Fracture-dominated flow in the Borrowdale Volcanic Group at Sellafield, NW England: the identification of potential flowing features, development of conceptual models and derivation of effective parameters
Published:January 01, 2003
P. J. Degnan, A. K. Littleboy, U. McL. Michie, C. P. Jackson, S. P. Watson, 2003. "Fracture-dominated flow in the Borrowdale Volcanic Group at Sellafield, NW England: the identification of potential flowing features, development of conceptual models and derivation of effective parameters", Hydrocarbons in Crystalline Rocks, N. Petford, K. J. W. McCaffrey
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Between 1989 and 1997 United Kingdom Nirex Limited (Nirex) studied in detail the geology and hydrogeology of a rock volume near Sellafield in Cumbria, NW England. The aim of the study was to determine the suitability, or otherwise, of the site as the location for a deep repository for intermediate-level and certain low-level radioactive wastes. An important factor in determining site suitability was the nature of groundwater flow in the potential repository host rock, the Borrowdale Volcanic Group. In the host rock, interpretation of borehole core, wireline logs and hydrogeologicai pumping test data indicated that groundwater flow was predominantly through a limited subset of discontinuities, mainly fractures, parts of which form networks of connected channels. Within this overall understanding of the nature of groundwater flow, there is a wide range of possible geometrical descriptions for the flow channels. Determination of one or more appropriate conceptualizations of the flow system must be soundly based on site characterization data as a prerequisite for any numerical modelling study. In the first part of this paper, details of the site characterization studies that were used to identify the location, orientation and mineralogical characteristics of discontinuities, and in particular the set of discontinuities referred to as Potential Flowing Features (PFFs), are provided. These features have either demonstrable present-day open porosity, or display evidence of geologically recent groundwater flow as part of the evolution of the current groundwater system. It is inferred that the PFFs observed in boreholes correspond to flowing features and that the borehole data can be used to infer the distribution and characteristics of flowing features that are present in the unobserved rock mass. On this basis, knowledge of the distribution, orientation and permeability associated with the PFFs provided the framework for developing conceptual models for groundwater flow. Numerical models were constructed to represent the flow. It is not computationally practicable to undertake regional scale groundwater flow and transport calculations in which small-scale variability is explicitly represented. Therefore upscaled effective parameters need to be derived as a precursor to running large-scale numerical model simulations. A summary of the Nirex upscaling procedure applied to the flowing feature network in the Borrowdale Volcanic Group is provided.
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Hydrocarbons in Crystalline Rocks
‘Commercial oil deposits in basement rocks are not geological “accidents” but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement rocks should be explored with the same professional skill and zeal as ccumulations in the overlying sediments’, Landes et al. (1960), AAPG Bulletin.
Given that most OPEC countries are currently at or within 5% Production capacity, there is a growing need to look for ‘new oil’ and other hydrocarbons in non-traditional sources.
While oil and gas fields in crystalline basement are still discovered mostly by accident, as shown in this book, such reservoirs can be very prolific, especially if the basement rock is highly faulted or fractured. The chapters in this volume cover a diverse range of topics related broadly to the theme of hydrocarbons in crystalline rocks, and challenge explorationists’ definition of basement rock, which needs to be less narrow and more responsive to new geological ideas.