, which highlights the characteristic difference to the shale (gas) in the U.S.A. It indicates about 10004000 m surface denudation across the Sichuan Basin since Late Cretaceous, based on about 100 modeled thermal histories from apatite fission track data. Different surface denudation and thickness of the terrestrial deposits played a firstorder influence on the Paleozoic depth. The bases of the Lower Silurian Longmaxi Formation and Lower Cambrian Niutitang Formation have depth more than 6 500 m and 9 000 m, respectively, of which the contour lines are characterized by NESW strike, and the depth decreases from NE to SW across the Sichuan Basin. The minimum paleodepth of the base of the Longmaxi Formation is ~6000 m in the Suijiang area, southern Sichuan Basin, and the maximum is ~9000 m in the northern Sichuan Basin. Furthermore, there are two areas, i.e. SuijiangYibinLuzhou area, and DazhouBazhong area, with NESW strike and smaller paleodepth of the base of the Longmaxi Formation, where the paleodepth increases southeastward and northward, respectively. The maximum paleodepth of the base of the Niutitang Formation is more than ~11000 m, in the western and northern Sichuan Basin, and the minimum paleodepth is less than ~8000 m, locating in the YibinZigongSuining area in the central and southern Sichuan Basin. The thermal maturation of the Longmanxi and Niutitang Formations are more than 2.4% and 2.8%, respectively, and it decreases from northeast to southwest across the Sichuan Basin and its peripheries. It indicates a close relationship between the thermal maturation of the Lower Paleozoic black shale and their paleodepth. Furthermore, the significant uplift, erosion, and strong deformation resulted in a dynamic stressstain state, changing temperature and pressure etc., and thus a dynamic reservation condition of the Lower Paleozoic shalegas. It should be noted that there was a correlation between the insitu success and setback of exploration in the Lower Paleozoic shalegas and their differential burial, uplift and denudation, and deformation across the Sichuan Basin and its peripheries, indicating a very important geologic feature of the burial and superimposition to the distribution of the Lower Paleozoic shalegas. Thus, not only should we pay more attention to what controls the highquality reservoir of shalegas, but also to what controls the highproductivity reservoir of shalegas under such a unique geologic feature in the South China.

Key words: substantial burial, significant uplift, strong deformation, lower Paleozoic shalegas, Sichuan Basin