Page 228 - Geology of Carbonate Reservoirs

DEPOSITIONAL RESERVOIRS 209

(the method of rock typing mentioned earlier). Where measured porosity and per-
meability values from core analyses are not available, porosity and saturation cal-
culations from wireline logs can be used with a moderate amount of confi dence in
some carbonate reservoirs except those dominated by vuggy, moldic, and fracture
porosity. If the pore system is depositional and intergranular, and if core analyses
are available for at least one core, then permeability can be estimated for correlative

intervals in wells without cores by using the best - fit equation for the linear relation-
ship between porosity and log of permeability, as described earlier.

8.2.4 Pore Scale Features
Pore scale features include physical characteristics of pores and pore throats such
as pore geometry, genetic pore type, pore/pore throat size ratio, and median pore
throat size. This information is gathered from thin section petrography, scanning
electron microscopy, and MICP and NMR measurements. Pore and pore throat
geometries are compared with petrographic or SEM image analyses and MICP data
to assess how different pore characteristics relate to capillary pressures, measured
porosity, and measured permeability. In this way, pore and pore throat geometries

can be compared with their potential to transmit or impede fl uid flow, thereby pro-

viding a ranking scheme for flow units or a basis for a rock - typing scheme. NMR
measurements can provide results to corroborate and extend the usefulness of the
flow unit ranking or rock - typing schemes. Once these pore scale characteristics have

been determined, it is necessary to scale up to flow unit and reservoir scale. This can

usually be done by finding key rock properties to serve as proxies for ranked fl ow
units or rock types. In some cases the proxy rock properties are simply genetic
classes of porosity or certain size and shape ranges of pores. These parameters are
easily measured with petrographic image analysis (PIA) techniques and can serve

as markers for correlating flow units, baffles, and barriers (Layman and Ahr,

2003 ).
8.3 DEPOSITIONAL RESERVOIRS

Depositional carbonate reservoirs are those in which the dominant pore system is
preserved more or less as it looked at the time of deposition. In truth, some diage-

netic modification of depositional pores is inevitable. The genetic classifi cation of
porosity types was designed to take this into account by including hybrids — dia-
genetically altered, depositional porosity — as a midpoint along the leg of the triangle
between depositional and diagenetic end - member pore types. The genetic classifi ca-
tion lists interparticle, intraparticle, fenestral, keystone, shelter, and “ reef ” porosity
as depositional pore categories. Reef porosity may be inter - and intraskeletal poros-
ity in reef organisms along with inter - and intraparticle porosity in detrital material
that fills spaces between skeletal masses. Some reef classification schemes were

described in Chapter 2 , including those by Embry – Klovan (1971) and Riding ( 2002 ).
The Embry – Klovan scheme is particularly useful for reefs that include large amounts
of rigid framework or large skeletal constituents. The Riding classification is more

useful for mudstone and cementstone reefs or buildups, as some workers prefer to
call them, but it does not include information about porosity or pore types. Each