Obtaining fracture information for low-permeability (tight) gas sandstones from sidewall cores

We illustrate a fracture characterization technique applicable to all tight gas sandstones. The technique uses microfractures and cements in the rock mass identified using SEM-based cathodoluminescence imaging to provide information on unsampled large fractures, including strike and cross-cutting relationships, intensity, and likelihood of preserved open fractures. We applied the technique in two tight gas sandstone wells in the Pennsylvanian Pottsville Formation, Black Warrior Basin, Alabama, USA. In one well, data was obtained entirely from drilled 1-inch diameter sidewall cores that were oriented using image logs and features visible in cores. We predicted fracture porosity preservation in large fractures using late cements in the rock mass as a proxy for fracture observation. Results from the technique are consistent with what is known of large fractures in this area. Where we predicted open, high intensity fractures, substantial gas flares were observed during drilling. By combining microstructural and diagenetic observations, it is possible to overcome the inadequate fracture sampling that plagues evaluation of natural fractures and that relies solely on observations of macroscopically visible fractures.