Page 217 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

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Case Study 6 Answer
The log package used in the well includes a Dual of coarse, sucrosic dolomite porosity. but rather the
Induction Spherically Focused Log (SFL*) with an SP log presence of vuggy porosity. You reach this conclusion
and a RR, curve (Fig. 99), and a Combination because you know that vuggy porosity has very low values
Neutron-Density log with a gamma ray log and caliper for irreducible water saturation, and therefore, also has very
(Figs. lOO and 101). low values for bulk volume water (see Chapter V1. Table 8).
On the Dual Induction Log, the R /R, quick look curve Conversely, because of higher bulk volume water values
XO!
has a strong deflection to the right, away from the SP log, when only intercrystalline porosity is present, you conclude
from 16,300 to 16,390 ft (Fig. 99). The rightward that the fine to medium grain sizes from 16,326 to 16,372 ft
deflection of the R,,/R, curve indicates hydrocarbons are (Fig. 102) represent a zone in the Chimney Hill Member
present. Further examination of the Dual Induction Log with predominantly intererystalline (i.e. sucrosic) porosity
shows resistivities decline precipitously at an interval from and only minor vugs.
16,325 to 16,370 ft (rig. 99). Such a rapid change to lower The low (less than 0.7) moveable hydrocarbon index
resistivities may be due to the presence of water or perhaps (S,/S,.) together with high moveable oil saturation
to a signficant change in lithology. percentages suggests hydrocarbons were moved by invasion
The SP curve in track #1 of the Dual Induction Log is of the mud filtrate. Relative permeability to water is low
used to find a value for Ry. The chart in Figure and varies from 0.06 to 0 (Fig. 103) and relative
13 helps you determine Ryy/Ry, from SSP; the permeability to gas (Fig. 104) is high with values ranging
value located on the chart is 12. Next. calculate a from greater than 20% to 100%. The relative permeability
value for R,, by the formula: Rye = Riy/(RmiRwe)- values for K,, and K,, are favorable indicators of
The resulting value for Ry, is 0.021. Then. you determine commercial production.
by the chart (Fig. 14) that Ry at Tis 0.025. Permeability (Fig. 105) averages 10 to 15 md and reaches
The caliper log on the Combination Neutron-Density Log a maximum permeability of 50 md. The bulk volume water
alerts you to the oveurence of invasion in the Hunton by crossplot (Fig. 106) confirms the presence of higher water
mudcuke development (Fig. 100). Another item of saturations due to a finer grain size and a lack of vuggv
significance interpreted from the Combination porosity, or the higher water saturation values may be
Neutron-Density Log is gas effected dolomite (see Chapter related to water saturation values above irreducible. where
IV). bulk volume water is greater than 0.015. Higher bulk
On neutron-density porosity logs recorded over a non- volume water values are from the zone occurring over an
gas-bearing dolomite, the neutron log will read a higher interval from 16,325 to 16,370 ft. This ts the interval which
porosity than the density log when the logs are run on a also happens to have the lower resistivities (Fig. 99). Bulk
limestone matrix. However, on this matrix through the volume water values of 0.015 or less (Fig. 106) are from
porous Hunton dolomite, both the neutron and the density zones in the Hunton with both vuggy and intercrystalline
logs record essentially the same porosity, with porosity porosity (see Chapter VI, Table 8).
values ranging from 6 to 10% (Fig. 100). These equivalent The upper zone from approximately 16,300 to 16.322 ft
porosity values can be explained by the presence of gas in of the Chimney Hill Member of the Hunton has several
the zone, because gas causes the neutron log to record too favorable indicators of a productive hydrocarbon zone.
low a porosity, and the density log to record too high a First, good permeability and porosity seem to be present.
porosity. The coming together of both the neutron and The porosity ranges from 6 to 10% and is intercrystalline
coarse-grained from 16.372 to 16,384 ft. The coarser grain support your decision to set pipe and to perforate the upper
and vuggy. There are low water saturations and low bulk
density curves, as noted in the Hunton dolomite, reflects a
volume water values; these, along with the high relative
gas effected dolomite (Fig. 100).
permeabilities to gas and the good reservoir permeabilities,
The density log (Fig. 101) has a high correction on the A,
curve from 16,304 to 16.386 ft. This high amount of
zone. The lower zone from 16,325 to 16.370 ft has higher
correction is the result of correcting the bulk density py, for
water saturations and higher bulk volume water values, and
elfect of mudcake.
therefore, it may be water productive. Because subsurface
Grain size, determined by a crossplot of water saturation
geology indicates the presence of downdip water, this zone
versus porosity (Fig. 102), over the porous Chimney Hill
might be avoided for perforating; however, the higher water
Member of the Hunton from 16,306 to 16,384 ft is:
coarse-grained from 16.306 to 16,324 ft; fine-grained to
from 16,325 to 16,370 ft may occur because of changes in
medium-grained from 16,326 to 16,372 ft: and
rock type. Evidence for a lithology change is the apparent
loss of vuggy porosity as indicated by the increase in BVW
size data points on the erossplot are probably not the result saturation and bulk volume water values over the interval

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