Petroleum accumulation feature and history of Upper Paleozoic, Ordos Basin

During the early burial period, the ground water of coal series in the Ordos Basin became weakly acidic, due to the release of organic acids from coal. The corrosion of oxygen-containing carboxylic acid then generated a large number of pores in aluminum silicate and made sandstone easily to be compacted. The following rise of formation temperature promoted the Al³⁺ and Si²⁺ complexation reduce to SiO₂, clay and ferrocalcite, which led to the regional sandstone densification. From the Late Jurassic to the Early Cretaceous, tectonic-thermal events resulted in the hydrocarbon generation of coal series and the vaporization of formation water, which promoted high-pressure fluid (methane and steam) flood the whole storage box. A basin-scale high-temperature and highpressure gas reservoir was founded. From the Late Cretaceous to Paleocene, the regional uplift and erosion of strata made the coal series burial depth decrease and the geothermal gradient decrease, so that vapor condensated into water, while the CH4 gas retained gradually. High-temperature and high-pressure gas reservoir was transformed into low-temperature and under-pressure reservoir.