AAPG Methods in Exploration No. 13, Chapter 18: Gamma-ray,
Spectral Gamma-ray, and Neutron-density Logs for Interpretation of Ordovician Volcanic
Rocks, West Cumbria, England , by David Millward, Simon R. Young, Brett Beddoe-Stephens,
Emrys R. Phillips, and Chris. J. Evans, Pages 251 - 268
from:
AAPG Methods in Exploration No. 13: Geological Applications of Well Logs, Edited
by M. Lovell and N. Parkinson
Copyright © 2002 by The American Association of Petroleum Geologists. All rights
reserved.
Chapter 18
Gamma-ray, Spectral Gamma-ray, and Neutron-density Logs for Interpretation of
Ordovician Volcanic Rocks, West Cumbria, England
David Millward
Simon R. Young
Brett Beddoe-Stephens
Emrys R. Phillips
British Geological Survey, Edinburgh, U.K.
Chris. J. Evans
British Geological Survey, Nottingham, U.K.
ABSTRACT
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.
