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Deep Burial and Substantial Uplift Features of ohe Lower Paleozoic Rocks in Sichuan Basin, Western China

Bin Deng, Shugen Liu, Zhiwu Li, Shun Liu, and Guozhi Wang
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation/Chengdu University of Technology, Chengdu, China

Being a typical superimposed basin, Sichuan basin as one of the major oil/gas basins in western China, has experienced three evolutionary phases, marine carbonate platform (ZT31), continental clastic basin (T32-E2) and uplift and modification (E2-Q), respectively. During the continental clastic basin episode, there were deposited very thick terrestrial strata across the basin, from the Upper Triassic Xujiahe Formation up to the Quaternary. Based on 212 boreholes and outcrop stratigraphic data, we compiled a thickness isopach map of the terrestrial strata (T3-K) in the Sichuan basin. In general, there are >3000-m thick terrestrial strata deposited in the western and northern parts of the basin, as the Dayi - Chengdu - Mianyang - Bazhong - Tongjiang - Daxian areas. However, significantly thinner terrestrial strata (less than 1500m) were deposited in the southern and eastern parts of the Sichuan basin. Furthermore, we constructed a map of surface denudation based on vitrinite reflectance (R0) profiles from 31 boreholes and ~100 apatite fission track ages, which unravels the uplift and modification process across the Sichuan basin, and indicates significant surface denudation of 1500~4000 m during the Cenozoic. Thus, we suggest that the most of marine strata in the Sichuan basin were buried with depth of ~5000m. If we consider the mean thickness with ~3000m for the middle-to-upper Paleozoic strata and with ~1000m for the lower-to-middle Triassic strata, the paleo-burial depth of lower Paleozoic Rocks will be more than ~8000m across much of the basin during pre-Cenozoic. Contrasting to the in-situ burial depth of the lower Paleozoic rocks with 2000-5000m; it indicates substantial uplift and exhumation took place across the Sichuan basin.

Such deep burial and substantial uplift features resulted in significant effects in the lower Paleozoic rocks. In particular, the changing temperature and pressure in such deep burial and substantial uplift processes probably accommodated serious change on the absorption and migration features of natural gas to have a key role in the accumulation and preservation of Lower Paleozoic (both conventional and unconventional) gas fields across the Sichuan basin.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013