Phase change of formation water and the formation of deep basin gas: A case from the Upper Paleozoic, Ordos Basin

The forming process and conditions of Upper Paleozoic gas reservoir in Ordos Basin was analyzed by comparing the water producing characteristics of abnormally high temperature gas reservoir in the Qianmiqiao buried hill with the simulated experiment results of gas/water phases under sealed conditions. The test results of vitrinite reflectance, apatite fission track and inclusion homogenization temperature indicate that the Upper Paleozoic in the Ordos Basin had an abnormally high geothermal field during the Late Jurassic to Early Cretaceous. Driven by the abnormally high temperature, the organic matter matured rapidly to generate a lot of natural gas; meanwhile, the formation water vaporized, with intersoluble gas and vapor to accumulate and generate abnormally high pressure. The gas (vapor) phase fluid migrated towards the upper formation within the compartment under abnormally high pressure, which lowered the pressure in the lower abnormally high temperature formation, sped up the vaporization of formation water and resulted in the accumulation of pressure for a new round of migration. In this circular way, the gas (vapor) phase fluid diffused to every part of the compartment, with inner temperature and pressure being balanced. The high temperature and high pressure gas reservoir at basin level was formed. The uplift and erosion during the Late Cretaceous-Paleogene led to the drop of the temperature and pressure of the Upper Paleozoic. The liquefaction of vapor lowered the vapor and gas concentration. Thus the basin level low-pressure gas reservoir was formed.