Chang 8 sandstone of the Upper Triassic Yanchang Formation in Huaqing area

Ordos Basin

is a typical set of reservoirs with low-ultra low porosity and permeability.Its average porosity is 8.27% and average permeability is 0.51×10

-3

μm

2

.The main type of pore space is residual inter-granular pore

and the main type of pore structure is tiny and micro throat.The Chang 8 reservoir has experienced complex diagenesis

such as compaction

cementation

and dissolution.The destructive diagenesis of compaction and cementation sharply reduce the primary pores of the reservoir

besides

some pores are filled with bitumen

resulting in the lowering of porosity and permeability and the tightness of the reservoir.Diagenis is a determining factor in reservoir tightening

while depositional environment is the basic controlling factor of the reservoir physical property.The underwater distributary channel sands in delta front have the best physical property

and the central part of the sandbody is better than its two sides.With the increasing of interstilital matter

the physical properties get poorer.