Structural Diagenesis Evolution Of The Paleogene Tight-sandstone Reservoirs In DN2 Gas Field,Kuqa Depression,Tarim Basin,China

Structural diagenesis mainly focuses on the fluid flow,mass transfer and chemical changes occurring during the deformation of sediments and sedimentary rocks.It has been demonstrated that structural diagenesis play an important role in understanding the formation and evolution of petroleum reservoirs.Polarizing microscope,cathodoluminescence microscope,image analysis,scanning electron microscope,XRD analysis,fluid-inclusion microthermometry have been employed to investigate the structural diagenesis and formation of Paleogene tightsandstone reservoirs in DN2 gas field.The reservoirs are pore-fracture reservoirs with reservoir spaces dominated by secondary pores,followed by primary pores and fractures.They underwent moderate compaction indicated by compactional indexes with an averages of 0.58.Authigenic minerals were recognized including carbonates,sulfates,silicon dioxide,feldspar,halite and clay minerals.The early-phase cements were related to the processes influenced by meteoric waters and lake waters;while the late-phase cements were precipitated from hot brines,which were probably derived from the Middle Cambrian evaporates and driven by tectonic compression.Fluid inclusion and illite K-Ar dating revealed that the brine upwelling occurred at about 11 Ma,recording the onset of extensive compression in the Dongqiu area.Three-stage fractures have been identified based on well cores and fluid-inclusion microthermometry and they were interpreted to form during the Late Miocene,the Pliocene,and the Pleistocene to present,respectively.Quantitative analysis on diagenesis indicated that compaction resulted in19.1% porosity losses,in which about 5.26% was related to tectonic compression.Cementation resulted in 13.86% porosity losses where cements precipitated from hot brines accounted for7.34%.Dissolution,diagenetic fracturing and tectonic fracturing contribute 2.98%,0.25% and0.5% porosity enhancement,respectively.During the Paleogene,the reservoirs experienced dissolution and cementation related to meteoric waters in graben basin setting.Since the Neogene,the basin evolved into a foreland basin because of collision between Indian plate and Eurasian plate.The reservoirs experienced dissolution under the influence of organic acid and compaction related to vertical stress and tectonic compression during A period of early diagenesis,followed by cementation related to brines driven by tectonic compression and subdued compaction during B period of early diagenesis,and then small-scale dissolution and cementation,tectonic fracturing and gas charging during mesogenetic diagenesis.The densification of the reservoirs occurred at late Miocene,preceding gas charging.Deposition is the internal cause of reservoir densification,vertical and tectonic compaction are major factors resulting in the porosity losses with cementation exacerbating the processes.