Page 60 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 41

All of the pore classifications discussed so far, though useful and informative, lack

information necessary to group pore types with common geological origins except
for purely depositional varieties. The value of a porosity classifi cation depends
largely on how reliably the pore types can be placed in correlatable stratigraphic

space at field scale. In other words, it depends on how easy it is to map fl ow units,

baffles, and barriers that are defined by the classified pore types. High correspon-

dence between porosity and permeability alone does not offer clues for ways to
correlate pore types at reservoir scale. Although the Lonoy (2006) scheme is useful,
it does not provide new information with which to predict the spatial distribution
of flow units, baffles, and barriers. That information must come from rock properties

that co - vary with porosity. They co - vary because the rock properties and pore types
were formed at the same time by the same geological processes. Usually there are
distinctive rock properties — signatures of a sort — left by the geological processes
that created the final set of rock and pore properties. It is those “ signatures ” that

can serve as “ tags, ” or proxies, for the pores that were formed concurrently with the
signature rock properties. It is those signatures — the rock or stratigraphic charac-
teristics — that can be correlated at reservoir scale.
Porosity classifications should be simple to use, they should illuminate the

genetic relationship between rock, pore, and petrophysical attributes, and they
should serve as aids in predicting the spatial distribution of reservoir fl ow units.
That is a big order. It requires a classifi cation based on integrated data from rocks,
pores, petrophysical attributes, and on ways that pore types can be correlated at
stratigraphic scale. Classification data must include time and mode of origin of

rocks and pores, as well as percentage abundance of pores along with attributes
such as average pore size and sorting. Rock texture, fabric, and mode of pore origin
are obtained from petrographic study of samples, usually from cores. Because cores
reveal fundamental rock properties of larger scale than cuttings, cored intervals
can be correlated directly with borehole logs and with the stratigraphic column.
Cores provide a greater volume of rock for more representative measurements of
petrophysical properties in reservoirs that have widely varying pore categories and
pore sizes. Without rock samples, pore characteristics remain unseen, and their
origin, geometrical properties, and distribution within the rock remain unknown.
Petrophysical attributes of different pore types are not visible in thin sections or
on sample surfaces but can be determined from capillary pressure measurements
that provide information about pore throat size distribution and aperture size
sorting. Oriented pore fabrics, especially fractures, vugs, and linear or planar

features, can be identified visually, and comparison of vertical and horizontal per-
meability measurements from core analyses provides additional clues about
directionality.
Information about the origin of both rocks and pores can be incorporated in a
genetic classification. If pores and rock matrix formed contemporaneously, as in

reservoirs with purely depositional porosity, the common and synchronous origin
of rocks and pores allows depositional facies to become proxies for porosity. Pores
not formed contemporaneously with deposition, as in diagenetic and fracture poros-

ity, must be interpreted differently. Methods for correlating flow units based on

nondepositional pore types may be significantly different from methods that simply

map facies and expect the maps to correspond with flow unit boundaries. Times
and modes of origin for diagenetic and fracture pores are important in a genetic