Characteristics of Fracture Distribution and Evaluation of Reservoir Effectiveness in the Chang 8 Formation of the Zhenjing Area, Ordos Basin

Abstract Objective This study aims to clarify the developmental characteristics and effectiveness of natural fractures in the tight sandstone reservoir of the Chang 8 Formation within the Zhenjing area, Ordos Basin. Thereby providing a geological basis for favorable zone evaluation and development planning. Methods Integrating core observations, field outcrop surveys, image logs, and three‑dimensional seismic data, we employ maximum‑likelihood fracture detection, statistical analyses of strike and dip, density calculations, and filling‑degree evaluations to systematically investigate fracture genesis, geometric distribution, and controlling factors. A comprehensive framework for fracture effectiveness assessment is subsequently established. Results Fractures in the study area are predominantly tectonic. Geometrically, high‑angle oblique and vertical fractures are dominant, with strikes mainly oriented NEE and NWW, reflecting the overprint of multiple tectonic episodes. The average density of fractures is 1.04 per meter. The overall development is relatively high, but it is relatively poor in the southeast area. The lengths are mostly concentrated between 10 and 20 centimeters, and the extensibility is good. Fracture distribution is jointly controlled by fault activity, lithological variations, and the thickness of rock layers, resulting in a heterogeneous pattern of " faults governing the plane distribution, lithology controlling the strata, and thickness dictating the density ". Effectiveness evaluation reveals that fractures in sandstone (63.87%) are significantly more effective than those in mudstone (43.83%). The smaller the angle between the fracture orientation and the current maximum principal stress direction, the larger the fracture opening, and the lower the filling degree, the higher the effectiveness. Increased burial depth generally reduces fracture aperture, although near‑vertical fractures are less affected. Cementation adversely affects effectiveness, whereas dissolution and micro‑fracturing can enhance connectivity. Conclusion Fracture effectiveness results from the synergistic interplay of tectonic stress, burial depth, and diagenetic evolution. The findings offer a geological foundation for fractured‑reservoir evaluation and well‑site deployment in the Chang 8 interval of the Zhenjing area.