The diversities and disparities of fracture systems in low-and ultra-low-permeability sandstone reservoirs in the Paleogene in DN gas field of Kuqa Depression

Tarim Basin

were scrutinized through a combination of analytic means such as core and thin section observation

SEM

as well as analyses of clay mineral XRD

fluid inclusion

and physical properties of oil-bearing formations

with regional geological understanding.The results show that the fracture systems are dominated by tectonic fractures

followed successively by dissolution fractures

diagenetic fractures

and overpressured fractures.The tectonic fractures have 3 types (shear fractures

extension fractures

and seismic-related fractures) and 3 development stages.It was the late Himalayan stage (i.e.the end of Pliocene

2.6-1.81 Ma) when certain tectonic compression process generated the most favorable fractures for hydrocarbon accumulations.The formation of the other three kinds of fractures

i.e.the dissolution fractures

diagenetic fractures

and overpressured fractures

is suggested to be happening during the period from the late Kuqua to Xiyu (2Ma-present) and associated with dissolution by hydrocarbon charging

shrinkage by dehydration of clay minerals

and a formation overpressure by intensive tectonic compression

rapid burial processes and pressurized fluids.The disparities of the fracture systems are mainly controlled by rock types

sedimentary facies belts

the single layer thickness and the distance to major controlling faults.The underwater distributary channels in the central part of the gas field are dominated by silty and fine sandstone where the fractures are highly developed and well productivity is high.The linear density of fractures are found to be negatively related to the thickness of single layers (a s-shaped correlation)

and decreases as the distance to the major controlling faults increases.The linear density of fractures in mudstone is less than 0.4 lines per meter and is observed to have no connection with single layer thickness.