Determining the Factors that Control Natural Fracture Characteristics in the Northern Appalachian Basin

Abstract:

Geoscientists know a lot about fracturing, for example, we clearly understand fracture mechanics, and there are multiple complex models for simulating fluid flow through fracture networks. However, there is currently no reliable way to accurately predict the location or characteristics of natural fractures in the subsurface. For that matter, it is unclear how important natural fractures really are for controlling fluid flow in the subsurface. In some settings, fractures are conduits for fluid flow (e.g., oil migration through open fractures), while in other settings, fractures may not significantly impact fluid flow (e.g., closed or mineralized fractures).

Here, we present a study that improves our ability to predict subsurface natural fracture characteristics by determining how tectonic history, stratigraphy, present day depth, and large scale geologic structures control fracture formation in the Marcellus shale. We conducted detailed fracture and stratigraphic analysis on seven cores from the Marcellus shale and one from Pennsylvanian-Upper Devonian formations in the northern Appalachian Basin. Fracture filling minerals were sampled and analyzed in two ways: fluid inclusion geothermometry was conducted to determine vein formation conditions, and trace metals were mapped using Laser Ablation ICP-MS. We found that tectonic history controls fracture orientation, stratigraphy determines the length and intensity of fracturing, and large-scale geologic structures overprint regional fracture trends. Moreover, past predictive efforts in the Appalachian Plateau were proven to have incorrectly identified which fracture sets are present both at depth and near the surface.

These findings take us one step closer to developing a predictive model for subsurface fracture occurrence, with potentially large applications for assessing fracture related fluid flow in a variety of geologic settings.