盐间超压裂缝形成机制及其页岩油气地质意义——以渤海湾盆地东濮凹陷古近系沙河街组三段为例

Formation of inter-salt overpressure fractures and their significances to shale oil and gas:A case study of the third member of Paleogene Shahejie Formation in Dongpu sag,Bohai Bay Basin

以渤海湾盆地东濮凹陷古近系沙河街组三段盐间富有机质页岩为例,综合运用薄片鉴定、扫描电镜、流体包裹体分析、测井资料分析、地层压力反演等手段,对盐间页岩超压成因及超压裂缝的发育特征、形成演化机制和期次及页岩油气地质意义进行研究。结果表明,东濮凹陷盐湖盆地普遍发育超压,盐膏岩层封堵及欠压实作用、生烃增压作用、黏土矿物转化脱水作用、断层封闭作用是形成超压的4个主要因素。盐间超压裂缝规模较小,平均长度356.2μm,平均地下开度11.6μm,但密集发育,平均面密度0.76 cm/cm^2,且多伴随油气充注,有效性好。超压缝主要形成于距今25~30 Ma和距今0~5 Ma的两次油气大规模充注时期。盐间超压裂缝对页岩油气具有储集空间和运移通道双重作用,对孔隙度的贡献率为22.3%,对渗透率的贡献率为51.4%,可连通多尺度裂缝系统,大幅改善储集层物性。在开发过程中,盐间超压裂缝可影响水力压裂缝的延伸和形态,形成复杂的高渗透性体积缝网,提升水力压裂效果,增加页岩油气产能。

Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation(Es3)of Dongpu sag in Bohai Bay Basin as an example,the origin of overpressure,development characteristics,formation and evolution mechanism,formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification,scanning electron microscopy observation,analysis of fluid inclusions,logging data analysis,and formation pressure inversion.The results show that overpressure is universal in the salt-lake basin of Dongpu sag,and under-compaction caused by the sealing of salt-gypsum layer,pressurization due to hydrocarbon generation,transformation and dehydration of clay minerals,and fault sealing are the 4 main factors leading to the occurrence of overpressure.The overpressure fractures are small in scale,with an average length of 356.2μm and an average underground opening of 11.6μm.But they are densely developed,with an average surface density of 0.76 cm/cm^2.Moreover,they are often accompanied by oil and gas charging,and thus high in effectiveness.Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging,approximately 30-25 Ma ago and 5-0 Ma ago.Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas.They contribute 22.3%to the porosity of shale reservoir and 51.4%to the permeability.They can connect fracture systems of multiple scales,greatly improving the quality of shale reservoir.During the development of shale oil and gas,inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures,giving rise to complex and highly permeable volumetric fracture networks,improving hydraulic fracturing effect and enhancing shale oil and gas productivity.