Abstract
Transitional shale gas is an important area for oil and gas exploration. It has a wide distribution area and large resource potential, accounting for about 25% of the total shale gas resources in China. The method of reservoir fracture prediction by seismic attributes has been relatively established. However, seismic attributes are often limited by factors such as seismic data frequency and resolution. Characteristics of transitional shale reservoirs, such as complex combinations of lithologies, frequent lithological changes, low resolution and frequent frequency changes of seismic data and the existence of strong reflection shielding in coal seams, greatly affect the prediction of fracture. A single seismic attribute technique is difficult to provide a comprehensive and accurate prediction of fractures in transitional reservoirs. In this study, the discontinuity information of large fractures, fine fractures, and fractures shielded by strong reflections from coal seams are extracted by variance attributes, amplitude contrast attributes and amplitude of diffracted waves, respectively. The discontinuity features extracted by the three methods are tracked using ant-tracking technique and fused. Fractures in the work area are systematically characterized. The pre-stack wide-azimuth gathers is analyzed by the near-offset Rüger formula method. Anisotropic gradients and anisotropic directions are obtained. The prediction results of the fracture are interpreted and verified with assistance. A combined pre-stack and post-stack seismic multi-scale fracture prediction methodology is developed. The strong reflection shielding effect of the coal seam is effectively removed. The characterization of fractures in transitional shale reservoirs is improved.