AAPG Methods in Exploration No. 13, Chapter 10: A
Sequence-stratigraphic Study of Mudstone Heterogeneity: A Combined Petrographic/Wireline
Log Investigation of Upper Jurassic Mudstones from the North Sea (U.K.) , by Joe H. S.
Macquaker and Clive R. Jones, Pages 123 - 141
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 10
A Sequence-stratigraphic Study of Mudstone Heterogeneity:
A Combined Petrographic/Wireline Log Investigation of
Upper Jurassic Mudstones from the North Sea (U.K.)
Joe H. S. Macquaker
Department of Earth Sciences, University of Manchester
Manchester, U.K.
Clive R. Jones
Exxon Production Research
Houston, Texas, U.S.A.
ABSTRACT
Subtle lithofacies variations in a mudstone-dominated succession are described using
combined optical, electron-optical (backscattered electron imagery), and electrofacies
methods. In this study, 110 samples were collected from Upper Jurassic strata from the
North Sea. Within this interval, seven lithofacies are described: clay-rich mudstones,
silt-rich mudstones, sand-rich mudstones, laminated mudstones, nannoplankton-rich
mudstones, concretionary carbonate rocks, and muddy sandstones. These are composed of clay
minerals, organic matter, pyrite, quartz, feldspars, and carbonate cements and have
characteristic wireline-log signatures (gamma-ray, neutron-porosity, and density logs).
The data demonstrate that in these relatively monotonous, mudstone-dominated successions:
(1) mudstone lithofacies vary greatly, particularly with respect to their detrital
component; (2) individual mudstone lithofacies have distinctive electrofacies; (3) there
are systematic fining- and coarsening-upward grain-size trends on a 3- to 10-m scale; (4)
there were significant breaks in sediment accumulation a short distance above or below
where the stacking patterns change; and (5) mudstone successions such as these, whose
internal variability is usually interpreted in terms of changing bottom-water oxygen
concentrations and surface productivity (primary organic productivity that occurs in the
surface layers of the sea), should also be considered in terms of varying rates of local
siliciclastic sediment supply.