by S.J. PIRSON* and AURELIAN NEGUT**
*The University of Texas, Austin, Texas
** Oil, Gas and Geology Institute Bucharest, Romania,
Fulbright Fellow at the University of Texas
Model laboratory experiments were performed to simulate stratified sedimentary
rocks drilled with fresh muds in which the only potential contrasts in existence
were redox (Eh) and diffusion (Ed) potentials.
It is shown that the conventional SP curve measures but a small fraction of
the total redox potential contrast between the formations whereas the redox
log recorded in the well-bore may measure a large fraction of the Eh contrast.
Experiments with simulated salt muds indicate that satisfactory measurements
are achieved with the redox log whereas the SP curve is completely supressed.
The SP curve appears as a subdued curve of the redox potential contrast between
formations except that the SP curve is not responsive to the presence of hydrocarbons
in clean sands nor in the water-oil contact.
By contrast the redox log may be run in salt muds (inorganic salts) and it
indicates the presence of hydrocarbons in formation and the oil-water contact.
However, the redox log may not be run satisfactorily when the mud is rich in
organic substances such as oil in quebracho.
Example logs illustrate the paper together with example surveys of redox potential
measurements in exploration for oil and gas fields, for uranium and for native
sulfur ore bodies.