龙马溪页岩弹性各向异性与矿物分布之间的关系探讨

岩石弹性各向异性特征是普遍存在的,但导致岩石弹性各向异性的原因复杂且仍然存在一定争议.本研究以龙马溪页岩为例,试图建立页岩弹性各向异性和矿物分布之间关系.研究使用无损超声波探测获取岩石弹性各向异性参数,并使用背散射技术获取岩石矿物分布特征.研究通过引进变异系数来描述矿物或孔裂隙在不同方向的差异性,并通过2个正交方向的变异系数建立微观异质性指标,用于表征微观尺度上矿物或孔裂隙在不同方向的差异程度.微观背散射图像显示,龙马溪组页岩主要由石英和伊利石构成,且这两种矿物和孔裂隙在X和Y面上都有定向排列特征.相应地,它们的变异系数在X面和Y面上均表现出随角度增大而降低的特征;而在Z面,石英、伊利石和孔裂隙变异系数变化不明显,这与Z面上矿物和孔裂隙无明显方向性的特点一致.无损超声波探测结果显示,波速在X和Y面上随角度增加而减小,这与主要矿物和孔裂隙变异系数变化趋势相同;而在Z面,波速变化不大,与主要矿物和孔裂隙变异系数变化不明显的特征一致.以上观测结果说明,宏观波速与矿物的微观变异系数明显相关,暗示岩石弹性各向异性与矿物分布直接相关.

Discussion on the relationship between elastic anisotropy and mineral distribution of Longmaxi Shale

Anisotropy is a common characteristic of rock elasticity. However, the factors inducing elastic anisotropy of rocks are complex and still controversial. This paper aims to inspect the relationship between elastic anisotropy and mineral distributions of rocks using the Longmaxi Shale from Chongqing, China. The mineral distributions on three orthogonal directions were obtained by AmicSCAN (a back scattered electron detector) on 3 mm×3 mm×1 mm samples, and the P-wave and S-wave velocities, which characterize rock elasticity, were measured by Panametrics-NDT transducer. A variation coefficient was introduced to characterize the variation of minerals and fractures in certain directions. The variation indices in two orthogonal directions were used to establish a heterogeneity index to characterize the difference of minerals and pores in different directions. The micro back scattered images showed that the Longmaxi shale was mainly composed of quartz and illite, and both the two principal minerals and pores are well-oriented on the X-plane and Y-plane. In addition, both the variable coefficients of the two principal minerals and pores decrease with the increase of the angle to the direction of the large principal stress on the X-plane and Y-plane. However, there are no differences in variable coefficients on the Z-plane. In term of acoustic characteristics, the velocities and anisotropy indexes show similar rules, which may be related to the heterogeneity of distributions of quartz, clay minerals, and pores. These results indicated that the elastic anisotropy was caused by minerals, pore and crack distribution.