Page 61 - Geology of Carbonate Reservoirs

42 CARBONATE RESERVOIR ROCK PROPERTIES

classification of carbonate porosity in order to compare pores formed by different

types of diagenesis or fracturing with other rock properties that reflect pore - forming
geological events — events that may leave traces other than pore characteristics in
the rock record. Those traces are geological clues that can help in correlating genetic

pore types from borehole to borehole. Modes of origin are defined explicitly from
sample examination, usually by thin section petrographic study. Relative timing of
pore origins is established by interpreting cross - cutting diagenetic features or frac-
tures. Once the distribution of genetic pore types is known, the different categories
can be compared with permeability, capillary pressures, borehole log data, well test
data, or production characteristics to establish links between pore types, petrophysi-
cal “ rock types, ” and their locations in the stratigraphic column. Before fl ow units

can be defined, ranked by quality, and correlated from well to well, pore types must

be classified according to their mode and time of origin.

2.4.2 A New Genetic Classification for Carbonate Porosity
Carbonate porosity is created or altered by (1) depositional processes, (2) diagenetic
processes, and (3) mechanical fracture. It is intuitive to plot those processes as end
members on a triangular diagram (Figure 2.13 ) with notations about hybrid pore
types along its sides (Ahr and Hammel, 1999 ; Ahr et al., 2005 ). Pores associated with
mechanically sedimented detrital deposits will conform to original grain texture and
fabric such that depositional facies maps are proxies for reservoir porosity maps.
In such cases, porosity is facies selective much as some of Choquette and Pray ’ s
(1970) fabric - selective pores may also be facies selective. Depositional pores related
to reef fabric and skeletal or microbial textures have properties that refl ect biologi-
cal processes rather than mechanical sedimentation of loose grains. Clearly, pore
geometry in reefs and mounds can be significantly different from that in detrital

carbonates.

Depositional porosity that has been altered by diagenesis is classified as one of
three hybrid pore types: hybrids of depositional and diagenetic processes, hybrids
of diagenetic and fracture processes, and hybrids of depositional and fracture pro-
cesses. If depositional characteristics are dominant but somewhat modified by dia-

genesis, depositional facies remain reliable proxies for porosity. If more than about
half of pores visible in thin section are determined to have been altered by diagen-
esis (diagenetic attributes dominate), the types of diagenesis that created the hybrid
pores must be determined because depositional facies are less reliable as proxies
for porosity and facies maps will not be reliable guides to the spatial distribution of
porosity at reservoir scale. In this case, it is necessary to determine the types of dia-
genesis that caused the alteration and at what times diagenesis was active during
the burial history of the rocks. This information can be used to identify rock proper-
ties that may be new proxies for porosity when depositional facies boundaries are

no longer reliable indicators of flow unit dimensions.
Diagenesis alters depositional porosity by dissolution, cementation, compaction
and pressure solution, recrystallization, and replacement. It may enhance or reduce
original porosity or it may create totally new pore types. Purely diagenetic porosity

cannot be identified from depositional attributes when alteration is pervasive and
dissects depositional textures and fabrics, rendering them useless as guides to fl ow
unit boundaries. Alteration such as cementation, compaction, replacement, or