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.