Abstract:

Carbonate cements are the most important cements in the Paleogene sandstone reservoirs of the Bozhong depression.Analysis on thin sections shows that the main cements of the middle and deep clastic reservoirs are siderites,dolomites,calcites,ferrodolomites and ferrocalcites.According to the statistical data,carbonate content gradually rises with the increase of depth.A sharp increase of carbonate content occurs in the Paleogene Dongying Formation.When the carbonate content exceeds 10%,it will have a great impact on physical properties of reservoirs.Analysis on thin sections and isotopes reveals that the siderites mainly occur as pores fillings and particle rim cements and were mainly formed in the synsedimentary period or at the early stage of the early diagenesis.The calcites mainly occur as particles or poikilitic cements.The relative calcite content has negative correlation with δ¹⁸O values,indicating that most of the calcites were formed late.The ferrocalcites mainly occur as particles.The ferrocalcite-dominant samples correlate with low δ¹⁸O values and relatively high temperature of precipitation.The dolomites mainly occur as intergranular pore-filling particles in patches,and its relative content has obvious positive correlation with δ¹⁸O value.Its temperature of precipitation is relatively low and the time of forming is relatively early.The ferrodolomites mainly occur as dispersed particles,and its relative content has obvious positive correlation with δ¹⁸O value and its temperature of precipitation is relatively low.

Key words: carbon and oxygen isotope, carbonate cement, clastic reservoir, Paleogene, Bozhong Depression