Diagenetic of the Mesozoic complex weathering crust reservoir rock in Fulin subsag,Jiyang Depression

The formation of weathering crust reservoirs and the analysis of their differences have become the current research focus of reservoir geology. The Mesozoic oil and gas exploration in the Fulin subsag of Zhanhua sag in the Jiyang Depression of Bohai Bay Basin has encountered its bottleneck. Only the weathering crust at top of the Cretaceous is currently penetrated by exploration wells and the weathering crust reservoir is characterized by complex lithology and high heterogeneity. In order to enhance exploration in the Mesozoic buried hill reservoir in the study area and even the Jiyang Depression,it's of great importance to figure out diagenetic evolution,reservoir space types and their combination relationships. The complex lithology of weathering crust reservoir rock of the Cretaceous Xiwa Formation in the Fulin subsag was studied by core and thin section observation combining with well logging data. The results show that the key lithologies of weathering crust include andesite and tuff,followed by dyke,cryptoexplosive breccia and glutenite. During the complex burial process,different tectonic units underwent the evolution of 'sag-uplift-uplift' or 'sag-uplift-sag',resulting in significant differences for diagenetic sequence and pore evolution of different reservoir rocks. The primary pore types and their genesis of igneous and clastic rocks are obviously different,but the present reservoir space is dominated by secondary pores,especially dissolution pores. The dissolved material includes porphyry,matrix and volcanic debris. The dominant fluid for dissolution is variable. In addition to the atmospheric fresh water rich in CO₂ during the uplifting and exposure,the organic acid discharged during the maturation of organic matter has a positive effect on improving reservoir quality. The formation of favorable weathering crust reservoir rock is controlled by lithology,burial evolution and diagenetic fluid. On the whole,the reservoir quality order of the Cretaceous Xiwa Formation in the Fulin subsag is as follow: Volcaniclastic rock,vesicular andesite,tuff,subvolcanic rock. Different weathering crust reservoir rocks in the study area underwent different diagenetic modification during the complex burial process.