Characterisics And Origin Of Fractures In Tight Sandstone Reservoirs With Low Permeability, Dabei Gas Field

In this paper, characterisics and origin of fractures in tight sandstone reservoirs with low permeability, lower Cretaceous Bashijiqike Formation(K₁bs), Dabei Gas Field in Kuqa Depression of Tarim Basin, are systematically studied by comprehensive use of outcrops, cores, thin-sections, logging and dynamic data.Tectonic fractures, including shear fracture and extension fracture, are the main types of fractures in the Cretaceous Bashijiqike Formation, Dabei Gas Field. Contractional fractures, crushed fractures and fractures formed by differential compaction are rare. Vertical and high dip angle fractures are dominated in the tectonic fractures, extend in the direction of NW-SE and are heavily filled. The fractures in the cores of Well DB102 and Well DB202 are typical of two sections or three sections filled structure. The untectonic fractures , on small scale, extend in little distance, are regularly filled with sedcondary minarals precipitation.The unsealed and unclosed fractures (or effective fractures) approximately parallel to the maximum principal compressive stress. Frctures are always present in the rocks of Dabei Gas Field, but not in the manner of uniformity. Sevearl geological parameters are important in controlling tectonic fractures spacing in the subsurface rock of study area: lithology, bed thickness, microfacies and structural position , and the dominant factor is structural position (or stress, structural movement). The developing degrees of untectonic fractures are mainly determined by lithology and rocks assemblage.After the cracks, the characteristics of fractures would be altered a lot by sealing, dissolution and transformation of stress filed. Based on the core fractures description, analysis of fluid inclusion homogenization temperature of the fillings in fractures, the acoustic emission tests of rocks, and combined with regional structural evolution history, the development periods of fractures are researched. The result indicates about 3 fracture generations (e.g. K+E, Late N₁, Late N₂), that are caused by Yanshan Movement and Himalaya Movement. The second and the third generation are the main fractures periods. Most of the third generation of fractures are effective, because of the miss of three main sealing stages (or mineral precipitation). Meanwhile, almost all the first generation and large parts of the second generation of fractures are sealliing, because of undergoing the main mineral precipitation stages.Contractional fractures are formed at Middle N₁ (6Ma±), Early N₂ (5Ma±), Late N₂ (3Ma±) and Q (2Ma-R), crushed fractures and fractures formed by differential compaction are formed at the period of the Cretaceous, E and Early N₁. Most of the untectonic fractures are possiblely filled now by secondary minerals precipitation.The effective open fractures, Dabei area, now extend in the main dirction of NW-SE, the extend dirction of fractures varies as the altitude of fault change. The intensity of effective fracture has a marked direct proportion to the dip degree of subsurface rock: (1) At the top of the anticline, dip of stratum is gentle, the density is small; (2) At the wing of the anticline, dip of stratum is steep, the density is large.