Henderson   Petrophysics

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;
	is curve separation
R	is measured formation resistivity
R ns	is resistivity of non-source shales
P	is porosity log reading
P ns	is the porosity log reading in non-source shales, and,
K	is a scale factor dependent on the porosity log measurement units.
TOC is calculated using;
	(2)
where;
TOC	is total organic carbon content in wt%, and,
LOM	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, Y_r and Y_p should both equal zero (they will overlie on a depth plot).

Y_r and Y_p profiles through the full range of clean formations to shales can be calculated by treating R_ns and T_ns as functions of shaliness:

Rsd	is resistivity of clean (no shale) formation,
RSh-ns	is resistivity of non-source shale,
PSd	is porosity log reading in clean formation,
PSh-ns	is porosity log reading in non-source shale, and,
Vsh	is shale fraction

In non-reservoir intervals (silt and shale), anomalous resistivity (Y_r > Y_p) is associated with solid organic matter. In reservoir rocks, Y_r > Y_p indicates the presence of hydrocarbons.

Continuous profiles of Y_r and Y_p can be used to calculate expected resistivity in water zones and non-source shales (R_o).

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, 8 Debbie Street,  Brisbane Queensland, Australia 4061. Telephone: (61) 7  3300 3980. Facsimile (61) 7 3300 9342.

"TOCP" - Petrophysical software distributed by Henderson Petrophysics.

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Scale Factors For TOC Calculation
LOG	Log Units	K
Sonic	usec/ft	-0.02
Density	g/cm3	2.5
Neutron	V/v	-0.04