Page 66 - Geology of Carbonate Reservoirs

TERTIARY ROCK PROPERTIES 47

throat geometries affect permeability greatly. Permeability depends on the geome-
try of pore throats rather than on the largest pore dimensions, although in some
cases, pore dimensions may vary predictably with pore throat dimensions. Reservoir
analysts must determine which geological characteristics of the reservoir rock serve
as “ markers ” for the pore types that consistently have the largest pore throats. This
task requires information from petrographic studies on reservoir rocks. When the
data are collected they are compared with other petrologic and petrophysical data
to formulate the geological concept that enables recognition and mapping of three -
dimensional fl ow units at fi eld scale.

2.5 TERTIARY ROCK PROPERTIES

Tertiary properties are measured indirectly by geophysical tools such as borehole
logging devices and to an extent by the seismograph. Neither borehole logs nor the
seismograph can make direct measurements of fundamental rock properties such
as depositional texture, sedimentary structures, mineralogical composition, or rock
fabric. In the case of borehole logs, the geoscientist or engineer makes inferences
about fundamental properties from log characteristics or from calculations based
on log data. Tertiary properties are twice removed from fundamental rock proper-
ties and once removed from dependent or secondary properties such as porosity,
permeability, and bulk density. Some of the more widely used measurements of
tertiary properties include electrical conductivity, electrical resistivity, acoustic

transmissivity (including seismic reflection characteristics), natural radioactivity,
nuclear magnetic resonance relaxation time, and photoelectric effect. One log that
does make direct measurements of borehole properties is the caliper log. It records
the diameter of the borehole. Otherwise, tertiary properties depend on porosity,

pore fluid composition and saturation, presence or absence of radiogenic materials,
and reaction of rocks and fluids in the borehole to external energy sources. Because

measurements of some tertiary properties require external energy sources for their
measurement (e.g., the photoelectric effect and acoustic logs), those properties are
sometimes called latent properties.

2.5.1 Borehole Logs and Carbonate Reservoirs
Logging is done by lowering a device called a sonde down the borehole and raising
it at a prescribed rate. As they are raised, the tools transmit data through cables to
recording devices at the surface. Some tools are designed for open - hole logging;
others are used in cased holes. Responses of logging tools vary with an array of
parameters such as size of the borehole, mud properties, speed of tool movement
up the borehole, and temperature; consequently, the novice interpreter should not
assume that log measurements always provide accurate and representative values.
Uncorrected logs usually require corrections before they are interpreted or com-
pared with direct measurements such as those made during core analyses.
Older, analog recording devices produce the familiar strip logs that have been
used for decades, but with today ’ s technology analog records can be digitized easily
and quickly. Digital data are acquired by modern logging tools and either processed

by computers at the wellsite or transmitted directly to a distant office or laboratory