Medium-term datum plane cycle and coal-accumulating processes of the Eocene Hulin Formation in eastern Heilongjiang

Abstract：Using the data of drill hole 86-46 in Dashiqiao, Hulin City, the high-resolution sequence stratigraphic division of the Hulin Formation in the Dunmi downfaulted basin, eastern Heilongjiang, was made, and three medium-term datum plane cycles were recognized. The three cycles are all made up of broadly symmetric structure consisting of two half-cycles of rising and falling, and they may be subdivided into tow basic types, namely, the rising half-cycle thicker than the falling half-cycle and the rising half-cycle less thick than the falling half-cycle. They represent three marked lake transgression-regression in the Dunmi downfaulted basin. On that basis, the authors discuss the relation between the medium-term datum plane cycles of the high-resolution sequences and coal-accumulation processes. Of the cycles, the falling half-cycles of the medium-term datum plane cycles shows the best coal-accumulating processes and are favorable horizons of important coal beds of commercial value, while the rising half-cycle of the cycle shows poor-poorer coal-accumulating processes. This study shows the falling half-cycles of the first and second medium-term datum plane cycles in the Hulin Formation are best and are our first selected horizons for coal mining and coal prospect expansion.