--> --> Contribution of Overall Basin Moderate Uplift on Coal-bearing Tight Sand Gas Accumulation— Case Study of Xujiahe Formation in the Sichuan Basin and Upper Paleozoic in the Ordos Basin

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Contribution of Overall Basin Moderate Uplift on Coal-bearing Tight Sand Gas Accumulation— Case Study of Xujiahe Formation in the Sichuan Basin and Upper Paleozoic in the Ordos Basin

Abstract

Tight sand gas, as a kind of important unconventional gas resources, distributes extensively in different petroliferous basins and is becoming a dominant element in natural gas production. In coal-bearing formations of Triassic System in the Sichuan Basin and the Carboniferous and Permian System in the Ordos Basin, tight sandstone and coal measures have formed gas-bearing assemblage in large scale in these two basins, which is a prerequisite to large scale tight sand gas generation and accumulation. Similar to most shale gas basins in North America, the Sichuan Basin and Ordos Basin all experienced overall moderate uplift in the Mesozoic and Cenozoic Era and a certain extent of stratigraphic denudation after deep burial process, which promoted the formation of tight sand gas resources. According to adsorption principles and simulation experiments, pressure drop and desorption may occur in coal measures during formation uplift; a great amount of free gas released then becomes the gas source at the uplift stage, which accounts for 28–42% of total gas expulsion from source rocks; meanwhile a great deal of free gas may also discharge from pore space inside source rocks in coal measures due to pressure drop and subsequent gas expansion. Gas expulsion strength is estimated accordingly to be 3–6×108 m3/km2 during formation uplift of Upper Paleozoic in the Ordos Basin and Upper Triassic Xujiahe Formation in the Sichuan Basin. Geochemical characteristics suggest natural gas released at uplift stage has lighter carbon isotope and low maturity compared with that formed in burial process, indicating that released gas was generated at an early stage and then discharged from source rocks due to formation uplift, pressure decline, desorption and volume expansion at the later stage. Studies on high-temperature and high-salinity inclusions and physical simulations in the conditions approximate to real geologic setting demonstrate that depressurization, desorption, expansion and subsequent massive gas expulsion and accumulation were likely to occur in the process of coal measures uplift. Gas accumulations during formation uplift mainly resulted from diffusion flow and gas charge due to close and extensive source-reservoir contact and drop in pressure difference. This new insight into gas accumulation may bring about some good changes in natural gas resources assessment in uplift provinces with coal measures and tight sandstone.