Page 312 - Petrophysics 2E

LOGDERIVED EVALUATION OF SHALY 283

porosity, grain density, and electrical properties, should be reported
where possible so that calculations of S, and oil-in-place can be adjusted.

LOG~DERIVED EVALUATION OF SHALY (CLAYEY)
RESERVOIR ROCKS

Several comprehensive reviews of the large number of studies on the
evaluation of shaly sands using well logs can be found in References
33 through 36. Important logging parameters were generated including
matrix density, hydrogen index (HI), cation exchange capacity (CEC),
and distribution of potassium, thorium, and uranium as shown by
natural gamma ray spectral log information, for the three most common
clay minerals, i.e. illite, kaolinite and montmorillonite (smectites), and
chlorite. However, because most shaly reservoir sands contain different
clay minerals in various amounts, no single clay parameter can be used
universally to characterize these sands [37]. Currently, the emphasis is
on the application of the Waxman-Smits model based on the continuous
computation of CEC per total volume, Qv, and core data over the logged
segments.
In many instances, however, core data over the logged zones of
interest are not available. To overcome this limitation, digital shaly sand
analysis techniques based on the Waxman-Smits model and variations
in the basic properties of various clay minerals were developed. Two
of these digital techniques, CLASS and CLAY, developed by Ruhovets
and Fertl and Berilgen et al., respectively, provide information on total
and effective porosity, total and effective fluid saturation distribution, silt
volume, amounts, types and distribution modes of clay minerals present,
and reservoir productivity [38, 391. The CEC and HI can be calculated
knowing three parameters: clay density, pel, neutron response to 100%
clay, NCl, and the clay volume, Vel. pCl and Ncl are best determined from
density, neutron, and natural gamma ray spectral data at every depth level
over the interval of interest, so that the unrealistic assumption that clay
properties in adjacent shale beds and the reservoir rock are identical is not
necessary [do]. The clay volume, Vel, which is essentially independent
of the clay types, is calculated from the potassium and thorium values.

FORMATION EVALUATION

The basic physical properties needed to evaluate a petroleum reservoir
are its permeability, porosity, fluid saturation, and formation thickness.
These parameters can be estimated from three common sources: core,