Reservoir and Gas Migration Characterization of Tight Sandstone in Kuqa depression of Tarim basin, China

Recent exploration in Kuqa depression of Tarim Basin, western China has discovered numerous tight sandstone gas reservoirs with reservoir porosity of less than 5% and permeability of less than 1mD at depth of over 5000 m. However, lacking of better understanding on the complicated structural geology conditions and hydrocarbon accumulation mechanism of tight sandstone in Kuqa area restricted the target optimization in exploration and evaluation for hydrocarbon resources. Apart from these factors mentioned above, pore structure of tight sandstone plays an important role in controlling the hydrocarbon accumulation. Pore structure of tight sandstone, however, rarely studied by combining the physical simulation experiments. Tight gas accumulation characterization in pore structures is of great importance for tight gas exploration and development. In order to getting a better understand on tight sandstone reservoirs and tight gas accumulation, this study focused on the pore structure of tight sandstone and gas migration in hydrocarbon-bearing reservoir in Kuqa depression of Tarim basin. Detailed analyses are carried out using microscopic observation, capillary pressure curve test, poro- permeability measurement and gas charging physical simulation experiments, taking tight sandstone gas potential area, Kuqa depression of Tarim basin as study area. The results indicate that (I) the main pore types of tight sandstone reservoir are dominated by dissolved pores and slot pores, and the size of majority pore-throat radius is about 0.1μm. (II)There is no obvious correlation between permeability and porosity, because the permeability is mainly controlled by pore structure and fracture.(III) Gas migration in tight sandstones requires certain starting pressure gradient, the higher the permeability is, the lower the starting pressure gradient is. (IV) The starting pressure gradient significantly decreases due to the development of fracture in the sandstone. (V) The process of gas migration and accumulation in tight sandstone can be regarded as individual gas bubble migration in free state in the beginning, gas bubbles accumulate to form continuous state, the continuous state gas start to migrate once it breakthroughs the capillary pressure of pore throat.