A PREDICTIVE MODEL FOR FRACTURE INTENSIFICATION DOMAIN VARIATIONS: STRAIN ENERGY DENSITY AS A FUNCTION OF FAULT SLIP

Our research integrates fault damage zone scaling studies and fracture spacing analyses for the purpose of determining the primary factors controlling variations in fracture intensification domain (FID) characteristics. Statistical analyses quantify the relationships among average fracture spacing within the FID (FID_SP), FID width (FID_W), fault slip, mineralogy and grain size. Equations derived from the models are then employed as predictive tools that describe the FID attributes with respect to fault slip and rock lithology. Additionally, the predicted fault/fracture relationships can be used to assist in characterizing unobserved faults in outcrop where only FID data are collected. Results from this research are applicable to projects dealing with subsurface fluid migration in fractures, such as ground water flow and contaminant migration.

We collected approximately 4000 fracture measurements from 34 field locations within Ordovician Utica and Schenectady units in the Mohawk Valley of eastern New York State. Observed faulting in outcrop included 17 minor normal faults and one thrust fault. Data were collected using the scanline, modified scangrid and abbreviated method. Fracture data were reduced and organized in Excel, categorized as FIDs or background frequency fracture sets and assigned distances to their associated faults. Additional data were also obtained from a review of related fault and fracture studies. Partial least squares statistical analyses were performed using XLSTAT to determine the significance of the predictor variables in the model. A Q² of 0.615 was achieved for a five component system with all variables exhibiting significant influence in the projection (error bars excluding 0). A positive correlation was found between FID_SP, fault slip, clay% and grain size. FID_w maintained a positive correlation with fault slip and grain size but was found to have a negative correlation with clay%. Fault slip has the greatest influence on both FID_W and FID_SP. These findings suggest that strain energy density, which is reflected in the width of the FID and the average fracture spacing within the FID, decreases with increasing fault slip even though total strain energy of the system increases.