北黄海盆地东部坳陷中生代火山碎屑岩成岩作用特征

Diagenesis characteristics of Mesozoic pyroclastic rocks in eastern depression,North Yellow Sea Basin

通过钻井岩性、岩石组分分析、狄更森图解分析和成岩作用分析,对北黄海盆地东部坳陷中生代火山碎屑岩的成岩作用特征进行了研究。结果表明:东部坳陷内A井区、B井区钻遇中生界上侏罗统—下白垩统陆相沉积-火山碎屑沉积序列,B井区凝灰质砂岩发育,火山碎屑含量较高;A井区主要发育碎屑砂岩。凝灰质砂岩中火山碎屑的体积分数为20%~35%,碎屑砂岩中也含有小于10%的火山碎屑。成岩作用研究表明,A区发育凝灰质溶蚀-绢云母蚀变和黏土矿物蚀变作用,同时酸性流体导致岩石中黏土矿物选择性生成或转化,这些黏土矿物早期占位,抵抗颗粒间压实,后期或溶蚀或转化,形成粒内溶蚀孔和部分基质溶孔,释放一定储集空间;B区发育的少量凝灰质砂岩以脱玻化-重结晶作用为主,导致微细粒凝灰质组分减少,降低喉道堵塞,提高连通性,晚期脱玻化导致硅质胶结作用增强。研究区储层具非均质性,凝灰质砂岩储层物性相对较好,储集空间以次生溶蚀孔为主,脱玻化孔及晶间孔贡献相对较少,构造-火山活动形成岩石裂隙-微裂隙。

Based on drilling lithology,rock components,Dickinson diagrams,and diagenesis analysis,the diagenetic characteristics of Mesozoic pyroclastic rocks in the eastern depression of the North Yellow Sea Basin were discussed.The results show that the Upper Jurassic-Lower Cretaceous continental sedimentary-volcanic sequence has been discovered in wells in area A and area B.The tuffaceous sandstone with high volcanic debris content dominates in area B,and clastic sandstone dominates in area A.The content of pyroclastic debris is about 20%-35%in tuffaceous sandstone and less than 10%in clastic sandstone.In area A,the clastic sandstone has undergone tuff dissolution-sericite alteration and clay mineral alteration.The clay minerals showed selective formation or transformation due to acidic fluids in rocks.The clay minerals occupied the initial pores to resist inter-particle compaction during early diagenesis,followed by the formation of intragranular dissolution pores and some matrix dissolution pores due to erosion and transformation,which benefited the reservoir spaces.In area B,devitrification and recrystallization are important diagenetic processes in tuffaceous sandstone,which reduced fine-grained tuff content,prevented pore-throat blocking,and improved pore-throat connectivity during early diagenesis.However,the devitrification products increase siliceous cementation in the late diagenesis stage.Reservoirs in this study area are heterogeneous,and the tuffaceous sandstone shows relatively good reservoir properties.The reservoir spaces in tuffaceous sandstone are featured by secondary corrosion hole dominating,relatively less devitrified holes and intercrystalline pores.Furthermore,tectonic-volcanic activity has increased rock cracks and micro-fissures.