Origins Of Fractured Veins And Characteristics Of Paleo-temperature And Pressure Of Fluid Inclusions In Marine Shales Of Fuling And Yichang Regions

The presence of overpressure in shale gas reservoirs is an important controlling factor for the enrichment and high yield of shale gas,evidence for favorable preservation conditions of shale gas reservoirs,the study on the present and paleo-pressure states and overpressure formation and evolution of shale gas reservoirs has been widely recognized by the industry,and the evidence of paleo-pressure state in shale gas reservoirs can be obtained through the study of paleopressure recovery of fluid inclusions trapped in shale fractured veins in different periods.This time,the Wufeng Formation of the Upper Ordovician and the Lower Silurian Longmaxi Formation in the Fuling region,the Lower Sinian Doushantuo Formation and the Lower Cambrian Niutitang Formation in the Yichang region were selected as the study object to research on the macroscopic and microscopic filling of the fractured veins,the characteristics of optics,cathodoluminescence and trace element geochemistry of the fractured veins,as well as,the forming time,forming stages and origin of the fractured veins.At the same time,the petrology,micro-thermometry,low-temperature phase transition and laser Raman spectra of the fluid inclusions captured in the fractured veins were studied.Further taking advantage of paleo-temperature and pressure characteristics of fluid inclusions developed in the fractured veins to analyze the paleo-pressure state of the shale layer where the fractured veins are located,at the same time,the current pressure state of the shale layers was compared to analyze gas saturation of shale layer,shale gas loss and preservation conditions in the geological history of Fuling and Yichang areas,and providing geological basis for further research on the relationship between shale gas preservation and enrichment and shale overpressure.The main achievements of the thesis are as follows:?1?According to the observation under the microscope and cathodoluminescence analysis,fractured composite veins?quartz veins+calcite veins?and quartz veins were developed in Wufeng Formation-Longmaxi Formation turbidite sand section in Well JY-A of Fuling region,the cathodoluminescence features of the fractured calcite veins are similar,and they are mainly dark red.Fractured composite veins?quartz veins+calcite veins?and fibrous calcite veins were developed in Wufeng Formation-Longmaxi Formation turbidite sand section in Well JY-B of Fuling region,and the cathodoluminescence characteristics of the calcite veins of the Longmaxi Formation are mainly orange,and Wufeng Formation are mainly tangerine.The fractured vein types of the Doushantuo Formation of Well EYY-A in the Yichang area is dominated by quartz veins,the cathodoluminescence characteristics of calcite,which develops between quartz veins,are complex,and they are mainly dark red,oxblood red and orange.The fractured vein types of the Niutingtang Formation of Well EYY-A is calcite veins,and the cathodoluminescence is characterized by two types:one is bright orange and the other is dark orange.?2?According to fractured calcite veins rare earth element analysis,the distribution patterns of rare earth elements of calcite veins,developed in Wufeng Formation-Longmaxi Formation turbidite sand section in Wells JY-A and JY-B of Fuling region,are all characterized by the relative enrichment of medium rare earth elements,with obvious Eu and Y positive anomalies,and the degree of Y positive anomalies of calcite veins in the Wufeng Formation are relatively higher than that of the Longmaxi Formation.There are two types of rare earth elements allocation model of calcite,developed in the quartz veins of the Doushantuo Formation in Well EYY-A of the Yichang area.The type I model is characterized by light-medium rare earth element enrichment,there are obvious Eu and Y positive anomalies,and the negative anomaly of Ce is not obvious in this model.The type II model shows the characteristics of heavy rare earth element enrichment,there are obvious Ce negative anomaly and Y positive anomaly,and Eu positive anomaly is not obvious in this kind of allocation pattern.The values of rare earth element charactermatic parameters of calcite belonging to the type I allocation model are different from those of the host rock,and this type of veining fluid is mainly affected by the effect of acidic hydrothermal fluid,and the veins formed in a weakly oxidized and weakly reduced fluid environment.The rare earth element geochemical characteristics of calcite belonging to the type II allocation model mainly inherited the rare earth element geochemistry of the host wall rock,and was less affected by the acidic hydrothermal fluid and precipitated in the relatively oxidized fluid environment.There are also two types of rare earth elements allocation model of calcite veins,developed in the Niutitang Formation in Well EYY-A of the Yichang area.Type I model is characterized by the enrichment of light-medium rare earth elements,there are obvious Ce negative anomaly,Eu and Y positive anomaly in this kind of model,and Type II model shows the equilibrium enrichment characteristics of rare earth elements,there are obvious Eu negative anomaly and Y positive anomaly,and the negative anomaly of Ce is not obvious in this model.The calcite belonging to the type I partitioning mode may be mainly affected by the action of the acidic hydrothermal fluid and precipitated in a relatively high temperature and oxidized fluid environment,and the calcite belonging to the type II partitioning model may be mainly influenced by low temperature mineralization and precipitated in a relatively reduced fluid environment.?3?According to analysis of laser Raman