生物扰动与成岩作用耦合对泥质碳酸盐岩改造机理--以中东美索不达米亚盆地白垩系为例

Transformation mechanism of argillaceous carbonate rock by the coupling of bioturbation and diagenesis:A case study of the Cretaceous System of the Mesopotamia Basin in the Middle East

以中东美索不达米亚盆地白垩系为例,基于岩心、铸体薄片及物性数据分析,研究生物扰动与成岩作用耦合对泥质碳酸盐岩的改造机理。研究表明,生物改造岩石的方式有3种:①生物机械改造地层,使岩石结构变得松散,增加粒间孔隙数量;②潜穴形成后回填粗粒碎屑,准同生期不饱和流体渗入发生溶蚀作用;③废弃潜穴发生化学蚀变,白云石化作用形成大量的晶间孔隙。生物扰动与溶蚀作用的耦合主要发生在准同生期,受岩石类型、海平面下降幅度、潜穴密度、充填物和水体环境等因素影响。潜穴物性优于基底,流体优先沿潜穴运移,潜穴溶蚀扩大后基底也会发生溶蚀。生物扰动与白云石化作用的耦合主要发生在埋藏阶段,潜穴中富集的有机质和还原细菌为白云石化提供了物质基础、还原条件和碱性环境。准同生期白云石化作用对潜穴物性的影响较小,当潜穴脱离沉积界面,封闭环境下发生交代,形成自形细晶白云石,发育晶间孔隙。生物对泥质碳酸盐岩的改造程度受岩石类型、潜穴密度和连通性、潜穴充填物和潜穴结构等因素控制。泥质组分含量增加,生物扰动对岩石物性的改善效果愈加凸显,当潜穴充填粗粒碎屑或细晶白云石,潜穴密度越大、连通性越高,迂曲度越低,泥质碳酸盐岩储集层物性越好。

The transformation mechanism of joint bioturbation and diagenesis was studied based on core,cast thin section and physical property data of Cretaceous strata in the Mesopotamia Basin,the Middle East.There are 3 ways of biological transformation of rocks:(1)The living creatures transformed formations mechanically to make the rocks looser and intergranular pores increase.(2)After formation,burrows were backfilled with coarse-grained debris,and then unsaturated fluid infiltrated into the burrows during the penecontemporaneous period,resulting in dissolution.(3)Chemical alteration occurred in abandoned burrows and dolomitization produced a large number of intercrystalline pores.The coupling of bioturbation and dissolution occurred mainly in the penecontemporaneous phase,and was controlled by rock type,sea level decline,burrow density,infillings,and water environment etc.As the burrows had better physical properties than the substrate,unsaturated fluid preferentially migrated along the burrows,leading to dissolution expansion of the burrows first and then dissolution of the substrate.The coupling of bioturbation and dolomitization occurred mainly in the burial phase.The rich organic matter and reducing bacteria in the burrow provided material basis,reducing conditions and alkaline environment for dolomitization.The metasomatism in the penecontemporaneous period had little effect on the physical properties of the burrows.When the burrows were separated from the deposition interface,equimolar metasomatism occurred in the burrows in a closed environment,forming euhedral fine-crystalline dolomite with intercrystalline pores.The transformation degree of bioturbation to argillaceous carbonate reservoir depends on rock type,density,connectivity,infillings and structure of the burrows.With the increase of the clay content,the improvement to rock physical properties by bioturbation becomes more prominent.When the burrows are filled with coarse-grained debris or fine-crystalline dolomite,the greater the density,the higher the connectivity,and the lower the tortuosity of burrows,the better the physical properties of the argillaceous carbonate rocks are.