FRACTURES AND FLUID FLOW IN THE MARCELLUS SHALE: USING TRACE ELEMENTS TO DECIPHER FLOW HISTORIES

Geologic fluid flow is controlled by the hydraulic properties of the rock matrix as well as the properties of any deformation features. In a fractured rock system, there are two sets of hydraulic properties, one for the country rock and the other for the fracture network. The fluid flow properties (e.g. hydraulic conductivity, flow anisotropy) of the fracture network are based on the geometry of the fracture network, the regional stress field, the hydraulic gradient, and diagenetic properties such as fracture mineralization. The properties of the rock matrix are determined by its porosity and permeability characteristics. The rock system as a whole reflects the simultaneous interactive effects of these two different sets of hydraulic properties (Eaton, 2006).

In the Marcellus shale of the Appalachian Plateau in New York, we have mapped a complex fracture network with fracture sets of varying orientation, opening mode and degree of mineralization. Minerals in these fractures record the microscale interactions of fluids with fracture surfaces and may be used to estimate the number of pulses of fluid migration and characteristics of the migrating fluid (Sanchez et al., 2009). In order to understand fluid flow through this network, we used cryogenic laser ablation - inductively coupled plasma mass spectrometry (CLA-ICP-MS) to measure trace element concentrations [TE] in mineralized fractures. Microsampling at a 10-micron resolution across fractures produced cross sections with three peaks of [TE] concentrations 6-8 times greater than background levels. The average wavelength of these cycles is ~1.3 mm and variations in [TE] between peaks indicate at least three distinct fluid flow events that caused mineralization. This suggests that mineralization does not necessarily close fractures; initial mineralization forms mineral bridges that prop open fractures and allow subsequent fluid flow and further mineralization. In addition, we analyzed mineralization in fracture sets with different orientations to determine flow anisotropy in the Marcellus shale, the relative timing of mineral growth, fluid flux through fractures and chemical characteristics of migrating fluids.