Stratigraphic controls on vertical fracture patterns in Silurian dolomite, northeastern Wisconsin

Abstract

Vertical opening-mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed Silurian dolomite of northeastern Wisconsin. Our two-stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons (e.g., organic partings or cycle-bounding mud horizons) in terminating opening-mode fractures. First, the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern. Both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces. The two methods show similar results, confirming the validity of qualitative visual inspection. The second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy. For example, 63% of cycle-bounding mud horizons within the inner-middle and middle shelf facies associations serve as mechanical interfaces. These empirical percentages are input to a Monte Carlo analysis of 50 stochastic realizations of mechanical stratigraphy. Comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33%. This method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces. The methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing.