The Structural-fluid Environment Of Deep Sandstones In The Kuqa Foreland Thrust Belt

Exploration of deep to ultra-deep sandstones have great potential and wide fields.However,it is characterized by highly compact,multi-type,unconventionality,and complex control factors.Natural fracture systems have a significant impact on reservoir performance,and it may form paths for fluid flow in tight sandstone reservoirs,and therefore plays an essential role in controlling high production rates.However,fractures completely filled with cement will just end up as barriers of fluid flow,and increase reservoir heterogeneity.Therefore,it is necessary to further explore the interaction mechanism between tectonic strain and fluid activity and their reservoir effect,and strengthen the study of structural diagenesis.The natural fracture systems in the Kuqa foreland thrust belt（KFTB）are subject to multiple superposition of differential extrusion,high temperature and pressure,and tectonic evolution and fluid-rock interaction under its constraint.The hierarchy and main control factors of structural-fluid interaction remain unclear.Besides,there is a lack of specific analysis on the geological phenomena related to structural-fluid interaction,and the physical and/or chemical mechanism in the process of preservation and transformation of open fractures is not illustrated thoroughly.Because of such problems,the diesertation takes the Cretaceous sandstone reservoir in the KFTB as an example,and carries out the structural-fluid environment analysis of deep sandstones.The study sampled sandstone from different structural positions,different formations,and different fracture systems.Crack-seal process of fracture was reconstructed by analyzing the homogenization temperature sequence of fluid inclusions in quartz bridge.Then researches on the in-situ oxygen isotope composition were carried out to reveal the source and effect of fracture diagenetic fluid.The preservation and reactivation mechanism of open fractures at great depth was clarified by quantitative and qualitative analysis,and the open fracture development model was established finally.The results show that:1)Quartz bridges were developed in specific fractures of Cretaceous sandstone in the KFTB.Quantitative analysis shows that the Cretaceous quartz bridge in the Kelasu structural belt can span 1 mm wide fracture.2)The hinge-perpendicular fracture system opened during the rapid burial period.The hinge-parallel fracture system opened during late rapid burial followed by exhumation period.3)During the opening of the hinge-parallel fractures,the ¹⁸O-enriched water in Paleogene evaporates driven by gravity and lateral compression inflow to the Cretaceous sandstones.¹⁸O exchange occurs during the precipitation of quartz bridge,which led to the difference of oxygen isotope between quartz bridges and quartz grains.4)Quartz and cementless can preserve porosity and permeability during compression.Shear reactivation can break the cement seal and reopen mineralized fracture.Open fractures at depth can be calculated by four parameters,including difference between maximum and minimum principal stresses,fracture dip,the angle between the fracture and the maximum horizontal principal stress,and critical dilation tendency（0.7）and critical slip tendency（0.6）.Finally,the dissertation establishes the development model of open fractures in deep sandstones in the KFTB,which could promote understanding of structural diagenesis and predict economic reservoir.