Material Sources Of The Main Diagenetic Products In The Paleogene Moderately-deeply Buried Sandstones In The Jiyang Sub-basin

The material sources,transport mechanism and cementation process of the main diagenetic products in sandstone are basic theoretical issues in the study of diagenesis of clastic reservoirs.The diagenesis of mudstone and sandstone,and the material source of the main diagenetic products in sandstone have been systematically studied.The article takes sandstones and mudstones of the 3th（Es₃）—4th（Es₄）member of the Shahejie Formation in the Jiyang Depression as the research object.And core observation,thin section identification,cathodoluminescence analysis,scanning electron microscope observation,energy spectrum analysis,homogenization temperature measurement,electron probe composition test,X-ray diffraction analysis and other methods are comprehensively used.The following achievements have been obtained.The diagenesis types of mudstones in the Es₃ and Es₄ in the Jiyang Depression include compaction,authigenic mineral precipitation,metasomatism,dissolution,clay mineral transformation and organic matter evolution.Authigenic precipitate minerals include carbonate,pyrite,etc.Carbonate minerals include calcite,ferric calcite,dolomite and ferric dolomite.And they are isolated form,laminated form and filling mudstone fractures.The main diagenesis of sandstone are compaction,cementation,dissolution and metasomatism.The types of cements include carbonat,siliceou,kaolinite and pyrit.There are also four types of carbonate cements:calcite,ferrocalcite,dolomite and ankerite.According to the fractures development and the scale of sandstone and mudstone,sandstone and mudstone combinations are divided into eight types,including sandstone lens and mudstone without fracture,sandstone strips and mudstone without fracture,thin bed sandstone and thick bed mudstone without fracture,sandstone and mudstone of similar thickness without fracture,thick bed sandstone and thin bed mudstone without fracture,sandstone strips and mudstone with fractures,thin bed sandstone and thick bed mudstone with fractures,and sandstone and mudstone of similar thickness with fractures.In different sandstone and mudstone combination,the types,period and compositions of carbonate cements being in sandstone and adjacent mudstone are same.There are two sources of carbonate cements in sandstone:endogenous and exogenous.The supply of endogenous carbonate cements in sandstone is less,and the source of carbonate cements is mainly provided by adjacent mudstone.The maximum amount of Ca CO₃ provided by the original formation water is only 0.00066%of the pore volume.The amount of Ca CO₃provided by the pressure solution of limestone gravel does not exceed 0.6%of the pore volume.The amount of Ca CO₃ provided by limestone gravel dissolved is no more than 0.15%of the pore volume.There are two main transport ways of exogenous materials:（1）diagenetic materials migrate from adjacent mudstone to sandstone by diffusion.（2）Diagenetic materials migrate from adjacent mudstone to sandstone in a episodic manner through overpressure fractures.There are two evidences for the transfer of diagenetic materials by external diffusion:（1）the content of carbonate cements in sandstone decreases with the increase of the distance from the interface of sandstone and mudstone.（2）The homogenization temperature of brine inclusions in the carbonate cements in sandstone increases with the increase of the distance from the interface of sandstone and mudstone.And there are two evidences for the episodic transmission of diagenetic materials through overpressure fractures:（1）The overpressure fractures communicate sandstone and mudstone.（2）There is little difference in the range of homogenization temperature of brine inclusions in the same type of carbonate cements in sandstone and mudstone fractures.