Patterns in
Ice-House Carbonates
Pollitt, David A.1,
Peter M. Burgess2, V. Paul Wright1, Giovanna Della Porta1
(1) Cardiff University, Cardiff, United Kingdom (2) Shell Internation
Exploration and Production, Rijswijk, Netherlands
Many 1D forward models of sedimentary cyclicity in carbonates have focused on strata produced
during greenhouse periods, and have attempted to address the dispute over the
relative importance of allocyclic versus autocyclic controls. There has traditionally been less
investigation and dispute over the origin of ice-house cycles, where prominent
exposure surfaces on sub-tidal facies are indicative
of relative sea-level falls. The understanding of ice-house cyclothems,
however, holds critical importance for super-giant fields in
The aim of this study is to use a 1D
forward model to investigate the impact of various controls on potential
reservoir layering and heterogeneity.
We present a robust 1D numerical forward
model incorporating euphotic, oligophotic
and aphotic carbonate production, depth-dependent
marine erosion, and linked sub-aerial pedogenesis,
dissolution and diagenesis. Results demonstrate how
modeled facies stacking-patterns depend on interactions
of production, erosion and dissolution.
The modeling suggests that high-order
orbital oscillations are unlikely to be manifest as stacked shallowing-upwards
cyclothems. Furthermore, they are unlikely to be
stacked to a ratio (e.g. 5:1) within a lower-order sequence, as some workers
have previously suggested. It is more likely that high-order oscillations are
manifest as a discrete facies changes, occurring
within longer-term cyclothems of varying thicknesses
and bounded by sub-aerial exposure surfaces. Statistical analysis suggests that
cyclothem thickness occur as a continuum of values,
lacking any evidence for a 3rd, 4th and 5th order heirarchy.
These model scenarios are tested against classic Four Corners Carboniferous
successions.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California