Influence factors of elastic anisotropy in orthorhombic shale with different maturity: rock physics modelling and orthogonal experiment analysis

Abstract Shales usually exhibit orthorhombic anisotropy. However, the degree of elastic anisotropy exhibits significant differences due to the stages of organic maturity, which remains poorly understood. The main controlling factors with varying organic maturity are still not well constrained. To address this knowledge gap, we developed a set of rock physics modelling method for orthorhombic shale based on the structure and distribution of organic matter with different maturity stages. Then we applied forward modelling analysis to examine the influence factors—organic matter content, clay content, lamination index, fracture density, and fracture orientation index—on the elastic properties and anisotropy of shale. Orthogonal experimental design was implemented to statistically rank their importance in relation to elastic anisotropy and isolate the main controlling factors governing elastic anisotropy in orthorhombic shales. Results show that elastic anisotropy magnitude is typically highest in immature shale and lowest in overmature shale. Organic matter content is positively correlated with elastic anisotropy at immature and mature stages. Clay content significantly enhances the elastic anisotropy magnitude, whereas the effects of lamination index and fracture properties vary across different maturity stages. Range analysis reveals that clay content exhibits the largest range values (Rε1, Rε2, Rγ1, Rγ2), indicating its dominant, first-order control on elastic anisotropy. Vertical fractures exert the greatest influence on anisotropy in directions parallel to lamination. These results provide valuable insights for the quantitative prediction of organic-rich intervals and the optimization of ‘sweet spot’ targeting in shale reservoirs.