Lithology, Lithofacies, or Architectural Elements for Reservoir
Modeling?
Florez-Nino, Juan-Mauricio1,
Omar Angola1 (1) iReservoir.com,
Understanding the difference between the
concepts of lithology, lithofacies,
and architectural elements is key to generate geologically sensible reservoir
models. The use of general lithology classifications
as sandstone and shale can be a too-simplistic approach, since limits the
ability to distinguish different types of sandstones in the model. In contrast,
reservoir models with too many lithofacies can become
too complicated, especially because of the lack of knowledge of the statistical
and geometrical parameters corresponding to each lithofacies,
and the difficulty of separating lithofacies using
well logs.
The architectural element constitutes a
fundamental concept for static reservoir modeling. An architectural element is
a rock body composed of lithofacies associations, or
a stack of them. Shapes and dimensions reported from outcrops commonly refer more
to architectural elements than to specific lithofacies
or general lithologies. Statistical parameters for geomodeling make sense when applied to architectural
elements, and can be misleading when applied to simple lithologies
or specific lithofacies.
Lithofacies, on the other hand,
constitute the key to understand the effect of textural variations on porosity,
permeability, water saturation and elastic properties. Grain size, sorting,
clay content, and cementation can introduce significant variations in rock
properties. We present examples, based on core and log data, illustrating the
additional uncertainty introduced by the variability associated with lithofacies.
An appropriate balance between
variability, associated with lithofacies, and
generality, implicit in the concept of architectural elements, constitutes a
key to obtain realistic and workable reservoir models. Paraphrasing Einstein:
reservoir models should be simple, but not simpler.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California