American Oil and Gas Reporter - July 2018 - 79

SpecialReport: New Tech & Geosciences
tinuous XRD, TOC and pyrolysis over the
Wolfcamp interval. Unfortunately, no wells
have mineralogical analysis with reservoir
properties. Also, no crushed rock analysis
using either the GRI or shale rock properties
methods was found.
The petrophysical model is developed
for highly heterolithic and complex lithology where the principal components of
the matrix include carbonate, quartz, dry
clay, kerogen, and accessory minerals
(pyrite, feldspar and apatite). The total
porosity component consists of claybound water, free water, irreducible water,
oil and gas, and soluble bitumen. Thermal
maturity is important in this conceptual
model since it determines how much of
the kerogen converts to bitumen and producible hydrocarbon, passing from the
matrix side of the model to become a
porosity component.
The model constructed is a basin- and
interval-specific, deterministic, complex
lithology, shaly-sand model designed to
consistently analyze an interval containing
"tight" and self-sourcing oil. The model
is iterative and corrects for hydrocarbon

calculations using variables specific to
thermal maturity. A simple thermal maturity
model is used based on vertical depth
with vitrinite reflectance data, pyrolysis,
and API gravity, validating this assumption.
Core data indicates a correlation between
porosity and saturation, while clay conductivity necessitates the use of a counterion saturation model. Complex lithology,
light organics and heavy accessory minerals
result in widely varying dry grain densities
that are accounted for in a density porosity
calculation.
The Wolfberry interval of interest is
comprised of several stacked pays, with
the principal target intervals including the
Middle Spraberry, Lower Spraberry, Wolfcamp A and Wolfcamp B. The geometry
and petrophysical properties within each
of these intervals vary laterally because of
sediment sourcing, paleodeposition, burial
history and thermal maturity. The thickness
of the intervals of interest is highly variable,
limiting the usefulness of property-thickness
mapping. While organic volumes (type II
and III kerogen) are moderately high and
present in all intervals, TOC averages 2.0

percent by weight and ranges from 0.25
to 6.50 percent with TOC content lowest
in limestones and higher in marls, silts
and mudstones.
Figure 2 displays the Wolfcamp type
log for the Midland Basin, showing
modern triple-combo log data on the
left and computed curves on right. Normalized gamma ray, corrected neutron
porosity (NPHIc) and photoelectric (PEF)
are shown along with permeability (mD).
Water saturation is shown shaded less
than 0.6 and initial water saturation is
conceptual only with no calibration (used
to identify free, producible water). The
porosity track is scaled from 0 to 0.20
and ranges between 0.01 and 0.12, and
typically averages 0.06 porosity unit.
The computer processed interpretation
plot (CPI) includes volume lithology
and fluids. All zones have sufficient
TOC to be considered source rock if
thermally mature.
Static Model Construction
The petrophysical analysis provides
an internally consistent set of rock prop-

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American Oil and Gas Reporter - July 2018

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