Hydrocarbon expulsion efficiency of the shale system is one of the most important issues in the shale oil and gas research

and also a key parameter for evaluating the potential of shale oil and gas resources.This paper chooses the mud shale system from Chang 7 Member of the Yanchang Formation

Ordos Basin

as the investigation object.It measures the expulsion efficiency of the shale system and also reveals the variation of oil-bearing properties among various lithologies of mud shale system

via systematic geochemical analyses.Their significance in shale oil production is also discussed.The result shows that at the medium maturity stage (R

o

=0.8%)

the hydrocarbon expulsion efficiency of Type Ⅰ organic mud shale and Type Ⅱ mud shale is 33% to 37% and 16% to 26%

respectively

while at the mature stage (R

o

=1.1%)

the expulsion efficiency of Type Ⅰ and Type Ⅱ is 64% to 67% and 54% to 58%

respectively.Thus we conclude that in the shale system with higher maturity (R

o

=1.1%)

the sandstone and silty mudstone with higher free hydrocarbon content and OSI index can be the favorable targets for shale oil exploration and development; the blocky mudstone with high free hydrocarbon content and OSI index would be a potential target for shale oil exploration

with integrated consideration of geological factors including oil-bearing properties of different lithologies

composition characteristics

gas/oil ratio

and fluidity of retained oil. However

the black shale (or pure shale section) might not become an effective target for shale oil exploration in the study area

restrained by some of its geological factors.