Based on the relationship between sealing mechanism and rock pore texture

the covering strata can be diyided into micro-pore type and micro-fracture type. The former has monopeak saturation-pressure ratio curve and the peak saturation-pressure ratio is greater than 3. 0?10

-2

MPa

-1

: the later has multi-peak curves of saturation-pressure ratio

the peak saturation-pressure ratio is less than 3. 0?10

-2

MPa

-1

. The micro-fracture cover beds are the evolution result of micro-pore cover beds

and are mainly buried in mid-deep depth (below oilwindow) : the micro-pore cover beds are mainly distributed shallower (above oil-window). The micro-pore type could not only be the direct cover bed of hydrocarbon

but also could form high quality regional covering strata in association with high energy fluid surface: the micro-fracture type could only be the direct cover bed.