Abstract
The Dongsheng Gas Field, a giant tight-gas field in the northern margin of the Ordos Basin in China, has the Lower Permian Shihezi Formation (P(2)x(1)) as the main gas production layer, with tight-sandstone reservoirs (average porosity and permeability of 8.6% and 0.82 mD, respectively). This study integrated basin modelling, fluid inclusion analysis, thin slice identification, and digital simulations of gas charging to explore reservoir densification, hydrocarbon charging history and accumulation dynamics. A net force concept during tight gas accumulation (Pn = Pd - Pr, where Pd is the overpressure of source rocks and Pr is the capillary force under 50% gas saturation) was established. Numerical simulations showed: Pn > 10.39 MPa (rapid charging) indicates commercial gas layers; Pn < 8.88 MPa (breakthrough stage) indicates dry/gas-bearing layers. The P(2)x(1) reservoirs experienced 1 oil-gas and 2 gas charging episodes (230-180 Ma, 180-120 Ma, 120-80 Ma), evidenced by source rocks, bitumen, inclusions and salinity variations. The 120-80 Ma episode (corresponding to post-120 Ma hydrocarbon peak) is the key accumulation period. During this key period, reservoirs south of the Poerjianghaizi Fault were tight, while the SGH zone had 13% porosity. Three charging stages (breakthrough: 6.81-12.83 MPa; rapid: 6.02-27.46 MPa; saturation: 24.86-49.96 MPa) were identified. Third-stage Pn and gas saturation enable gassiness prediction. This study provides insights into tight gas accumulation mechanisms and gas distribution prediction.