New method for logging identification of natural fractures in shale reservoirs: The Fengcheng formation of the Mahu Sag, China

Natural fractures are one of the critical types of reservoir space in lacustrine shale formations. The enrichment, preservation, and production of shale oil are closely related to these natural fractures. However, the development of various sedimentary structures in lacustrine shale can interfere with fracture identification. In this study, we combined image logging and array sonic logging methods to examine the response characteristics of natural fracture development zones in the borehole, near-wellbore, and far-wellbore regions. We integrated a new logging identification method for natural fractures and explored the relationships among mineral composition, fracture types, and reservoir quality, providing a reference for selecting sweet spots in lacustrine shale reservoirs. Core samples, thin sections, and scanning electron microscope observations of the Fengcheng Formation shale reveal characteristics of multi-scale natural fracture development. High-angle and oblique fractures, which are relatively easy to identify, were recognized using core-calibrated image logs and appear as sinusoidal features on image logs. Low-angle fractures, however, require correlation with core observations for accurate identification. Using array acoustic logging data, including shear leaky and reflected waves, we obtained radial profiles of fast and slow shear waves based on dispersion variations, enabling the determination of fracture development zones. The crossing of slowness curves of fast and slow shear waves along the radial direction and strong responses of red energy clusters in radial profiles indicate the presence of fractures. Additionally, processing of shear-wave reflected wave information remote acoustic reflection imaging profiles, allowing the identification of reflectors distant from the borehole. This resulted in a new multi-scale observation method for natural fracture identification by coupling acoustic and electrical imaging data. Applying this method, we identified natural fractures and established relationships among mineral composition, fracture types, and reservoir quality. The study indicates that high-angle and oblique fractures tend to develop in interbedded felsic shale and mudstone, while low-angle fractures are more likely to form in interbedded felsic shale, dolomitic shale, and mixed shale. Reservoir quality deteriorates with increasing clay mineral content but improves with higher felsic minerals and dolomite content. The findings of this research provide a valuable reference for improving natural fracture logging identification methods and offer guidance for achieving efficient development of lacustrine shale oil.