Characteristics and dominant controlling factors of tight sandstone reservoirs in the 5th member of Xujiahe Formation in the central of western Sichuan Depression was researched through core observation

logging interpretation

phy-sical property analysis

thin section examination and scanning electron microscope

in order to provide the basis for deployment of exploration and development of new sequences.The tight sandstone reservoirs occur in the 5th member source rocks and consist of delta front underwater distributary channels

channel bars

sheet sand and shallow lake sand bars

with single layer thickness ranging from 2 to 25 m.They belong to super-low porosity and low permeability reservoirs

with an average porosity of 2.06% and permeability of 0.162×10

-3

μm

2

respectively.High contents of plastic particles

clay matrix and carbonate cements are the principal factors that led to the densification of the 5th member reservoirs.The medium-fine grain calcarenaceous sandstones and carbonate lithic sandstone deposited in relatively stronger hydrodynamic environments are favorable for the preservation of primary pores and exchange of fluids

thus are reservoirs with relatively high quality.Dissolution of feldspar resulted in the low content of feldspar and the formation of some dissolution pores.And the unique cracks caused by hydrocarbon generation pressurization in source rocks were helpful for the organic acid to enter into the residual inter-granular pores

enlarging pre-existing dissolution pores.Therefore

the delta front underwater distributary channels and channel bars sandbodies adjacent to the source rock with a certain range of thickness are the most favorable targets for tight sandstone reservoirs exploration.