STATISTICAL ANALYSIS OF FRACTURE VARIATIONS NEAR FAULTS IN UTICA BLACK SHALE

Fracture analysis performed on outcrops typically focuses on documenting fracture orientation, fracture spacing, planarity, abutting relations, mechanical unit height/fracture height as well as lithologic and rheologic variations. This current fracture-spacing analysis examines the possibility of fault influence on local fracture spacing and specifically focuses on faults that have produced fracture intensification domains (FIDs). The research investigates the potential relationships between the magnitude of slip on a fault and the average spacing of fractures within the FID (FID_SP), average FID width (FID_W) and number of fractures within an FID as a proxy for total fracture surface area (FID_A).

Characteristics of 3,678 fractures and 17 faults have been measured from 39 outcrops within the Ordovician foreland basin of the Mohawk Valley, New York State. Lithologic units included in the study range from Utica black shale and interbedded limestone/black shale to Schenectady Formation sandstone and interbedded siltstone/sandstone. Field work thus far has focused on minor normal faults with observable displacement and FIDs. We also collected fracture data on one N/S striking thrust fault but the lack of reliable displacement measurements excludes it from the statistical analysis. Variables recorded in outcrop include fracture spacing within the FID, total FID width, distance of fractures to the fault, observable height of the fractures, lithologic unit(s) fractured, throw and heave on the fault, observable height of the fault and observable length of the fault. The fault slip parameter represents the vector sum of the measured throw and heave.

Data compiled from outcrop measurements are used to perform multivariate statistical analyses utilizing the R programming language. Preliminary results suggest that fault slip is more strongly correlated with FID_W (R² value of 0.4075) and FID_A (R² value of 0.4616) than FID_SP (R² value of 0.0346). These findings suggest that greater slip on a fault will be more strongly associated with the formation of wider FIDs and increased total fracture surface area than to tighter fracture spacing within the FID.