咸化湖盆成岩致密带形成机制及其地质意义

The Forming Mechanism and Geological Significance of Diagenesis Tight Zone in Saline Lacustrine Basin

针对成化湖盆成岩早期储层致密的问题,开展了这一成岩致密带的形成机制及其地质意义的研究。研究中以渤南洼陷沙四上亚段为例,综合物性分析、铸体薄片、全岩矿物X衍射等分析化验资料,认识到渤南洼陷沙四上亚段的成岩致密带上碳酸盐岩胶结物发育,胶结率普遍大于0.5,压实率低于0.2;早期较强胶结作用使原生孔隙所剩无几,造成后期有机酸与可溶矿物的接触面积变小,大量溶蚀发生的时间滞后,从而在2 800~3 200 m形成致密带;致密带上储层孔隙度普遍小于12%,有利储集体应为致密带上、下的扇三角洲前缘、混积岩滩坝及近岸水下扇扇端;致密带物性较差,不但可阻止油气二次运移,使其上、下油气聚集方式存在差异,而且致密带内、外油气富集程度和产能也存在差异。

Under the influence of an alkali(first)-acid(second)diagenetic environment,tight reservoirs developed in the earlystage of diagenesis in the saline lacustrine basin.To reveal the formation mechanism and petroleum geological significance ofthe diagenesis tight zone,we took Ess4of Bonan Sag as an example and analyzed physical property,casting sheet,and mineralX-ray diffraction data.Research has shown that the cementation rate of carbonate cements in the diagenesis tight zone of Es4isgreater than0.5and the compaction rate is less than0.2.Primary cores were lost due to strong cementation in the early stage ofthe alkaline diagenetic environment;the contact area between organic acid and soluble minerals reduced later,and the occurrenceof large-scale dissolution lagged in time.Therefore,the diagenesis tight zone was formed at2800–3200m.Reservoir porosityin the diagenesis tight zone was generally less than12%,hence,the more favorable reservoir is located above and below thediagenesis tight zone,in delta front,mixed sedimentary rock beach-bar,and near-shore subaqueous fan front environments.Theinferior physical properties of the diagenesis tight zone not only stops secondary migration of hydrocarbons,so that hydrocarbonaccumulation modes differ above and below the zone,but also causes the degree of hydrocarbon enrichment and deliverabilityto differ in the inner and outer parts of the zone.