Sedimentation Of Mixed Fine-Grained Sedimentary Rocks In The Second Member Of The Kongdian Formation In The Cangdong Depression,Bohai Bay Basin

Cangdong depression in Bohai Bay Basin is one of the key areas of continental shale oil exploration in China.Mixed fine-grained sedimentary rocks are widely developed in the shale formations in the second member of the Kongdian Formation（Ek₂）within the Cangdong Depression,which are superior source rocks with abundance organic matters.Minerals in mixed fine-grained sedimentary rocks are of complex compositions,and multiple sources.The content of particles with diameter less than 0.0625 mm in mixed fine-grained sedimentary rocks is more than 50%.The sedimentary structures are diverse,the sedimentary process is complex,and their genesis is not well understood.Investigation of the sedimentary environment,sedimentation and the controlling factors of the mixed fine-grained sedimentary rocks in Ek₂ in the Cangdong Sag is conducive to analyzing the origin of fine-grained sedimentary rocks,studying the difference of the spatial-temporal distribution of different lithofacies,and delineating favorable exploration targets.On the basis of detailed examination of cores and thin sections,XRF（X-ray Fluorescence）,SEM（Scanning Electron Microscopy）,XRD（X-ray Diffractionelement）data,geochemical measurement data,the mixed fine-grained sedimentary rocks in Ek₂ have been subdivided based on their mineral compositions and sedimentary structures.The evolution of sedimentary environments,sedimentation and their controlling factors was revealed by lithofacies types and changes of geochemical parameters.The result shows that the mineral compositions in Ek₂ are variable with a high content of analcite,felsic minerals,carbonate minerals and minor amount of clay minerals.The sedimentary structures can be divided into laminated structures and massive structures.On the basis of mineral contents and sedimentary structures,the mixed fine-grained sedimentary rocks in Ek₂ are divided into three categories:dolomite fine-grained sedimentary rocks,felsic fine-grained sedimentary rocks and homogeneous（mixed）fine-grained sedimentary rocks,which are further sub-divided into 21 sub-categories.The characteristics of the widely distributed nine lithofacies and their vertical stacking and distribution characteristics are described.The mixed fine-grained sedimentary rocks in Ek₂ were mainly developed under an arid climate in a semi-deep to deep lake,with brackish and anoxic water with strong terrestrial input.The change of sedimentary environment was not drastic,and fluctuated within a certain range,forming fine-grained sedimentary rocks characterized by mixing.The evolution of sedimentary environment can be divided into 15 stages.When the climate was relatively dry,the terrigenous input decreased,the salinity increased,and carbonate minerals developed,forming dolomite fine-grained sedimentary rocks.When the climate was relatively wet,terrigenous input increased,and felsic fine-grained sedimentary rocks developed.In the broad transition interval between dry and wet climatc settings,homogeneous（mixed）fine-grained sedimentary rocks developed.The sedimentation of mixed fine-grained sedimentary rocks in Ek₂ within the Cangdong Depression mainly includes suspension sedimentation,gravity flow,seismicity-induced,evaporation and biochemical processes.Under the control of climate change,water conditions and terrigenous input,suspension sedimentation,evaporation and biochemistry developed in different degrees,which determined the mineral composition of each lithofacies.Laminated structures were mainly formed in quiet water,and sudden gravity flow sedimentation and seismicity lead to the deposition of massive lithofacies and deformation of sedimentary structures.Finally,a sedimentary model of mixed fine-grained sedimentary rocks controlled by climate change,water conditions,terrigenous input and sudden events was established.