准噶尔盆地中部下乌尔禾组深层陆相页岩孔隙结构分形特征及其地质意义

Fractal characteristics of pore structure of deep continental shale of Lower Wuerhe Formation in central Junggar Basin and its geological significance

为明确准噶尔盆地中部下乌尔禾组深层陆相页岩孔隙结构以及分形特征,以东道海子凹陷下乌尔禾组页岩为研究对象,在深入剖析页岩矿物及地球化学特征的基础上,采用场发射扫描电镜、低温N2吸附等研究方法,定量表征下乌尔禾组页岩孔隙结构特征,并基于FHH模型计算页岩孔隙的分形维数,揭示总有机碳含量(TOC)、矿物组分、孔隙结构参数和分形维数的关系及其地质意义。研究结果表明,研究区下乌尔禾组页岩主要发育无机孔和微裂缝,孔径分布呈多峰型,以平行板状或窄缝状孔隙为主;页岩孔隙发育受TOC和石英、长石、黏土矿物含量的控制,导致孔隙结构之间差异性较大,非均质性强。研究区下乌尔禾组页岩孔隙具有双重分形特征,其中表面分形维数(D_(1))为2.4522~2.5948,平均为2.5409;结构分形维数(D_(2))为2.6045~2.7748,平均为2.7056。TOC与分形维数呈负相关,孔隙结构参数(比表面积和孔体积)和矿物组分(石英、长石以及黏土矿物)含量与分形维数呈正相关。脆性矿物(石英、长石)和黏土矿物含量的增加有助于微纳米尺度孔隙以及微裂缝的发育,比表面积和孔体积增大,分形维数也增加,孔隙非均质性越强,孔隙结构越复杂。

In order to clarify the pore structure and fractal characteristics of the deep continental shale of the Lower Wuerhe Forma-tion in the central Junggar Basin,the Lower Wuerhe Formation shale in the Dongdaohaizi Sag was taken as the research object.On the basis of an in-depth analysis of shale minerals and geochemical characteristics,the pore structure characteristics of the Lower Wuerhe Formation shale were quantitatively characterized by using field emission scanning electron microscopy and low-temperature N2 adsorption experiments.The fractal dimension of shale pores was calculated based on the FHH model,and the rela-tionships among TOC content,mineral composition,pore structure parameters,fractal dimension,and its geological significance were revealed.The results show that the Lower Wuerhe Formation shale mainly develops inorganic pores and micro-fractures,and the pore size distribution is multi-peak type,mostly parallel plate or narrow slit pores.The pore development of shale is controlled by TOC and the content of quartz,feldspar,and clay minerals,which results in significant differences and strong heterogeneity among pore structures.Shale pores of the Lower Wuerhe Formation in the study area have double fractal characteristics,in which the surface fractal dimension D_(1) varies from 2.4522 to 2.5948,with an average value of 2.5409.The fractal dimension D_(2) of the structure ranges from 2.6045 to 2.7748,with an average of 2.7056.TOC is negatively correlated with fractal dimension,while pore structure parameters(specific surface area and pore volume)and mineral composition(quartz,feldspar,and clay mineral con-tent)are positively correlated with fractal dimension.An increase in the content of brittle minerals such as quartz,feldspar and clay minerals contributes to the development of micro-and nano-scale pores and micro-fractures.This results in an increase in specific surface area,pore volume,and fractal dimension.As pore heterogeneity strengthens,the complexity of the pore structure also in-creases.