Controlling Effects of Overall Basin Uplift on Tight Sand Gas Resources — Case Study of Xujiahe Formation in the Sichuan Basin and Upper Paleozoic in the Ordos Basin

Summary URTeC 1577339 Tight sand gas, as a kind of important unconventional gas resources, distributes extensively in different petroliferous basins in China. In 2010 the annual tight sand gas yield in China went beyond 23 billion cubic meters, nearly one quarter of which came from tight sandstone of the 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 tight sand gas generation and accumulation. Similar to most shale gas basins in North America, the Sichuan Basin and Ordos Basin all experienced overall uplift in the Mesozoic and Cenozoic Era and a certain extent stratigraphic erosion 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 0.3-0.6 billion cubic meters per square kilometer during formation uplift of Upper Paleozoic in the Ordos Basin and Upper Triassic Formation in the Sichuan Basin. Studies on high temperature and salinity inclusions and physical simulations in the conditions approximate to real geologic setting demonstrate that 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; Upper Paleozoic gas charge from diffusion flow may reach 130,000 billion cubic meters in the Ordos Basin, which compensates the loss of gas diffusion in reservoirs and is crucial to the generation of large gas provinces. This new insight into gas accumulation may bring about some changes in natural gas resources assessment in uplift provinces with coal measures and tight sandstone; those districts considered to be unprofitable in the past due to large uplift need to be reevaluated for their actual resource potential and exploration prospect.