Characterization Of Fluid Inclusions In Chang 7 Shale Oil In Ordos Basin

The Ordos Basin is extremely rich in hydrocarbon resources and is one of the largest hydrocarbon-bearing basins in China,as well as a key area for hydrocarbon exploration and development in China.In this paper,the petrographic characteristics,homogeneous temperature and salinity of fluid inclusions in the Chang 7 shale oil in the Upper Triassic Yanchang Formation in the southwestern part of the Ordos Basin were analysed with the help of core observations,logging data,common thin section identification,fluorescence thin section identification,fluorescence spectroscopy analysis,cold-hot bench experiments,laser Raman analysis and other analytical and laboratory tools,focusing on the petrographic characteristics,homogeneous temperature and salinity of fluid inclusions in the Chang 7 shale oil,and combined with the simulated burial-thermal evolution history,the study area was The timing and timing of oil and gas formation in the study area,as well as the characteristics of the palaeo-pressure distribution,were studied and described in conjunction with the simulated burial-thermal evolution history.The main results and insights achieved are as follows:A large number of inclusions are developed in the sandstone interlayer of the Chang 7section shale formation,and fluid inclusions are seen to be assigned to four mineral types and productions,i.e.quartz grain microfractures,quartz grain secondary enlarged edges,cut-through quartz grain microfractures,and group distribution within quartz grains,with an average size of2-15μm.According to the composition and phase characteristics of the inclusions,they can be classified into three categories:brine inclusions,hydrocarbon-bearing brine inclusions,and hydrocarbon inclusions.The hydrocarbon inclusions fluoresce yellow-green under blue light excitation,and are tentatively judged to be Phase I sub oil and gas inclusions.The fluorescence spectra of the hydrocarbon inclusions are clearly characterised by the main peakλ_max of the oil inclusions fluorescence spectrum between 542.4 and 549.1 nm,withλmax corresponding to yellow-green fluorescence;the spectral parameters red-green entropy Q and QF₅₃₅ values are widely distributed,reflecting the complexity of the crude oil components.The uniform temperature range of oil inclusions is 70-120°C,with the main peak segment at 90-100°C.The uniform temperature range of brine inclusions associated with hydrocarbon inclusions is 70-140°C,with the main peak segment at 90-110°C.The stripping thickness of the study area at the end of the Early Cretaceous is 400-1500 m.Based on this,the burial-thermal evolution history of the study area is simulated,and the combination of fluorescence spectral characteristics and the homogeneous temperature clarifies the formation characteristics of the study area as continuous filling and one-stage formation,and the maturity of hydrocarbon source rocks may affect the duration of formation.The salinity of fluid inclusions in the study area is high,with salinity mainly concentrated in the intervals of 6-16%and 18-22%.The recovered palaeo-pressures range from 30.89 to 36.42MPa,with an average palaeo-pressure of 34.18 MPa,an average palaeo-pressure coefficient of1.18 and an average residual palaeo-pressure of 5.13 MPa,showing an overall palaeo-overpressure.There are multiple centres of high palaeo-pressure coefficients,with the centres of high values gradually decreasing towards the surrounding areas,showing that the palaeo-pressure coefficients are spreading along the areas with low values.The spreading from high pressure coefficients to low pressure coefficients indicates the general direction of oil and gas transport.