Logging evaluation of acoustic anisotropy and its relationship with “sweet spots” in lacustrine shale oil reservoirs: The Fengcheng Formation of the Mahu Sag, China

Abstract

China's lacustrine shale oil reserves are abundant, making it a key area for future exploration and development. Most lacustrine shales feature a mix of mineral compositions and interlayer sedimentary structures. High-quality reservoirs exhibit significant heterogeneity, which influences the stress distribution during fracturing, leading to complex fracture network patterns. This complexity presents challenges for the comprehensive well logging evaluation of the geological-engineering "double sweet spots" in shale oil, severely restricting efficient development. This study focuses on the impact of shale sedimentary layering on the radial slowness of dipole shear waves. It employs rock physics experiments combined with advanced well logging techniques to explore the relationship between reservoir anisotropy caused by sedimentary layering and reservoir quality, thereby establishing a logging evaluation method for vertical identification of "sweet spots" in lacustrine shale oil. The shales in the Fengcheng Formation of the Mahu Sag into three types according to sedimentary structure scale: laminated, interlayer, and massive. Each type has different mineral compositions, affecting reservoir quality and fracturing potential. Laminated shales develop more fractures under stress along the beddings, showing moderate anisotropy, with reservoir capacity dependent on intercrystalline porosity within carbonate layers. Interlayer shales easily form complex fracture networks, exhibiting significant anisotropy, and their reservoir capacity depends on the porosity within sandy bands. Massive mudstones have the fewest fractures under stress, appearing isotropic with reservoir capacity dependent on matrix pore size. The intensity of reservoir anisotropy correlates positively with storage capacity and the propensity to form irregular and complex fracture networks during hydraulic fracturing. In sections without natural fractures, a larger difference between fast and slow shear waves corresponds to a radial profile shift towards warm tones, indicating stronger anisotropy and better reservoir quality, thus forming complex fracture networks during fracturing. Conversely, a smaller difference leads to a profile energy shift towards cooler tones, indicating stronger isotropy and poorer reservoir quality, hindering the formation of complex fracture networks during hydraulic fracturing. In sections with natural fractures, the difference between fast and slow shear waves exhibits erratic behavior, showing a cross-pattern in radial profiles, indicating strong anisotropy. The presence of natural fractures can synergize with induced fracture networks to form more complex systems, significantly enhancing reservoir productivity.