Source Rock Analysis From Well Logs
The method presented here to identify organic-rich zones and calculate TOC (Total
Organic Carbon) content is based on the EXXON technique published by Passey et. al.
(1990). The EXXON method was designed for use in shales. We have modified the
model to create baseline conditions through the full range of clean sand to shale.
Deviations from the baseline conditions can be used to identify both source rocks and
hydrocarbon reservoirs.
The following expression (after Passey et. al.) describes the separation of baselined
resistivity and porosity log curves;
(1)
where;
R
R ns
P
P ns
K
is curve separation
is measured formation
resistivity
is resistivity of non-source
shales
is porosity log reading
is the porosity log reading in
non-source shales, and,
is a scale factor dependent on
the porosity log measurement
units.
TOC is calculated using;
(2)
where;
TOC
LOM
is total organic carbon content
in wt%, and,
is the level of organic
maturity. This can be
estimated using the LOM vs.
vitrinite reflectance crossplot
Equation (2) predicts zero TOC where there is no curve separation (baseline
conditions). In practice, however, all shales have some organic carbon content, so it is
necessary to add 0.2 to 1.6 percent to predicted TOC (Passey et. al., 1990). The
baseline TOC content of shales is usually determined from laboratory measurements or
using local knowledge.
In practice it is rare to have both TOC laboratory measurements and reliable organic
maturity data. In these situations it is possible to chose a value for LOM that will result
in a match with available TOC data.
Baseline Conditions in Lean Shale and Water-Bearing Reservoirs
The EXXON Dlogr technique can be modified to create baseline conditions through the
full range of shale to clean sand. Equation (1) can be re-arranged to individually
describe deviation from baseline conditions for the resistivity log (Yr) and the porosity
log (Yp):
In the absence of solid organic matter or hydrocarbons, the baseline conditions for the
resistivity and porosity logs become;
and
so;
In the absence of organic matter or hydrocarbons, Yr and Yp should both equal zero
(they will overlie on a depth plot).
Yr and Yp profiles through the full range of clean formations to shales can be calculated
by treating Rns and Tns as functions of shaliness:
Rsd
RSh-ns
PSd
PSh-ns
Vsh
is resistivity of clean (no shale)
formation,
is resistivity of non-source shale,
is porosity log reading in clean
formation,
is porosity log reading in nonsource shale, and,
is shale fraction
In non-reservoir intervals (silt and shale), anomalous resistivity (Yr > Yp) is associated
with solid organic matter. In reservoir rocks, Yr > Yp indicates the presence of
hydrocarbons.
Continuous profiles of Yr and Yp can be used to calculate expected resistivity in water
zones and non-source shales (Ro).
In the absence of hydrocarbons and organic material:
(3)
and,
(4)
Separation of Ro and Rt (where Rt > Ro) can be used to identify organic-rich shales and
hydrocarbon zones in reservoirs.
References
Archie, G. E.: "The Electrical Resistivity Log as an Aid in Determining Some Reservoir
Characteristics", Petroleum Transactions of the AIME 146 (1942).
Hood, A., Gutjahr, C. C. M., Heacock, R. L.: "Organic Metamorphism and the
Generation of Petroleum", AAPG Bull., V.59, June 1975.
Passey, Q. R., Creaney, S., Kulla, J. B., Moretti, F. J., Stroud, J. D.: "A Practical Model
For Organic Richness From Porosity and Resistivity Logs", AAPG Bull., Dec. 1990.
"QUICKLOG" - Petrophysical software distributed by Henderson Petrophysics .
"TOCP" - Petrophysical software distributed by Henderson Petrophysics.
Scale Factors For TOC Calculation
LOG
Log Units
K
Sonic
usec/ft
-0.02
Density
g/cm3
2.5
Neutron
V/v
-0.04