AAPG Bulletin, V. 84, No. 3 (March 2000), P. 345-359.
The Effect of Ductile-Lithic Sand Grains and Quartz Cement on
Porosity and Permeability in Oligocene and Lower Miocene Clastics, South China Sea:
Prediction of Reservoir Quality1
R. H. Worden,2 M. Mayall,3 and I. J. Evans4
©Copyright 2000. The American Association of Petroleum Geologists. All rights
reserved.
1Manuscript received June 8, 1998; revised manuscript received June 14, 1999;
final acceptance September 3, 1999.
2School of Geosciences, The Queen's University Belfast, Belfast BT7 1NN, United
Kingdom; e-mail: r.worden@qub.ac.uk
3BP-Amoco, Sunbury Research and Engineering, Chertsey Road, Sunbury on Thames,
Middlesex TW16 7LN, United Kingdom.
4BP-Amoco, Sunbury Research and Engineering, Chertsey Road, Sunbury on Thames,
Middlesex TW16 7LN, United Kingdom. Present address: BP-Amoco, Sherwood House, Holton
Heath Trading Park, Poole, Dorset BH16 6LS, United Kingdom.
ABSTRACT
Ductile, clay-rich sand grains control porosity and permeability in the fluvio-deltaic
Oligocene and lower Miocene sandstones of the South China Sea. Ductile grains account for
between approximately 5 and 50% of the original sand grain population. There is a
pronounced loss of porosity with increasing burial depth in the basin. At depths of less
than 3000 m this is due solely to ductile grain compaction where the rate of porosity loss
with depth increases with increasing ductile grain content. At depths greater than 3000 m,
the steep porosity loss with depth is due to combined ductile grain compaction and quartz
cementation. The amount of quartz cement increases with increasing burial depth; however,
cleaner sandstones tend to have greater amounts of quartz cement at any given depth below
3000 m. This leads to convergence of porosity evolution for the clean and ductile-rich
sandstones below 3000 m. There is a rapid loss of permeability with decreasing porosity
because compaction of ductile grains smears them between rigid quartzose grains leading to
blocked pore throats. A consequence of this process is that the lowest permeabilities are
found in sandstones with the highest ductile grain contents. Quartz cement does not have a
clear and discernible control on permeability. The pronounced loss of porosity with
increasing depth and permeability with decreasing porosity leads to low permeability at
relatively shallow burial depths. Reservoir quality is thus controlled by the nature of
the primary sand and depth of burial. The sediment supply system led to systematic changes
in ductile grain content across the basin with ductile content increasing into the more
distal part of the sediment system. The consequence is that depth of economic basement (in
terms of porosity or permeability) can be predicted as a function of ductile grain content
and burial depth for prospects across the basin.