Coal petrography variation in the sequence stratigraphic frame of the Jurassic coal measures of Dameigou mine area in Northern Qaidam Basin

In order to reveal the relationship between the base level in the bog of inland basin and the coal petrography variation,principle of the sequence stratigraphy has been used to study the coal petrography variation in the sequence stratigraphic frame of the Jurassic coal measures of Dameigou mine area in Northern Qaidam Basin.A total of 9 sequence boundaries have been identified(including regional unconformity surface,incised valley,regional tectonic stress transforming interface and color of strata discontinuous surface),which divided Jurrasic coal measures into 3 second-order sequences and 8 third-order sequences.The major coals(coal F) in the research area were formed in the environment of highstand swamp in the abandoned braided river plain,the coal macerals are characterized by high inertinite(average 59.1 %) and low vitrinite(average 23.9 %).According to the variation of coal maceral in the vertical profile,coal F has been divided into 1 fourth-order sequence and 5 fifth-order sequences.In the vertical profile,vitrinite content of coal F has a trend of increase as a whole,which reflects the dynamic background of gradual rise of base level rise of the lacustrine transgressive system tract of the third-order sequence SIII6.