Characteristics of Sequence Stratigraphy and Coal Accumulation Controlling Factors of Lower-Middle Jurassic Coal-bearing Series,South Margin of Junggar Basin,NW China

Combined with few well drilling and well logging data,and results of previous studies,sequence stratigraphy characteristics and coal accumulation controlling factors of Middle-Lower Jurassic coal-bearing series in south margins of Junggar Basin have been analyzed on the basis of sequence stratigraphic study in outcrops. Middle-Lower Jurassic coal-bearing series in south margins of Junggar Basin can be divided into one complete 2ndorder tectonic sequence,where four 3rdorder sequences and twelve system tracts were developed inside. The results show that paleotectonic setting( weakly extensive and gentle subsidence) and paleoclimate( warm,humid,and reducing condition) were the fundamental conditions for coal-forming in Middle-Lower Jurassic. While,relative lake level changes of the 2ndorder tectonic sequence and rate of relative lake level change in the 3rdorder sequence were the major controlling factors for coal accumulation. Commercial recoverable coals,which were characterized by large thickness and stably lateral distribution in outcrops,were all developed in highstand system tract( HST). During the depositional period of SQ1,relative lake level of the 2ndorder tectonic sequence was descending,leading a lower paleo-water table,where the coals were mainly developed in the lower part of HST( ascending rate of the relative lake level in the 3rdorder sequence was moderate). The depositional period of SQ3 was the maximum flooding period of Early-Middle Jurassic, when the relative lake level of the 2ndorder tectonic sequence ascended to the highest point. coal beds were mainly developed in the middle-upper part of HST( descending rate of the relevant lake level in the 3rdorder sequence was moderate).