鄂尔多斯盆地中部延长组长7段致密储层成因及控制因素

Origin and dominated factors of Chang 7 Member tight reservoirs in Yanchang formation,central Ordos Basin

鄂尔多斯盆地铁边城地区长7段储层总体上经历了压实、2期油气充注及其引发的2期成岩响应。砂岩厚度不同,油气充注与致密化过程也不同。研究表明:薄层砂体(0.5~2.6 m)的致密化过程为压实-Ⅰ期油气充注—压实-Ⅰ期方解石胶结,储层致密机理为压实和早期方解石胶结;厚层砂体(大于2.6 m)的致密化过程为压实-Ⅰ期油气充注-压实-Ⅰ期方解石胶结-Ⅱ期油气充注-Ⅱ期方解石胶结,储层致密机理为压实和早晚2期方解石胶结。富软碎屑砂岩仅受压实作用就可致密。除压实作用外,油气充注控制了储层的成岩过程。油气充注时,酸性流体溶蚀改造储层;成藏后,油水分异导致水层中的方解石沉淀填充孔隙而变致密,油层中水岩反应被抑制而残留孔隙。油气充注为方解石胶结提供了碳源,且与沉积差异一起控制了方解石胶结层的分布。沉积作用、压实作用、胶结作用及油气充注是长7段储层致密化的控制因素。

In general,the Chang 7 Member reservoirs in Tiebiancheng,Ordos Basin have experienced compaction,two stages of hydrocarbon charging and two stages of diagenetic response induced by hydrocarbon charging.Notably,the hydrocarbon charging and the reservoir densification process vary with the sandstone thickness.The research shows that:the reservoir densification process in thin bedded sandstones(0.5-2.6 m)is concluded as compaction-hydrocarbon chargingⅠ-compaction-calciteⅠcementation,and the reservoir densification mechanism is compaction and calciteⅠcementation.The thick bedded sandstones(>2.6 m)is concluded as compaction-hydrocarbon chargingⅠ-compaction-calciteⅠcementation-hydrocarbon chargingⅡ-calciteⅡcementation,and the reservoir densification mechanism is compaction and two episodes of calcite cementation.The soft-rich clastic sandstone can be compacted only by compaction.Hydrocarbon charging events control the whole diagenetic process in the reservoirs except the compaction.During hydrocarbon charging,acid fluid dissolved the reservoirs.After hydrocarbon accumulation,the oil-water distribution resulted in calcite precipitation in water layers which involved into tight sandstones,and remained pores in oil layers due to the inhibition of the water rock reaction.Hydrocarbon charging not only provides carbon source for calcite cementation,but also controls the spatial distribution of calcite-cemented layers together with sedimentary varieties.Consequently,sedimentary,compaction,sedimentary,cementation and hydrocarbon charging are the factors for the densification of Chang 7 Member sandstone.