Sequence stratigraphy and coal accumulation of the Lower Cretaceous in Yimin Sag

Yimin Sag of Hailaer Basin is a continental down faulted coal accumulating basin. Yimin Formation and Damoguaihe Formation are the main coal bearing strata. The lithofacies type,sedimentary facies,sequenc-es-paleogeog- raphy and coal accumulation function features were studied based on the rock coring and drilling information. Accord- ing to two typical interface in the research area of the regional unconformity surface and river incised valley washed surface,the Yimin Formation and Damoguaihe Formation were divided into four third level sequences. A single factor analysis and multifactor comprehensive mapping method was applied to recover the paleogeographic pattern within every third level sequence as an analyzing unit. The study results showed that the paleogeographic unit of the coal meas- ures in Damoguaihe Formation mainly had braided river delta. From Sequence I to II,the paleogeographic unit in the area varied mainly with the braided river delta-lake to mainly with the lake,which informed the evolution process of general shoaling. The paleogeographic unit of the coal measures in Yimin Formation mainly had fan delta and braided river. The coal measures were mainly discovered in Sequence III,and only few coal measures were found in Sequence IV,which informed that the coal accumulation function would be steadily weaken in Yimin period. The coal accumula- tion function was under control by the down faults,the thickness of the seam would steadily reduce from the center of the sag to the South-eastern and the North western of the sag. Based on the statistical comparison and analysis of the seams in sequence III,the response mechanism of peat accumulation and sequence strata in continental basins was dis- cussed,and the response model of continental peat-sequence strata was improved. The most favorable coal forming en- vironment in the Early Cretaceous of Yimin sag was located in the fan delta-braided river environment. The coal seam in sequence III and IV was shallow in depth and thicker in thickness,which was suitable for open cut mining. The stra- tigraphic II coal strata were deeply buried and covered by thick mudstone,which was a potential area for coalbed methane exploration.