Page 68 - Geology of Carbonate Reservoirs

TERTIARY ROCK PROPERTIES 49

discussions on the theory and methods of log interpretation. For the nonspecialist,
a brief review of the types of modern wireline logs, their applications, and their
limitations is presented in Morton - Thompson and Woods (1992) . Acoustic imaging
and NMR logs have only recently come into widespread use and are not included
in Alberty ’ s 1992 compilation. Hodgkins and Howard (1999) present an illustrated
discussion on NMR logging in Gulf of Mexico sandstone reservoirs and the hand-
book by Asquith and Krygowski (2004) presents a variety of methods for calcula-
tions from borehole logs including image and NMR logs. Rider ’ s (1996) book is a
well - illustrated review of virtually all types of wireline logging procedures, the types
of records that are generated by the logging devices, and how geological interpreta-
tions are made from the resulting records. Both Asquith and Krygowski (2004) and
Rider (1996) include discussions on acoustic and nuclear magnetic resonance
imaging, which are not included in the list of logs in Table 2.2 . A particularly useful
section in Rider ’ s book describes geological interpretations of wireline logs in
sequence stratigraphic context.
Traditional methods of interpretation, particularly on older analog records,
involve reading values from analog wiggle traces and then making calculations
to determine rock and reservoir properties. Graphical methods involve cross -
referencing the values read from wiggle traces on nomograms to obtain estimates
of rock or reservoir properties. Borehole log data from carbonate reservoirs pro-
vides an indispensable aid for stratigraphic correlation, for calculating values for
porosity, for estimating lithological composition, especially in mixtures of carbon-
ates and evaporites, for determining fluid saturation, formation resistivity, borehole

diameter, structural and stratigraphic dip, and, especially in the case of imaging
devices, to detect fractures. Calculations using wireline log data are routinely made
to determine saturation ( S w ), formation water resistivity ( R w ), porosity ( ϕ ), density
(ρ ), and lithology. In the case of terrigenous sandstones, additional inferences can
be made about depositional environments based on the shape of the resistivity and
SP or gamma ray traces. Gamma ray, or SP, and resistivity readings are sometimes
interpreted to represent grain - size trends in siliciclastic sandstones; therefore, by
extension, to represent depositional facies characteristics. Resistivity and gamma
ray curves that depict “ bell ” or “ funnel ” shapes (Figure 2.15 ) are interpreted as

fining - upward and coarsening - upward sandstone textures, respectively. The former
could be indicative of a channel - fill sequence, the latter a deltaic sequence. Such

interpretations based on log curve shapes enable geologists to create “ electrofacies
maps ” that depict fluvial channels, deltas, and turbidites, among a variety of related

facies types. The validity of these interpretations depends on the assumption that
shapes of the gamma ray and resistivity curves are proxies for grain size trends, and
that the logging engineer made no errors while running and recording the log. In
fact, gamma ray and resistivity devices do not measure grain size; they measure
natural radioactivity and electrical resistivity. The proxies for fine grain size, higher

gamma ray and resistivity values, depend on the presence of clay minerals in the
sandstones.
The shapes of gamma ray and resistivity log traces from carbonate reservoirs do
not indicate anything about depositional environment, particle characteristics, or
pore types. Carbonate reservoir properties are influenced by depositional, diage-

netic, or fracture processes, or combinations of those processes as reflected in pore
type and pore geometry. It is generally impossible to distinguish depositional or