Fan deltas in the 3rd member of Hetaoyuan Formation in Shuanghe oilfield have the characteristics of shelf-type fan deltas

where the strtigraphic sequences

as a whole

are developed in anticyclic sequences

but the single layer appear to be in positive rhythm. This kind of sequences are neither typical fluvial deposits

nor typical river mouth bar deposits. They are deposited as a result of axial efflux from river channel to lake basin under the condition of shelf-type fan delta with midly sloping lake floor. The energy and speed of axial efflux gradually decrease toward the axial direction and both sides

and the grain sizes of sediments decrease along with lowering of water flow energy

the deposits get thinner and evolve from subaquous distributary channel deposits into distributary mouth bar deposits. The deposits in the leading edge of mouth bar spread out and become sheet sand. In fact

the river mouth bar is a complex composed mainly of river mouth bar

and 6 microfacies

including core

lateral margin

margin

front edge

sheet sand and interbar. The bars get higher vertically and migrate laterally when the main stream line or location of channel migrate

resulting in plenty of sequence patterns.