Page 55 - Basic Well Log Analysis for Geologist

TEE DVUONTANEOUS POTENTIAL LOG

value for Ry, calculated from SSP. Therefore, to determine Review - Chapter I
R,, from SP it is best, whenever possible, to use the SP
1. The spontaneous potential log (SP) can be used to: (1)
curve opposite Anown water-bearing zones.
detect permeable beds; (2) detect boundaries of permeable
beds; (3) determine formation water resistivity (Ry); and (4)
Volume of Shale Calculation
determine volume of shale (V,,) in a permeable bed.
The SP log can be used to calculate the volume of shale in 2. The variations tn the SP are the result of an electric
a permeable zone by the following formula: potential that is present between the well bore and the
formation as a result of differences in salinities between Rr
Vo, (in Ye) = 1.0 — np and R,.
3. The SP response in shales is relatively constant and its
Where: continuity of amplitude is referred to as the shale baseline.
Vo. = volume of shale In permeable beds the SP will do the following relative to
PSP = pseudo static spontaneous potential (SP of the shale baseline: (1) negative deflection to the left of the
shaly formation) shale baseline where Ry > Ry; (2) positive deflection to
SSP = Static spontaneous potential of a thick clean the right of the shale baseline where Ry < R,,; (3) no
sand or carbonate deflection where Ry = R,.
4. The SP curve can be suppressed by thin beds.
SSP = —K & log (RyRy) shaliness, and the presence of gas.
ll
K = 60 4+ (0.133 * T,)
The volume of shale in a sand can be used in the
evaluation of shaly sand reservoirs (Chapter VI) and as a
mapping parameter for both sandstone and carbonate facies
analysis (Chapter VIT).

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