Page 45 - Geology of Carbonate Reservoirs

26 CARBONATE RESERVOIR ROCK PROPERTIES

Allochthonous Autochthonous
Original components Original components organically
not organically bound bound during deposition
during deposition
> 10% grains > 2 mm

Matrix Supported By By By
supported by >2 mm organisms organisms organisms
component that act as that that build
baffles encrust a rigid
and bind framework

Floatstone Rudstone Bafflestone Bindstone Framestone

Figure 2.8 The skeletal reef classification of Embry and Klovan (1971) . Note that there is
no provision for nonskeletal mounds such as microbialite buildups, “ mud mounds, ” or “ algal

mounds. ” Dunham (1970) and Ahr (1971) addressed the use of the term “ reef ” for nonskeletal

buildups and Riding (2002) developed a classification for various nonframebuilt mounds.
(Adapted from an illustration in Tucker and Wright (1990) .)
depositional rock properties and fractures or diagenetic attributes. For example,
dolomicrites are more brittle than pure lime micrites and fracture more readily;
therefore fracture intensity should be greater in the former than in the latter. Meta-
stable (aragonitic or Mg calcitic) grains are more susceptible to diagenesis than
stable calcitic grains so that porosity may be the result of selective removal, recrys-
tallization, or replacement of original minerals. We will see that several carbonate

porosity classifications do not include the mode of origin of rocks; consequently, it

is difficult to use those classifications to distinguish pore types that formed as results
of depositional processes from those that were modified or created by postdeposi-

tional diagenesis or fracturing. It is equally difficult to distinguish flow units by their

geological origin, without which it is difficult if not impossible to predict their spatial
distribution at stratigraphic scale. This book presents a genetic classifi cation of
porosity linked to the complete geological history of reservoir rocks as an alterna-
tive. It is based on the idea that there are three end - member pore types in carbonate
reservoirs: depositional, diagenetic, and fracture pores. These different processes
impart distinctive characteristics to both rock matrix and pores. Because the distinc-
tive characteristics were imparted to pores and rocks at the same time and by the
same processes, key rock properties may act as “ markers ” or proxies for pore types
that can be identified and traced at stratigraphic scale. To the extent that the proxies

are identifiable and mappable, so will be the accompanying pore types and, of capital

importance, their petrophysical attributes.