Page 118 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION

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sandstones. but also occur in limestones and dolomites. Where:
Whenever shale is present in a formation, all the porosity Psonic = SOnic log derived porosity corrected for shale
tools (sonic, density, and neutron) will record too high a Ato, = interval transit time of formation
porosity. The only exception to this is the density log. It will Atha = interval transit time of the formation’s matrix
not recorc too high a porosity if the density of shale is equal (Table 6)
to or greater than the reservoir’s matrix density. Also, the Aty = interval transit time of fluid (189 for fresh mud
presence of shale in a formation will cause the resistivity log and 185 for salt mud)
to record too low a resistivity. Hilchie (1978) notes that the Aty, = interval transit time of adjacent shale
most significant effect of shale in a formation is to reduce Vo, = volume of shale
the resistivity contrast between oil or gas, and water. The
Density Log (Dresser Atlas, 1979):
net result ts that if enough shale is present in a reservoir, it
may be very difficult, or perhaps impossible, to determine if
pen _ (fe Pr _ Var Ce i”)
a zone is productive. Hilchie (1978) suggests that for shale
Pima ~ Pr Pina ~ Pr
to significantly affect log-derived water saturations, shale
Where:
content must be greater than 10 to 15%.
Vs, = volume of shale
Remember that all shaly sandstone formulas reduce the
pen = density log derived porosity corrected for shale
water saturation value from the value that would be
Pma = Matrix density of formation
calculated if shale effect was ignored. However, this
Pp» = bulk density of formation
lowering of water saturation can be a problem in log
py = fluid density (1.0 for fresh mud and [.1 for salt
evaluation, because. ifa geologist overestimates shale
mud)
content, a water-bearing zone may calculate like a
Psp = bulk density of adjacent shale
hydrocarbon zone.
The first step in shaly sand analysis is to determine the Combination Neutron-Density Log (Schlumberger, 1975):
volume of shale from a gamma ray log (see Chapter V).
Volume of shale from a gamma ray log is determined by the Vion
obs, corr oby a (Seer) x 0.30
chart (Fig. 38) or by the following formulas (Dresser Atlas,
1979):
Op won = bp ~ (ae )s O13 « Vy
0.45
Older rocks (consolidated):
—
+ bp cor
Vy, = 0.33 [22 < Iex) — 1.0] np = ON corr ee
fo
2.0
Tertiary rocks (unconsolidated):
Where:
Vy, = 0.083 [23.7 « Ica) — 1.0]
Nn corr = Neutron porosity corrected for shale
Where: peor = density porosity corrected for shale
Vy, =volume of shale Va, = ~volume of shale
Oy clay = Neutron porosity of adjacent shale
Iop =gamma ray index
dy; = neutron porosity uncorrected for shale
lor = aa GRinin bp = density porosity uncorrected for shale
Rinax a GRnin ox. == neutron-density porosity corrected for shale
“ons commonly used shaly sand equations are:
Where: Next, after the volume of shale has been determined and the
GRay, =gamma ray maximum (shale zone)
log derived porosity has been corrected for volume of shale,
GRopin =Zamma ray minimum (clean sand)
the water saturation can be calculated. Three of the more
GRjg
==gamma ray tog (shaly sand)
(Simandoux, 1963):
After the volume of shale (V,,) is determined, it can then
-~ py.
yy
be used to correct the porosity log for shale effect. The O.4™&« R, Vin A vs] : Sh?
-
be
Ray / \ Ru
formulas for correcting the sonic, density, and Combination Sy = [eS] x fet SOP) te - Rx Ry
Neutron-Density logs for volume of shale are: (Fertl, 1975: where a = 0.25 Gulf Coast; a = 0.35 Rocky
Sonic Log (Dresser Atlas, 1979): Mountains):
.
Mtoe
=
bag = (Mer Aba x 100.) y, (Atm Sw = i “VW [ REMY i] 80)
Mo
Ate — Atma
At,
Ata
>
—
Ate
A
+
R t