The Evolution Model Of Diagenetic Fluid In Dawsonite-Bearing Sandstone,Shangdian-Pingfangwang Area,Jiyang Depression

Shangdian-Pingfangwang area in Jiyang Depression is characterized by double injection of CO₂ and oil-gas,and has experienced a relatively complex diagenetic fluid evolution process.Dawsonite is the tracer mineral for CO₂ migration,accumulation and escape.The study on the evolution of diagenetic fluid characteristics in dawsonite-bearing sandstone is of great significance for revealing the interaction mechanism of CO₂-fluid-sandstone,and provides a natural analogy for CO₂ capture and storage.Combining sedimentary petrology,fluid inclusion geology,stable isotope geochemistry and hydrogeochemistry,the petrology and fluid inclusions characteristics of dawsonite-bearing sandstone in Shangdian-Pingfangwang area were studied by using microscopic observation of thin sections,cathodoluminescence analysis,fluorescent observations,inclusion microthermometry and carbon and oxygen isotope test,and the diagenetic sequence and period of fluid charging were determined.Combining the characteristics of formation water and ancient fluid in period of dawsonite formation,the diagenetic fluid evolution model in Shangdian-Pingfangwang area was built.The diagenetic sequence of dawsonite-bearing sandstone in Shangdian-Pingfangwang area was:quartz overgrowth,authigenic kaolinite,first-stage calcite,dawsonite,second-stage calcite,ferrocalcite,dolomite,and ankerite.Hydrocarbon inclusions in the Fourth Member of Shahejie Formation in Pingfangwang area were mainly developed in carbonate cements,and the homogenization temperature of concurrent brine inclusions was 90-110℃.The oil and gas charging occurred in the period of carbonate mineral precipitation after CO₂ injection,and the charging time was 2.6-0 Ma.The present formation water of dawsonite-bearing sandstone is characterized by high HCO₃^-,low Ca²⁺and low Mg²⁺.According to the research of carbon and oxygen isotopic characteristics of dawsonite,theδ¹³C of the CO₂ forming dawsonite was-7.18‰～-1.61‰,which was mantle-derived inorganic origin and had the same source as the CO₂ in the Pingfangwang gas reservoir.Theδ¹⁸O of the water medium during the formation of dawsonite was-3.76‰～+1.84‰,which had the characteristic of entrapped underground brine.The paleo-fluid temperature in the period of the formation of dawsonite ranged from 69.30℃to 96.82℃,with an average of 81.72℃.The evolution of diagenetic fluid in Shangdian-Pingfangwang area can be divided into five stages:the first stage was the alkaline fluid environment of original sedimentary water;the second stage was the weak alkaline fluid environment in the period of organic acids formation;the third stage was the acidic fluid environment in the early stage of CO₂ injection;the fourth stage was the alkaline fluid environment in the late stage of CO₂ injection;the fifth stage was the weak alkaline fluid environment in the stage of oil and gas charging.The evolution model of diagenetic fluid is in good agreement with the characteristics of high HCO₃^-,low Ca²⁺and low Mg²⁺in present formation water of dawsonite-bearing sandstone in Shangdian-Pingfangwang area.