Page 55 - Geology of Carbonate Reservoirs

36 CARBONATE RESERVOIR ROCK PROPERTIES

Archie ’ s classification is the pioneering effort to relate rock properties and res-
ervoir petrophysics but because Archie was an engineer rather than a geologist, his

classification does not deal with pore origin or modification. The major contribution

of the Archie classification is that it relates petrophysical properties, including capil-
lary pressures, electrical properties, and saturation characteristics, to different rock
types. For the first time, groups of capillary pressure curve shapes could be compared

with their corresponding groups of carbonate pore types. But by today ’ s standards
these pore types do not have an obvious relationship to depositional textures,
diagenetic fabrics, or pore types by geological origin; consequently, the Archie
classification is more interesting from a historical standpoint than it is from a

practical one.

Archie ’ s goal was to develop a classification that emphasized pore structure, fl uid
fl ow characteristics, fl uid distribution, and the fl ow of electricity. He avoided terms
that denote rock composition or that suggest a geological origin for the porosity.
His visionary work resulted in a combined petrological – petrophysical classifi cation
system for carbonate porosity. He did not consider it important to identify the geo-
logical thread that binds pore origin to rock origin, although many carbonate pore
types are altered or created long after the host rock was deposited. Unfortunately,
when the mode of origin — the genesis — of carbonate pore types is not part of the

porosity classification, the job of establishing correspondence between pore types

and rock matrix properties is all the more difficult. Even with the present technol-
ogy, it is difficult to correlate individual porous zones from well to well, and always

difficult to map the spatial distribution of flow units at field scale. Performing these
tasks requires data on how porosity relates to other rock properties that serve as
proxies or markers for effective porosity. The Archie classification does not provide

that kind of information.
2.4.1.3 The Choquette –Pray Classifi cation
Choquette and Pray (1970) recognized the need to incorporate time and mode of
origin in their classifi cation of carbonate porosity. Their classifi cation (Figure 2.11 )
is practical. They recognized 15 basic pore types and organized them into three
classes depending on whether they are fabric selective, not fabric selective , and fabric
selective or not . The origin of fabric - selective pores could be depositional, diagenetic,
or both, although that point is not emphasized in the classifi cation. Intergranular
pores in an oolite grainstone, intercrystalline pores in a crystalline dolomite, or
grain - moldic pores in a skeletal packstone are examples of fabric - selective pores
that have different origins. Non - fabric - selective porosity includes fractures or dis-
solution cavities of various sizes that cut across rock fabric. Fabric selective or not
is a category that includes mainly penetrative features such as animal or plant
borings and burrows and desiccation cracks. Breccias may be conglomeratic residue
from solution collapse, the products of clast - producing erosion followed by resedi-
mentation, or the result of tectonism.
Time of origin and direction of pore - altering, diagenetic processes are incorpo-
rated as “ genetic modifiers. ” Paralleling Archie (1952) , terms for pore size and

abundance also modify root names for pore types. Times of origin are designated
as primary and secondary. Primary origin includes pores formed by depositional and
early postdepositional processes. Secondary porosity is interpreted to be later