The Marcellus shale is naturally fractured black shale that underlies much of Pennsylvania, as well as parts of New York, Maryland, Virginia, West Virginia, and Ohio. In New York, the Marcellus shale exposed at the surface is comprised of the Devonian Hamilton Group, made up of siliciclastic sedimentary rocks of which the Marcellus shale is the stratigraphically lowest subgroup. The Marcellus subgroup is comprised of three formations: the Union Springs Formation, overlain by the Oatka Creek Formation and its lateral stratigraphic equivalent, the Mount Marion Formation. The Union Springs is made up of black shales and dark grey limestones, and is separated from the black shales of the Oatka Creek and Mount Marion Formations by their basal member, the Cherry Valley limestone (Ver Straeten et al., 1994; Baird et al., 1999).
Regional Map and Cross Section of the Marcellus shale.
The Marcellus Shale as a Natural Gas Play
There is great interest in developing natural gas resources in the Marcellus shale. Black shales, such as the Marcellus shale, are potential domestic hydrocarbon plays because of their high organic content, and their burial depth which is deep enough to stimulate the cracking of kerogen into natural gas. Economically viable production of natural gas shales, such as the Marcellus shale, requires enhanced methods of oil and gas recovery such as hydraulic fracturing and horizontal drilling. The combination of these techniques has had mixed success at extracting economic quantities of natural gas from low permeability shale deposits, which have poor country rock permeability and transmissivity, but can contain natural fractures.
Hydraulic fracturing involves the injection of fluids and propping agents into the subsurface to break open low permeability formations, stimulate fluid flow through natural and induced fractures and increase fracture related permeability (e.g. increased fracture aperture, fracture size, and fracture network connectivity) thus enhancing hydrocarbon production (Jennings, 1990). The use of hydraulic fracturing is limited by: 1) inefficient resource recovery, 2) the potential for groundwater contamination from drilling fluids or mobilized hydrocarbons (Harrison, 1983), and 3) our inability to develop accurate models for fracture fluid flow in the exceedingly complex fracture network present in black shale.
Analyzing the Controls on Fluid Flow: Matrix and Microstructure
Fluid flow within the Marcellus shale is determined by the characteristics of the rock matrix, and the characteristics of any deformation features. In low permeability formations such as shale, fracture networks act as pathways for fluid flow making the geometry and hydraulic properties of fracture networks integral to geologic fluid flow in an area.
In this study, we will conduct a detailed micro-analysis of the shale matrix and the deformation features within the Marcellus shale. By understanding the properties of the rock and looking in detail at the veins, fractures, and other deformation features we will gain an understanding of how these features behave in the subsurface.
Natural fractures in the Union Springs member of the Marcellus Shale
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