鄂尔多斯盆地北部盆-缘过渡带致密砂岩气成藏动力机制OA

[Objective]Due to the westward shift of natural gas exploration strategy in the Hangjinqi area,the First Member of the Shihezi Formation in the Xinzhao East Zone(located in the western part of the Hangjinqi area)has become the main target for natural gas exploration at this stage.Compared with other zones in the Hangjinqi area,the Xinzhao East Zone has the largest burial depth,the poorest physical properties of the Upper Paleozoic sandstone reservoirs,and most of its reservoirs have been highly densified.Existing exploration results confirm that,under conditions of high densification,the First Member of the Shihezi Formation in the Xinzhao East Zone can still form large-scale natural gas charging and accumulation.This study aims to identify the coupling mechanism of driving and resisting forces during the main accumulation period and its controlling effects on gas accumulation,and to reveal the dynamic mechanism of tight sandstone gas accumulation in the transition zone of the northern margin of the Ordos Basin.[Methods]In this study,the quasi-continuous tight sandstone gas of the First Member of the Shihezi Formation in the Xinzhao East Zone of Hangjinqi area,located in the northern basin-margin transition zone of the Ordos Basin,was taken as the main research object.Guided by the theory of reservoir-forming dynamics,reservoir petrology,reservoir-forming chronology,and basin simulation technology were comprehensively used to analyze the mechanism and process of reservoir densification of the First Member of the Shihezi Formation in the Xinzhao East Zone.The charging period and time of natural gas were determined,the relationship between reservoir densification and natural gas accumulation was summarized,and the charging dynamic-resistance evolution process of tight sandstone reservoirs was quantitatively reconstructed.The dynamic-resistance coupling mechanism and its reservoir-controlling effect of tight sandstone gas accumulation in the main accumulation period were summarized.[Results]The results showed that the proportion of tight reservoirs in the First Member of the Shihezi Formation in the Xinzhao East Zone of the Hangjinqi area was more than 50％,and two types of tight sandstone reservoirs(compaction-dominated and quartz cementation-dominated tight types)and one type of dissolution-dominated non-tight reservoir were developed.The main accumulation period of natural gas in the First Member of the Shihezi Formation in the Xinzhao East Zone was the end of the Early Cretaceous(110-100 Ma),and the natural gas was characterized by mixed charging of CO2 and CH4.The tight sandstone reservoirs of the two origins were characterized by densification followed by accumulation,and the natural gas accumulation driving force was greater than the resisting force during the main accumulation period in the Middle and Late Early Cretaceous.[Conclusion]It is proposed for the first time that the gas accumulation net driving force(difference between driving and resisting forces)is greater than 7 MPa,which is a necessary condition for natural gas enrichment in the First Member of the Shihezi Formation in the Xinzhao East Zone of the Hangjinqi area.High net accumulation driving force and favorable sealing conditions may be the key factors for natural gas enrichment in the Xinzhao East Zone of the Hangjinqi area.This study can provide references for the exploration and development of similar oil and gas reservoirs in the basin-margin transition zone of large depression basins,and enrich the theory of tight-low permeability oil and gas accumulation in the basin-margin continuous-discontinuous accumulation transition zone of large depression basins.