Abstract: Carbonate cements were pervasively developed in deep clastic strata of the Mesozoic formations in the Bohai Bay Basin.Based on results through thin section identification,X-ray diffraction,Scanning Electron Microscopy (SEM),elements analysis,and numerical basin modeling and laser micro-sampling analysis for carbon and oxygen isotopes,we made a comparative study on the types,occurrences,and genetic mechanisms of carbonate cements of various periods,and their impacts on reservoir physical properties in the C12 and Q17 structures.The results show that the differences of carbonate cements of the two structures have led to different reservoir physical properties,and the generation and distribution of carbonate cements were mostly under the control of burial period,leaching,tectonic conditions,and diagenetic settings.The C12 structure features high content of carbonate cements,and its reservoir pores are filled with both early and later generated carbonate cements.Furthermore,the cements of early generation experienced no dissolution at later stages,while secondary pores developed only near the top Mesozoic unconformity.The reason lies in the impact of repeated deep burial processes,sealing diagenetic conditions,and intensive evaporation of the braided rivers,as well as the early faulting,which resulted in the intense carbonate cementation and then the initiation of reservoir tightening in the C12 structure.In comparison,the Q17 structure is relatively good in reservoir physical properties.Although the carbonate cementation is also intensive in the structure,characterized by early-stage carbonate cement concentration in local intervals of fine-grained sandstones,the thick-bedded coarse-grained sandstone intervals of better original sediment facies have usually low carbonate content or experienced carbonate dissolution at early stages,resulting in sporadic cements of ferron dolomite.This is due to its shallow burial history,open diagenetic settings,continuous fault activities,and wide-spread leaching.The occurrence of ferron dolomite was associated with formation water flows and hydrocarbon charging on the sandstones.Based on these observations,we proposed a concept of differential diagenesis,and discussed its mechanism from both the macro-structure and micro-diagenesis perspectives.

Key words: leaching, diagenesis, burial condition, differential diagenesis, carbonate cementation, tectonic environment, Mesozoic, Bohai Sea