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.