Sequence-paleogeography and coal accumulation features ofJiuqiao Sag Yimin Formation in Hailaer Basin

Due to Jiuqiao Sag of Hailaer Basin was a continental down faulted coal accumulating basin and the Lower Cretaceous Yimin Formation was the main coal bearing strata, in order to solve the problem to seek a superiority coal accumulation area, the rock coring and drilling information, logging information and others were applied to study the lithofacies type, sedimentary facies, sequences-paleogeography and coal accumulation function features. The paper provided the regional unconformity surface and river incised valley washed surface as the represented two typical sequence interface and thus the Yimin Formation was divided into three third level sequences. A single factor analysis and multifactor comprehensive mapping method was applied to recover the palaeogeographic pattern with the three level sequence as a mapping unit. The study results showed that the palaeogeographic unit of the coal measures in Yimin Formation mainly would have rivers, braided river delta and lake as well as others. From Sequence I to Sequence III, the palaeogeographic unit in the area mainly with the delta - lake to mainly with the river - delta showed the evolution process of the general shoaling and the material sources were mainly came from the northwest and southeast of Hailaer Basin. From the Sequence I to the Sequence III, the thickness of the seam would steadily reduced and the coal accumulation function would be steadily weaken.