Low Permeability Reservoir. Santanghu Basin, Permian,

Santanghu Basin, lying in the Kazakhtan-Junggar plate, is a supermposed basin that deposited terrestrial sediment of Mesozoic- Cenozoic. The basement of the basin is consist of Precambrian crystal basement and Paleozoic folding basement, while the cover has undergone the evolution of intra- continental rife in Permian, down warped basin in Triassic- Cretaceous period and inter-mountains basin in Tertiary .The main sediments are huge-thick volcanic rock, shallow intrusive rock and tuff in early Permian, dark mudstone ,marl and muddy dolostone in Lucaogou Formation in Middle Permian, huge thick volcanic rock, intruder in Tiaohu Formation in Middle Permian, miscellaneous sandy gravel, sandstone and gray mudstone in Triassic (the red is under the gray, and huge-thick dark gray, gray sandy gravel, sandstone, mudstone with coal layer in Jurassic.The oil and gas was mainly accumulated in Tiaohu and Malang depressions in Santanghu Basin. Oil-bearing reservoir is mainly in Mid-Permian and Mid-Later Jurassic. Mid-Permian could be divided into Lucaogou Formation and Tiaohu Formation, this two formations are mainly distributed in Malang depression, while it was not found in most wells in Tiaohu depression. It was developed mostly deep lake facies on the south of basin and coastal-shallow lake facies on the north of basin.The reservoirs in Permian is consist of carbonate rock, tuff, clastic rock and volcanic rock. The reservoir types in Lucaogou Formation is carbonate rock, clastic rock and tuff, while the reservoir types in Tiaohu Formation is mainly tuff and volcanic rock. All of these reservoirs have bad physical property with lower porosity and lower permeability. Mainly reservoir paces is cracks (structure crack, diagenetic crack), the following is primary intergranular pores, primary intragranular pores, intergranular solution pores, intergranular solution pores, intercrystaline pores and so on. This layer was in later diagenetic A stage.There are three kinds of crack include of structure crack, diagenetic crack and synaeresis crack in this area. The main crack of Tiaohu Formation are straight oblique-cracks and level crack, there are many scores in the crack .The length of cracks is long, and they cut across several layers. The average surface density of the cracks is 0.101 slips /m. The cracks in Lucaogou Formation is mostly net cracks ,and there are solution on crack surface. These cracks occurred mainly in carbonate rocks on which the average surface density of the crack is 0.143 slips /m. The development of the crack improve the permeability of the non-sedimentary reservoirs.After synthetic evaluation to the Permian reservoir, it was considered that there were deep lake facie in Mazhong-Heidun-Shibandun structure zone, it was suitable to depositing thick carbonate rock. These cracks were developed in the zones where the stress was concentrated. These zones are fairly good oil and gas exploration areas.. The following is Niujuanhu structure zone where the solution pores are well developed.