Sedimentary And Geochemistry Characteristics Of Coal-bearing Black Shales Of Permo-carboniferous In The Eastern Ordos Basin

The paper takes eastern Ordos basin as the study area and late paleozoic Carboniferous – Permian coal-bearing shale as study objects to recognize its sedimentary facies, microfacies and divide sequence by the means of sedimentology, geophysics, petrology, geochemistry and coal geology, etc. Moreover, core samples collected in the study area were analyzed the characteristics of petrology, elementary and organic geochemistry. The main contents and understanding are as follows:Using the knowledge about rock color, lithology, sedimentary structure, paleontology, logging, and combining with rare earth element geochemical data,the target layers in the study area were divided into three sedimentary system groups:continental facies, transitional facies and marine facies, then distinguished from five sedimentary facies including rivers, deltas, barrier coast, lakes and shallow shelf,and several subfacies and microfacies. Considering the theory and methods of sedimentology and sequence stratigraphy, especially based on identifying sequence boundary levels and flooding surface, the strata taiyuan group and shanxi formation were divided into two ultra-long-term base-level cycles and 5 long-term cycles, namely SLSC1, SLSC2 and LSC1 ~ LSC5 respectively.The results of X-ray diffraction(XRD) analyses show quartz and clay minerals play a dominant role, taking a proportion of 33% and 50% respectively. In terms of clay minerals, they are constituted by kaolinite(39%), mixed-layer illite(34%), illite(18%),and montmorillonite(0%),indicating the sedimentary environment are mainly warm, humid climate, and in the acidic media.In order to recovery the paleosalinity, paleoclimate and redox environment in the study area,X-ray fluorescence spectrometry(XRF) is required for black shale samples to obtain major and trace elements to observe changes of their element contents and ratios among them, which Sr / Ba ratio is in a wide range, varying from 0.23 to 1.41, suggesting the water environment of the study area consists of freshwater, brackish and saline water salinity, and this is primarily related to transitional deposit environment; while Sr / Cu values are between 1 and 10,reflecting a warm and humid climate.The results of analyzing of rock pyrolysis, hydrogen index and oxygen index demonstrate that the keregon types of samples belong to typeⅡand type Ⅲ;organic carbon content values range from 0.32% to 19.67%,which majority are more than 1.5%. Especially, TOC content is generally greater than 2.5% at the bottom of shanxi formation in the central area.Vitrinite reflectance( Ro) is between 0.7%~2.6%,mostly disturbed between 1.6% and 2.1%,indicating organic matter have reached maturity- over mature stage with high hydrocarbon potential.