四川盆地乐地1井须家河组泥页岩段孔隙结构及分形特征差异性对比

The difference of pore structures and fractal features in the Xujiahe Formation shales from Well LD-1,Sichuan Basin,China

研究四川盆地上三叠统须家河组不同层段黑色泥(页)岩的孔隙结构和孔隙分形差异性特征及其控制因素。对乐地1井须家河组全取心段泥页岩地层进行系统采样,进行总有机碳含量、有机质成熟度、X射线全岩和黏土矿物含量、场发射扫描电镜/能谱分析、等温氮气吸附等测试,并采用基于FHH模型的分形维数计算模型,得到不同层位和不同孔径范围的页岩孔隙双重分形维数。结果表明:随着埋深增加,不同沉积环境下的页岩样品孔隙结构参数发生显著变化,表现为页岩孔隙BET比表面积逐渐增大、BJH平均孔径逐渐减小和BJH总孔体积不断增大的特征,且须一段泥页岩孔隙的比表面积和总孔体积比上覆的2个层位发生数量级的倍增。须家河组各层位黑色泥页岩均具有双重分形特征,存在明显的孔径分界点,且各段都具有大孔隙分形维数均值D_1均大于相应层位小孔隙分形维数均值D_2的特征,表明大孔隙比小孔隙的结构更复杂,体现了层内不同孔径范围孔隙复杂程度的差异性;而页岩随TOC含量和Ro的增大,须一段D_1、D_2值均比上覆页岩层位大,体现了层间差异性。孔隙结构参数、TOC含量和Ro是影响页岩孔隙复杂程度的主要控制因素。具备高D_2值(吸附能力)和低D_1值(渗流能力)的页岩储层最优。

Samples of Upper Triassic Xujiahe Formation black shale from Well LD-1 are systematic collected in order to study its pore structure,pore fractal characteristics and controlling factors.These samples are analyzed by means of total organic carbon content,organic matter maturity,X-ray whole rock and clay minerals content,FE-SEM/energy spectrum,isothermal nitrogen adsorption and so on.Fractal dimension calculation method based on FHH model is used to obtain the double fractal dimension of shale pore for different diameter and different range.It shows that with the increase of buried depth,significant changes occur in pore structure parameters for varying sedimentary environment,characteristic of gradual increase of BET specific surface area for shale pore,gradual decrease of BJH average pore size and increase of BJH total pore volume.The shale pore specific surface area and total pore volume of the 1 st Member of Xujiahe Formation redouble in comparison with the overlying two shale layers.Black shale of Xujiahe Formation has double fractal characters and obvious cut-off aperture,and the large average pore fractal dimension D 1 is larger than small pore fractal dimension D 2 in corresponding horizons for all members of the Xujiahe Formation,indicating that the texture of large pore is more complicated than that of the small pore.However,with the increase of buried depth,the extent of TOC and R o,the D 1,D 2 of the Member 1 of Xujiahe Formation are larger than the overlying rock layers.Therefore,pore structure parameters,extent of TOC and R o are the main controlling factor affecting the shale pore complexity.Shale with high D 2 value(adsorption)and low D 1 value(seepage ability)forms favorable shale reservoir.