Different Evolution Mechanism And Main Control Factors Of Ultra-Deep Sandstone Reservoirs In Kuqa Depression

The Kelasu Structural Belt in the Kuqa Depression of the Tarim Basin is an important gas source for China’s"West-to-East Natural Gas Transportation Project".The buried depth of the study interval of the Cretaceous Bashiqike Formation is generally more than 6000m.The structural deformation there is strong,structural fractures are typically developed,and the pore throat structure of the reservoir is complex.The reservoir heterogeneity in different sections results in large differences in gas productivity affects the scale efficiency output,and restricts the stable supply of natural gas.Taking the Bashijiqike Formation as the research object,this paper systematically and quantitatively analyzed the tectonic diagenesis effect of Bozi,Dabei,and Keshen sections of the Kelasu structural belt by comprehensively using field digital outcrop modeling,drilling,core,logging,field emission scanning electron microscope,cathodoluminescence,laser confocal focusing,electron probe,CT scanning,high-pressure mercury injection,and other experimental analysis methods,The genetic mechanism and control factors of differential evolution in ultra-deep scale high-quality sandstone reservoirs have been clarified,and a genetic model for ultra-deep high-quality reservoirs has been established,providing a useful reference for differential productivity planning.The ultra-deep sandstone reservoirs of the Bashiqike Formation in the Kelasu structural belt have developed four stages of diagenetic transformation:early alkaline cementation,early epigenetic to middle burial dissolution,middle and late burial compaction tectonic compression,late tectonic transformation,and dissolution.The types of cementation mainly include siliceous(quartz)cementation,feldspar(potassium feldspar,albite)cementation,and carbonate(calcite,dolomite)cementation.The dissolution process is mainly acidic,with less alkaline dissolution,and mineral particles and cement can be dissolved.The compaction process includes burial compaction of the overlying strata and lateral compaction caused by tectonic movement compression.The role of structural transformation mainly refers to the change in reservoir quality caused by natural fractures formed by multiple tectonic movements,which is a key influencing factor for the heterogeneous distribution of ultra-deep sandstone reservoirs.The ultra-deep structural fractures are characterized by"grouping and clustering".Large fractures are composed of multiple micro-fractures,which can cut through and bypass mineral particles of the reservoir and differentiate into smaller fractures.The distribution of structural fractures is related to their location and is controlled by multiple factors.Under multiple tectonic movements,the development of structural fractures near the front wing of the anticline and secondary faults is high,but the opening is small,and the extension is short.The density of fractures in the axial structure of the anticline is relatively low,but they have a large opening and long extension and are primarily arranged in parallel.In the strong deformation zone of the Kelasu tectonic belt,stress and strain control the distribution and arrangement of structural cracks.In weak deformation areas,lithology,layer thickness,and faults control the scale and density of structural fractures.The structural fractures of the Bashiqike Formation were mainly formed during the Himalayan movement,which can be divided into early slow compression,middle medium speed compression,and late strong rapid compression.While Cretaceous sediments started,the ancient landforms of the Kelasu structural belt presented a paleo-geographic pattern of high-north,low-south.These alternating east-west uplifts or depressions control the distribution of sedimentary facies in different sections and affect the type and intensity of reservoir structure and diagenetic evolution.The Bozi section is sheltered and protected by the ancient uplift,and the thrust and pore reduction caused by tectonic compression is relatively small.Moreover,due to relatively weak compaction and cementation,the primary pores of the reservoir are well preserved.The strata in the Dabei section are mainly subjected to compression and torsion deformation caused by the strike-slip of the ancient uplift.The reservoir structure has the smallest amount of compression and pore reduction,and the damage to pores by diagenetic compaction and cementation is moderate.However,structural fractures and relatively strong dissolution have a certain improvement effect on the reservoir space.The Keshen section is a forward compression without the obstruction of ancient uplifts,with the largest reduction in pore volume caused by structural compression and being subjected to strong compaction and cementation.The development of primary pores in the reservoir is the worst.At the same time,due to its location in the feldspar-rich mother rock area,the widely developed structural fractures and acidic dissolution caused by hydrocarbon fluid injection make secondary pores in the reservoir the most developed.When sedimentation of the Bashiqike Formation performing,with the continuous fluctuation of the lake level and the migration of the lake shoreline,the Kuqa Depression formed a huge delta front sand body with an area of approximately 16000km~2 due to the wide and gentle slope of the lake sedimentary basin background.Although the burial depth exceeds 8000m,the long-term shallow burial and short-term rapid deep burial history result in better preservation of reservoir pores.Besides,the thick gypsum salt layer overlying the target layer forms a detachment effect,effectively avoiding the pore reduction effect caused by strong structural compression.In addition,natural fractures generated by multi-stage tectonic compression have developed a widely distributed network of fractures in the reservoir,which not only can connect the dispersed pores and significantly improve the permeability of ultra-deep sandstone reservoirs but also facilitate the injection of acidic fluids to generate dissolution pores and increase pore pressure through oil and gas injection,ultimately improving the effectiveness of reservoir space.Therefore,multiple factors control the scale development of ultra-deep high-quality sandstone reservoirs.