ABSTRACT
A practical method, the z\log R technique, for
identifying and calculating total organic carbon in
organic-rich rocks has been developed using well logs.
The method employs the overlaying of a properly
scaled porosity log(generally the sonic transit time
curve)on a resistivity curve(preferably from a deep-
reading tool), in water-saturated, organic-lean rocks,
the two curves parallel each other and can be overlain,
since both curves respond to variations in formation
porosity; however, in either hydrocarbon reservoir
rocks or organic-rich non-reservoir rocks, a separation
between the curves occurs. Using the gamma-ray
curve, reservoir intervals can be identified and eliminated
from the analysis. The separation in organic-rich
intervals results from two effects: the porosity curve
responds to the presence of low-density, low-velocity
kerogen, and the resistivity curve responds to the formation
fluid. in an immature organic-rich rock, where
no hydrocarbons have been generated, the observed
curve separation is due solely to the porosity curve
response. In mature source rocks, in addition to the
porosity curve response, the resistivity increases
because of the presence of generated hydrocarbons.
The magnitude of the curve separation in non-reservoirs
is calibrated to total organic carbon and maturity,
and allows for depth profiling of organic richness in
the absence of sample data. This method allows organic
richness to be accurately assessed in a wide variety
of lithologies and maturities using common well logs.
