Quantitative characterization of fracture spatial arrangement and intensity in a reservoir anticline using horizontal wellbore image logs and an outcrop analogue

Using image logs from horizontal wells in the asymmetric East Painter Reservoir anticline in Wyoming, scanlines and petrographic observations from nearby outcrops, and methods that allow statistical quantification of fracture spatial arrangement patterns (including clustering), we show that in early Jurassic Nugget Formation sandstone degree of clustering and intensity of fractures varies with structural position. In the backlimb, fractures are markedly more clustered than random, and hierarchical and regularly spaced clusters are present, but intensity is low. In the forelimb, arrangements are mostly indistinguishable from random, but intensity is high. Based on image log response, the backlimb contains a higher proportion of open fractures, although the forelimb has greater numbers of open fractures. Spatial arrangement and intensity patterns are similar in a forelimb outcrop. Petrography reveals that quartz seals fractures less than 0.15 mm wide. Wider fractures are open with quartz rinds or are quartz lined and sealed with calcite or filled by quartzose cataclasite. The proportion of open fractures depends on quartz and calcite deposits and closure by reactivation (shearing) of pre-existing quartz -lined fractures. Based on increased abundance and greater strike dispersion, results suggest that in forelimbs, high fracture intensities reflect shear on preexisting fractures, possibly resulting in more numerous but less spatially correlated open fractures. The most prolific gas and water production wells are in the backlimb, sug-gesting that clustered but sparse quartz-lined open fractures are more effective fluid conduits than closely spaced partly damaged/sheared arrays.

Automated summary

Using various geological observations and statistical methods, this study investigates the spatial arrangement and intensity of fractures in the East Painter Reservoir anticline in Wyoming. The results show that the degree of clustering and intensity of fractures vary with structural position, with the backlimb exhibiting more clustered fractures but lower intensity compared to the forelimb. Petrographic analysis reveals that quartz plays a role in sealing narrow fractures, while wider fractures can be sealed by quartz, calcite, or quartzose cataclasite. The findings suggest that shear on preexisting fractures in the forelimb may lead to numerous but less spatially correlated open fractures.