Research On Sedimentary Evolution Characteristics Of The Jurassic Shaximiao Formation In The Sichuan Basin

The Jurassic Shaximiao Formation is the key exploration target of natural gas in the Sichuan Basin,which has made a breakthrough in the central and western Sichuan Basin in recent years.But the weakness of study on sedimentary evolution characteristics of the Sichuan Basin and origin of sandbodies in the central Sichuan Basin,which hindered the natural gas exploration in the Sichuan Basin.This study investigates the provenance characteristics,sedimentary facies and its evolution characteristics within the Shaximiao Formation of the central Sichuan Basin and the Sichuan Basin,based on previous research results,field reconnalssance,core observation,microscopic analysis,logging data and seismic data interpretation.The main achievements are as follows:（1）The Shaximiao Formation in the Sichuan Basin was the development stage of the Dabashan Foreland Basin.The subsidence center was located in the northeast of the basin,whereas the depocenter was located in the south of the basin,which caused the direction of water catchment was from the northeast to southeast.The sediment sources of the basin mainly came from the Daba Mountains in the northeastern basin,the Micang Mountains in the northern basin,the Longmen Mountains in the western basin,and the Xuefeng Mountains in the eastern basin.The Daba Mountains and Micang Mountains supplied the detrital material for the central area of the Sichuan Basin.（2）The main depositional environment in the Sichuan Basin during the Jurassic Shaximiao Formation included fluvial,delta,lacustrine facies and so on,which based on the analysis of outcrop,stratigraphic characteristics and seismic data.The climate changed from wet to dry resulted in lake shrinkage of the Sichuan Basin during the J₂s¹ to J₂s².The deposition setting was changed from underwater to subaerial environment in the central Sichuan Basin.（3）From the analysis of sedimentary characteristics in the central Sichuan Basin,it is found that within the Shaximiao Formation,form bottom to top,the thickness of sandbodies was gradually thinning,the wave-derived sedimentary structures were gradually decreasing,and the color of mudstone and sandstone was gradually turning red.All these indicated that the climate gradually turned arid from early to late,and the supply of the detrital material was weakening.Combining the sedimentary distribution characteristics of the Shaximiao Formation in the Sichuan Basin and the sandbodies evolution characteristics in the central Sichuan Basin,it was believed that the sedimentary evolution of the Shaximiao Formation in the Sichuan Basin had gone through four stages.In the J₂s¹ and J₂s^2-1,the river-normal delta-lacustrine sedimentary system mainly developed.In the J₂s^2-2,it gradually transformed into a river-shallow water delta-lacustrine sedimentary system.In the J₂s^2-3,it turned to the fluvial sedimentary system.Finally,it evolved into the distributive fluvial system in the J₂s^2-4.