spectroscopy,petrology and low temperature phase transform of fluid inclusions in fractured veins,two types of fluid inclusions,namely gas-liquid two-phase brine inclusions and methane inclusions,are mainly developed in the fractured veins in Wufeng Formation-Longmaxi Formation turbidite sand section in Wells JY-A and JY-B of Fuling region,the gas-liquid two-phase brine inclusions generally contain methane,some methane inclusions contain a small amount of CO₂,some are approximately pure methane inclusions,and individual methane inclusions contain bitumen,which are trapped in CH₄-NaCl-H₂O immiscible dissolved fluid system.Three types of fluid inclusions,namely gas-liquid two-phase brine inclusions,methane inclusions and bitumen inclusions,are mainly developed in fractured veins in the Doushantuo Formation and Niutitang Formation in Well EYY-A of the Yichang area,among them,bitumen inclusions are mostly found in the quartz veins of the Doushantuo Formation,and individual bitumen inclusions contain methane,and the three types of fluid inclusions coexist in the same primary fluid inclusion combination,indicating that the inclusion combination is trapped in hydrocarbon-salt immiscible dissolved fluid system.?4?According to analysis of microscopic temperature,the homogenization temperatures of brine inclusions developed in fractured quartz veins and calcite veins in the Wufeng formation in Well JY-A in Wuling area are mainly distributed at 237²71?and 216²74?,respectively;The homogenization temperatures of brine inclusions developed in the quartz veins and calcite veins in the turbidite sand section of the Longmaxi Formation are mainly distributed at 230²60?and 206.6²66?,and the salinities are mainly distributed at 4.02⁵.86%,?The average value is 5.23%?and4.80⁵.26%?average 5.07%?,respectively.The homogenization temperatures of the gas-liquid two-phase brine inclusions developed in the fractured quartz veins of the Wufeng Formation and Longmaxi Formation in Well JY-B are distributed at 234²55?and 229²44?,respectively,and the salinities are distributed at 5.0⁷.4%?average5.9%?,4.3⁴.8%?average 4.5%?,respectively;the homogenization temperatures of the gas-liquid two-phase brine inclusions developed in the fibrous calcite veins of the lower segment of the Longmaxi Formation are mainly distributed between 165 and 209?,and the salinity distribution is between 0.2 and 1.4%,with an average value of 0.8%.The homogenization temperatures of the brine inclusions developed in the quartz veins of the Doushantuo Formation in Well EYY-A of the Yichang area is mainly concentrated in three temperature zones,which are 150¹80?,180²10?and 220³00?,and the temperature range of 180²10?is the most important,followed by the temperature range of 150¹80?;The salinity is concentrated in three intervals,which are 2.6⁶.2%,11.6¹5.4%and 17.4²3.2%,respectively,mainly 11.6¹5.4%and 17.4²3.2%.The homogenization temperatures of brine inclusions developed in the calcite veins of the Niutujitang Formation is mainly concentrated in two temperature ranges of170²20?and 220²70?,and the temperature range of 170²20?is the most important.?5?According to analysis of the laser Raman spectroscopy and microthermometry,the homogenization temperatures of the methane inclusions,developed in the quartz veins of Wufeng Formation and Longmaxi Formation in Well JY-A in the Fuling area,are-98.7^-86.1?and-99.4^-87.0?,respectively,the corrected laser Raman scattering peak wave numbers of methane inclusions are 2910.929²911.819 cm^-1 and2910.779²911.470 cm^-1,respectively,and the densities of methane inclusions are0.234⁰.302 g/cm³ and 0.226⁰.304 g/cm³,respectively.The homogeneous temperatures of the methane inclusions developed in the quartz veins of the Wufeng Formation and Longmaxi Formation in well JY-B are-97.8^-86.8?,-96.3^-95.9?,respectively,and the corrected laser Raman scattering peak wave numbers of methane inclusions are 2910.929²911.623 cm^-1 and 2910.779²911.623 cm^-1,respectively,and the densities of methane inclusions are 0.244⁰.299 g/cm³ and 0.244⁰.302g/cm³,respectively.The corrected laser Raman scattering peak wave numbers of methane inclusions,developed in the quartz veins of the Doushantuo Formation in Well EYY-A in Yichang area,are 2910.7793²911.6131cm^-1,and the densities of methane inclusions are 0.244⁰.323 g/cm³.?6?Taking advantage of the state equation of CH₄ system,combined with single well burial-uplifting-thermal evolution history,the trapping pressures of methane inclusions developed in the quartz veins of the Wufeng Formation and Longmaxi Formation turbidite sand section in Well A of Fuling area are obtained,and they are 92.72¹60.64MPa and 85.74¹62.00 MPa,respectively,and the corresponding pressure coefficients are 1.46².53 and 1.37².59,respectively;The trapping pressures of methane inclusions in the quartz veins of the Wufeng Formation and Longmaxi Formation turbidite sand section of well JY-B are 99.62¹55.50 MPa and 98.22¹57.25 MPa,respectively,and the corresponding pressure coefficients are 1.51².36 and 1.52².43,respectively.These shows that the entire shale gas layer was in a medium-strong overpressured state when the Wufeng-Longmaxi formation of the Pingqiao and Jiaoshiba block in Fuling area approached the maximum depth and the formation of the fractured veins at the early stage of the Yanshanian uplifting or before the Yanshanian uplifting.The average trapping pressures of methane inclusions in the quartz veins of the Doushantuo Formation in Well EYY-A of Yichang area are between 88.62 and170.16 MPa,and the corresponding pressure coefficient are 1.37².64,this indicates that the shale layers of the Doushantuo Formation in Yichang area developed overpressure due to thermal cracking and the entire shale gas layers were in a medium-strong overpressure state in the process of burial depth.