Page 144 - Basic Well Log Analysis for Geologist

LITHOLOGY LOGGING AND MAPPING TECHNIQUES

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Figure 48. Determining Alpha (a) from an SP log. Two different cutoffs are demonstrated: 50% alpha (cvs) and 75% alpha (c5)-
The alpha percentage is determined as an inverse function of shaliness (100% alpha is shale-free; 50% alpha is 50% shaly).
Given: You must first determine SSP (see text for formula). Ry = 0.83 at 120°F Ry = 0.04 at 120°F, Ty = 120°F
Procedure:
1. By formula, we determine that SSP = — 100mv. Plot a scale of 1OOmv on the SP Jog. using the shale baseline
as the zero point; then use the SP scale to establish the value of your scale increments (in this case, cach
increment is 20mv).
2. In this exercise, a thickness and depth will be determined for both asy and a75, so draw vertical lines through
your SSP scale approximately halfway (50%) and three-quarters of the way (75%) across, and drop your
vertical lines to intersect the SP curve at the desired depth range.
3. From the intersections, follow the values horizontally to the depth scale on the log. From this log-depth scale
you can count depth-increments to determine alpha thickness. as well as the top and bottom boundaries of the
given alpha zone.
In this example a7s is the thinner of the two, and measures 20 ft, from approximately 5,935 to
5,955 ft; whereas a@sq is thicker and measures 39 ft, from approximately 5,931 to 5,970 ft.

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