Gamma-ray, Spectral Gamma-ray, and Neutron-density Logs for Interpretation of Ordovician Volcanic Rocks, West Cumbria, England
Published:January 01, 2002
David Millward, Simon R. Young, Brett Beddoe-Stephens, Emrys R. Phillips, Chris J. Evans, 2002. "Gamma-ray, Spectral Gamma-ray, and Neutron-density Logs for Interpretation of Ordovician Volcanic Rocks, West Cumbria, England", Geological Applications of Well Logs, Mike Lovell, Neil Parkinson
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Borehole geophysical logs are a major source of information about rocks of the Ordovician Borrowdale Volcanic Group, which is concealed beneath a Carboniferous and/or Permian-Triassic cover sequence in west Cumbria, northwestern England. The volcanic succession is dominated by more than 1000 m of pyroclastic rocks, composed of thick andesitic, dacitic, and rhyolitic ignimbrites; volcaniclastic sedimentary rocks, intrusive andesite sheets, and thin dikes of basalt and rhyolite are subordinate constituents. Comparison of lithologic logs with gamma-ray, spectral gamma-ray, and neutron-density geophysical logs from 22 fully cored boreholes provides a robust interpretation of many aspects of the volcanic facies, lithostratigraphy, geochemistry, and mineralogy. Distinctive gamma-ray and spectral gamma-ray responses characterize volcanic units that vary from substantial lithostratigraphic units several hundred meters thick to marker beds only a few meters thick. Gamma-ray and spectral gamma-ray logs indicate the bulk chemical composition of the rocks, and show that some ignimbrite sheets are internally homogeneous, whereas others are zoned geochemically. Detailed comparison of the same lithologic units and their gamma-log responses in multiple boreholes facilitates interpretation of complex lateral variation in thickness and facies. Wireline-log responses allow recognition of dikes where these differ distinctly in composition from country rock, and they facilitate correlation among closely spaced boreholes. Bulk-density and neutron-‘porosity’ logs aid in identification and characterization of mineralogically and chemically altered zones within cleaved, unwelded lithofacies. In rocks of this kind, this combination log is also an indicator of lithology.
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Geological Applications of Well Logs
This book provides examples of the use of well logs across a broad range of applications. The “Technology and Techniques” section includes papers on logging in high-angle wells and on logging and imaging while drilling. The “Sedimentology and Stratigraphy” section, introduced by John Doveton (University of Kansas), includes studies from the North West Shelf of Australia, Kalimantan, and Iberia as well as from the North Sea, where important new contributions have been made on the wireline signature of mud rocks. The section on “Fractures and the Stress Field,” with a review paper by Colleen Barton (GMI), includes imaging examples from the North Sea and papers on the characterization of fractures for nuclear-waste disposal. Case histories in the volume come from a wide range of petroleum-related applications and from geotechnical and groundwater studies. Most of the papers were presented originally at the “Geological Applications of Wireline Logs” conference convened by the Geological Society (London) in 1999. The objective of the conference was cross-fertilization of approaches among a range of specialists who use well logs in their work. All users of log data should find inspiration in this volume.