Page 53 - Geology of Carbonate Reservoirs

34 CARBONATE RESERVOIR ROCK PROPERTIES

Porosity Range Qualitative Description
5% or less Poor
10% Fair
15% Good
20% or more Excellent

Note that porosity together with permeability determine reservoir quality. Porosity
is a measure of capacity to store fluids; permeability is a measure of capacity to

transmit fluids. Pores are connected by pore throats , which strongly infl uence perme-

ability and which are related to pore size, shape, and arrangement. If one rock is
composed of spherical, well - sorted grains of a given size, intergranular pores will be
only a fraction of the grain size and the attendant pore throats will be even smaller
but generally uniform in size. Rocks with poorly sorted grains will have poorly
sorted pore and pore throat sizes. Sorting and distribution of pore throat sizes in
rocks determines pore – pore throat size ratio and accessibility of fluids to the pore

throat system in rocks. Imagine molecules of oil migrating through pores and pore
throats. If pore throat sizes are uniformly distributed, the oil molecules will encoun-
ter more - or - less the same size pathway for migration and the threshold force (pres-
sure) required to move through the rock will be more - or - less uniform for any given
pore throat size. If the pore throats are poorly size - sorted, the migrating oil mole-
cules will encounter different sized pathways with different pressure thresholds to
pass through. In most terrigenous sandstones and in some carbonate grainstones,
intergranular pore sizes have a strong statistical correlation to grain size. In such
cases, pore throat sizes may correlate well with pore sizes, as illustrated by a plot of
the log of permeability as a function of porosity. In the ideal case, data points on

this “ Φ – k ” plot will form a straight line cluster. In most carbonates, pore and pore
throat geometries are not as predictably regular as this ideal case; therefore it is
helpful to be able to identify the pore types that have the most influence on perme-

ability. In order to understand pore characteristics and how pore types are related
to other readily identifiable rock properties, a system for classifying carbonate

porosity is necessary.

2.4.1.1 Porosity Classifi cations

Classifications are schemes for sorting or grouping things. They vary from simple
sorting of items into groups with common characteristics to tiered schemes in which

characteristics are grouped in sets, or which take on modifiers to aid in identifying

complex or obscure relationships. For example, a simple classification for porosity
might designate three categories of pores as interparticle, vug, and fracture. A tiered
classification scheme might group according to average pore size, pore shape, pet-

rophysical characteristics, and mode of origin. Classifications are ways of organizing

material into useful forms. Some schemes aid in interpreting the origin of pore types.
However, a weakness in much of our traditional approach is that we treat rocks and

reservoirs as separate entities. The links between rock classifications and pore clas-

sifications are poorly represented in most geological literature. Even less well rep-
resented are the relationships between rock and pore classifications and reservoir

petrophysical characteristics. The concept of “ rock typing ” — identifying reservoir