The Shanxi Formation of Ordos Basin is regarded as an exploratory target for it is rich in both coal and natural gas.The second member of the Formation (Shan-2) pertains to delta sedimentary system of a lake basin.The distributary channel sandbodies in the Shan-2

3

sub-member are the major pay zones of the Zizhou gas field.The Shanxi Formation in the gas field and its adjacent areas can be divided into 2 long-term basal level cycles;4 medium-term basal level cycles and 8 short-term basal level cycles.The structures of 5 short-term basal level cycles have been identified.During the division of medium-short term cycles

the writers not only employed identification methods of sequence boundary

lake flooding surface

and deposition cycles

but also took into conside-ration the positions of coal beds from delta sedimentary system in high-resolution sequence stratigraphy.Shoreline trajectory analysis was combined with research of coal sedimentary mode to exactly reconstruct the evolution of the coal deposition environment in the Shan-2

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sub-member in the areas.The Shan-2

3

sub-member is the product of delta deposition environment with intermittent lake transgression-regression as the feature.There had been 4 short periods of lake transgression-regression and created environments for the deposition of 4 coal layers in the lower deltaic plain.Among the 4 short-term transgression-regression cycles

the last one is the most extensive and during the time the lower deltaic plain turned to swamp peat

forming the thick 5

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coal-bed that is distributed almost in the whole study area.