Page 62 - Geology of Carbonate Reservoirs

DEPENDENT OR DERIVED ROCK PROPERTIES 43

Interparticle
Intraparticle
Fenestral
Shelter / Keystone
Depositional
Reef
Intraskeletal
Depositional Interskeletal
aspects dominate Stromatactis vugs
Constructed voids
Detrital Infill
Hybrid 1 Hybrid 3
Diagenetic Depositional character
aspects dominate influences fracturing

Diagenetic Fracture
Enhanced Hybrid 2
Dissolution
Replacement Diagenesis influences fracturing
Recrystallization
Reduced
Compaction
Cementation
Replacement
Recrystallization
Figure 2.13 A genetic classification for porosity in carbonate rocks by the author. Pores are

created by three end - member processes that include depositional, diagenetic, and fracture
mechanisms. The end - member processes are independent but hybrid pore types exist between
them because more than one mechanism can affect the formation of a given pore system at
different times during its genetic history. For example, depositional porosity altered by dia-
genesis, but with depositional texture, fabric, or bedforms still recognizable, is classified as a

hybrid in which depositional attributes are dominant. Fracturing, especially fracture intensity,

is influenced by mineralogy and crystal size (diagenetic influence) and by bed thickness and

grain size (depositional attributes). Complete porosity analysis must include the total amount
(percent) of porosity present, and ideally the amounts of separate versus touching vugs. Pore
geometry is important too, because some pore types can be identified by their size and shape

with NMR measurements (Genty et al., 2007 ). A version of this classification appears in Ahr

et al. (2005) .

recrystallization may have reduced porosity to the extent that previous fl ow units
become baffles or barriers. Alternatively, diagenesis may enhance or create new

porosity by dissolution or by some types of replacement or, rarely, by recrystalliza-
tion. For example, dissolution creates caves, connected vugs, and karst features
including solution - collapse breccias that do not correspond to depositional facies
boundaries. Instead, they may correspond to positions of ancient water tables,
positions on antecedent structure, or to the locations of ancient mixing zones.
Diagenetic - fracture hybrids are those fractures that form preferentially in rocks
with diagenetically altered mineralogy and texture. Dolostones behave as more
brittle material than limestones such that fracture intensity is higher in dolostones
than in limestones with the same crystal size and bed thickness. As we will see in
later chapters, fracture porosity and permeability are computed differently than
porosity in unfractured rock matrix; therefore it is important to recognize the geo-
logical factors other than stress concentration and stress orientation that infl uence
the degree and type of fracturing.