Sequence of Tight Sandstone Reservoir Densification and Oil Charging in the Yanchang Formation of the Southern Ordos Basin, China

The origin of tight reservoirs in the Yanchang Formation of the Ordos Basin and their relationship with hydrocarbon charging remain unclear. Based on petrological observations, physical property analysis, fluid inclusion system analysis and in situ U-Pb dating, the sequence of tight sandstone reservoir densification and oil charging was determined. Through petrological observations, fluid inclusion analysis and physical property analysis, it is concluded that compaction and cementation are the primary causes of reservoir densification. When the content of calcite cement is less than or equal to 7%, compaction dominates densification; otherwise, cementation becomes more significant. However, determining the exact timing of compaction densification proved challenging. Microscopic observations revealed that oil charging likely occurred either before or during the densification of the reservoir. According to in situ U-Pb dating and the porosity evolution curve, cementation densification occurred between 167.0 ± 20.0 Ma and 151.8 Ma. Temperature measurements of the aqueous inclusions indicate that oil charging occurred between 125.0 and 96.0 Ma, suggesting that densification preceded oil charging. This study provides valuable insights for the future exploration of tight oil reservoirs in the Ordos Basin.