Depositional environment and its control on mechanical stratigraphy: an integrated study of process sedimentology, composition and geomechanics

Abstract

Mechanical stratigraphy describes the mechanical properties associated with various strata in the subsurface and can be used to optimize hydraulic fracture completion designs, aid in targeting and placement of lateral wells, and determine wellbore stability. In unconventional plays, such as the Bakken Formation, where lateral wells and hydraulic fracturing are key to economic oil production, understanding the mechanical stratigraphy is crucial. This study examines the geologic controls on mechanical stratigraphy using quantitative measures of sedimentology, composition and geomechanical properties to develop a predictive framework for optimizing hydraulic fracturing. Additionally, high resolution data collected along a horizontal core allows for a comparison of data and quantification of heterogeneity in both the lateral and vertical directions, as well as an assessment of the anisotropy. Results indicate that the depositional environment controls the types of physical sedimentary processes acting to deposit sediments, the amount and distribution of bioturbation working to disrupting the sediments, and diagenesis that alters the sediments after burial. These processes ultimately determine the final composition and fabric of the rock, which are the primary drivers of many material properties. Rock fabric describes the distribution and orientation of planes of weakness in a rock due to bedding, laminations, pre-existing fractures, depositional interfaces or other sedimentary structures and is directly related to fracture complexity. In more homogenous rocks with few planes of weakness, low fracture complexity is observed in core and a hydraulic fracture will likely propagate out in one direction with minimal energy loss resulting in low complexity. In a more heterogeneous rock that has many planes of weakness, the energy of a hydraulic fracture is lost at these interfaces, resulting in higher complexity, where the fracture branches, change directions, steps over, or comes to a stop. Finally, statistical analyses of the data indicate that overall, there is more vertical heterogeneity than horizontal heterogeneity. This is likely to translate into more fracture complexity in the vertical direction compared to the horizontal direction, and therefore hydraulic fractures may be more likely to be contained in the vertical direction.