Analysis Of Salt Tectonic Characteristics And Genesis Mechanism Of Early Permian Hollow Valley Satge In The Pre-Caspian Basin

The Pre-Caspian basin is a large superimposed saline basin formed at the beginning of the early Paleozoic and is also one of the large-scale sedimentary basins with the richest oil and gas resources in the world.The lower Permian hollow valley saline is mainly composed of salt rock and anhydrite interlayer,which are widely distributed throughout the basin,and the salt rock layer is extremely thick with various structural types and distributions.The factors affecting the structural deformation are diverse and complex due to the particularity of the properties and deformation of salt rock.This study aims to investigate the formation mechanism and distribution differences of giant thick salt structures in the Pre-Caspian basin.By using two-dimensional seismic profile data and regional geological data,this study summarizes the evolution process and salt structure characteristics of the basin from the perspective of the whole basin.The equilibrium profile recovery is employed to discuss the evolution process and trigger mechanism of salt structure formation in the basin.The sedimentary mode of the giant thick salt rock and the dominant factors of the formation of salt structure in different periods are analyzed.The influence of salt structure on oil and gas reservoirs is analyzed based on the results of oil and gas exploration,and the trap types related to salt structure are summarized.The results of the study show that:(1)The salt structure in the basin is widely distributed,with a huge thickness and drastic changes.The degree of salt rock deformation decreases from the center of the basin to the edge of the basin.The salt structure deformation in the central basin is the strongest,piercing the upper cladding,and the maturity of the salt structure is high.The deformation of the salt structure of the basin margin is weak,and the maturity of the salt structure is low.The whole presents a zonal feature in which the degree of salt tectonic deformation gradually increases from the basin margin to the center of the basin.(2)The salt-forming model of the Pre-Caspian basin is a deep basin drying model similar to the Sabha model,with stable settlement and a relatively closed structural environment.Phased recharge of seawater and arid climate are necessary conditions,leading to the deposition of an initial salt rock layer up to 6km thick.The main deformation period of salt structure in the Pre-Caspian Basin is the Triassic-Cretaceous,and the initial deformation factor of the salt rock layer is the extrusion of the pre-existing slope and the basin margin.The differential loading of the overlying stratum is the leading factor of the structural deformation of the salt rock layer,and stretching and buoyancy also have a certain effect on the formation of salt structure.(3)The deformation of the salt rock layer has minimal impact on the underlying pre-salt strata,and their continuity remains relatively intact.However,the upper salt strata are severely deformed due to strong interspersed cutting,resulting in poor continuity.These deformed upper salt strata are mostly filled in the micro-basin between the salt structures,and there are faults present on the top of the salt structures.(4)The continuous and huge thick tight salt rock layer in the Caspian basin acts as a natural cover layer,preventing the upward movement of oil and gas.The pre-salt formation should be the key target of future oil and gas exploration.After the local rock salt deformation disappears,the upper and lower strata are connected,which is conducive to the upward migration of oil and gas into reservoirs,forming a target area for oil and gas exploration on salt.Most of the reservoir types related to salt structure in the basin are salt piercing and shading type,unintegrated surface shading type,and formation tip extinction type trap.This study analyzes the morphological characteristics,distribution characteristics,and formation mechanism of salt structure in the basin and their influence on oil and gas migration and aggregation.It also provides a more specific guidance for oil and gas exploration in the Coastal Caspian Basin.