A comprehensive fracture characterization method for shale reservoirs

Opening-mode fractures play a crucial role in shale reservoirs, as they serve as flow channels and provide storage space for hydrocarbons. The shale reservoirs in the Permian Lucaogou Formation of the Junggar Basin, NW China, record multi-stage tectonic and diagenetic processes that created multi-stage natural fractures, thereby contributing to the oil content differentiation within the formation. Effective identification and characterization of natural fractures is vital for the efficient recovery of shale oil in the Jimsar Sag. We combine outcrop observations, drill core analyses, thin section examinations, and well log analyses to determine the characteristics of fractures in the shale reservoirs and their modes of development. We also establish multi-parameter evaluation index criteria and an evaluation system for fractures using statistical analyses. The shale reservoirs of the Lucaogou Formation in the Jimsar Sag host three main types of fracture: tectonic fractures, diagenetic fractures, and overpressure fractures. Conventional well logging, imaging logging, and core observations demonstrate that the fractured shale reservoir section has gamma-ray counts (GR) of >75 API, shallow laterolog resistivities of <80 Ω·m, neutron densities of <2.40 g/cm³, neutron porosities of >27%, and interval transit times of >23.77 μs/m, fracture density exceeding 3 fractures/m, and average porosity ranging from 0.2% to 0.3% in the lower sweet spots. The lower sweet spot (P₂l₁²) of the Lucaogou Formation exhibits the highest degree of fracture development. Our detailed characterization reveals high fracture permeabilities and porosities in the upper and lower sweet spots (P₂l₂² and P₂l₁²), with higher values in the latter. In addition, we present a novel rose diagram method to represent various fracture parameters. The best-developed tectonic fractures in the Lucaogou Formation strike ENE–WSW, have an average linear density of 1.65 fractures/m, an average aperture of 0.25 mm, an average length of 8.7 cm, and the highest proportion of unfilled fractures. Our study shows that a combination of field observations, drill core analyses, microscopic observations, and well logging provides a solid foundation for investigating the mechanisms of fracture formation in shale reservoirs.