Page 59 - Geology of Carbonate Reservoirs

40 CARBONATE RESERVOIR ROCK PROPERTIES

intraparticle, or intercrystalline pores, but they were enlarged by dissolution to
become vugs. Dissolution does not follow a predictable pattern in most cases; con-
sequently, the size, shape, and spatial distribution of vugs may be quite irregular.
They may begin as fabric - selective dissolution or non - fabric - selective enlargement
of fractures by leaching. Flow between separate vugs has to pass through matrix
porosity and permeability to drain the vugs; therefore the contribution of separate
vugs to total reservoir porosity and permeability can be estimated if matrix charac-
teristics and total porosity are known. Clearly, the only way to obtain that kind of
information is by direct observation of rock samples — such as cores — that are large
enough to display vugs that may be centimeter scale in size. Fluid fl ow through
touching vugs is much less affected by matrix permeability and behaves more like

flow through open fractures. Because most vugs, particularly touching vugs, are
larger than rotary drill cuttings, they may be overlooked during sample examination,
which again emphasizes the importance of examining full - diameter cores when
working with carbonate reservoirs.
Non - vuggy or interparticle pores are classified by Lucia as visible or not visible

in cuttings. Visible pores are grouped according to particle size as fi ne ( < 20 μ m),

medium (20 - 100 μ m), and large ( > 100 μ m). There are no genetic modifiers or catego-
ries for time and direction of pore alteration, as in the Choquette and Pray (1970)
scheme, but Lucia provides a basis for estimating the displacement pressure (mercury
injection capillary pressure) for each particle size range in interparticle porosity.
This is important because it offers clues about the ease with which fl uids can move
through rocks of different particle (grains or crystals) sizes. The relationships
between porosity, permeability, and particle characteristics were further investigated
by Lucia (1995) and expanded upon in his 1999 book Carbonate Reservoir Charac-
terization . His 1995 paper is an extension of the original classification and presents

discussions on pore characteristics in limestones, dolostones, and grain - dominated
and mud - dominated fabrics. Especially useful are the discussions on petrophysical
attributes of the different rock and pore types.
Lucia ’ s classification is an objective rather than a genetic classifi cation; conse-

quently, it does not provide information about rock and pore characteristics with

common geological origins. Notwithstanding, the Lucia classification is an excellent
and practical method that focuses on relationships between rock and petrophysical
properties.
Recently, Lonoy (2006) incorporated basic elements of Lucia ’ s (1983, 1995) and

Choquette and Pray ’ s (1970) classifications into a porosity evaluation scheme based
mainly on statistical correspondence between porosity and permeability in various
Lucia and Choquette – Pray pore types. Lonoy (2006) added subdivisions of pore

categories in both classifications to create additional varieties of interparticle and

intergranular pore types. He added refinements to better distinguish macro - and
micromoldic pores, and created additional categories for microporous, mud -
supported fabrics. Lonoy ’ s (2006) study provides valuable information about cor-
respondence between porosity and permeability in different carbonate pore types
and it should provide useful information for refining pore volume estimates used in

volumetric calculations. For its merits, it does not provide information on how to

improve our ability to correlate flow units, baffles, and barriers in stratigraphic space

and it does not focus on combined petrophysical and conventional geological rock
typing techniques.