Page 174 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

Case Study 2 Answer
The very complete log package run in this well includes on the density log (Fig. 63) as a decrease in bulk density
an electric log suite for resistivity measurements. and both a (py). Finally, they are identified on the sonic log by an
Combination Neutron-Density Log and a sonic log for Increase in the interval transit time (At).
porosity measurements. A Cyberlook* or computer A check of the Cyberlook® Log (Fig. 66) verifies that the
processed log is also part of the package. It is used fora lower Mission Canyon zones from 9,370 to 9,415 ft have
quick look examination of the well to identify zones with higher water saturations. You note in track #3 of the
low water saturations (i.e. possible productive zones). Cyberlook* Log an increase in water saturations with
Beeause the borehole has experienced some caving increasing depth.
problems, your assessment begins with a careful check of Further verification of water problems from a depth of
the caliper log (Fig. 61). The caliper log shows a relatively 9.370 to 9.415 ftcomes from a Pickett crossplot (Fig. 67).
constant hole diameter and no intervals of significant hole On the plot, data points with water saturations above 35%
enlargement due to washout. The constancy of the hole are mostly from lower zones
diameter means log measurements should be reliable. As you continue your evaluation of the Mission Canyon,
Your next evaluation step includes an examination of the you decide to compare your observations of the core
resistivity logs (Fig. 61). The salt saturated drilling mud lithologies with lithologies derived from log data. Your
(Ria = Ry) in the well has necessitated using a Dual study of the core indicates it is microcrystalline dolomite,
Laterolog* with a Microspherically Focused Log (MSFL*). limestone and anhydrite. To compare this information with
The MSFL* measures the resistivity of the flushed zone fog data, you construct a MID® plot. a neutron-density
(R,,). while the Laterolog* shallow (LLS) and Laterolog* porosity crossplot, and an M-N* plot. (Ordinarily you
deep (LLD) measure the resistivities of the invaded (Rj) and would probably construct only one of these lithology plots
uninvaded (R,) Zones, respectively. when evaluating a well, but all are presented here as a
You begin scrutinizing these resistivity logs to identify learning expericnce.)
invasion profiles. Invasion profiles help you locate zones By crossplotting (Atyy)y” Versus (Pm), on a MID* plot
which merit a more detailed analysis. (Fig. 68), you determine that the interval you are studying
Between depths of 9,308 and 9.415 ft. the resistivity has a matrix which varies from dolomite to dolonutic
logs —MSFL*, LLS. and LLD—read different values for limestone. And. because the average (At),), is 44.4 pesce/ft
R,,. Ry. and R.; and the curves separate. The curve (Fig. 69) and the average (py) is 2.82 gm/ce (Fig. 70). the
separation suggests that invasion has taken place and interval has an average lithology of limey dolomite.
hydrocarbons are present in porous and permeable zones The neutron-density crossplot (Fig. 71) shows porosities
occurTing intermittently over the interval.* However, you varying from 4 to 17%. The clustering of points between
note that the lower porosity zones from 9,370 to 9,415 ft dolomite and limestone supports a judgement that lithology
have less separation between the Microspherically Focused isa limey dolomite.
Log* (MSFL* reading R,,: see Fiz. 61) and the deep An M-N®* plot (Fig. 72) suggests the presence of
Laterolog* (LLD reading R,: see Fig. 61). The lessening of secondary porosity because many data points are plotted
separation in the lower zones indicates higher water above the caleite-dolomite lithology tie-line. Once more,
saturations (remember: higher water saturations mean lower like the MID* and the neutron-density crossplots, the M-N*
hydrocarben saturations). plot indicates a lithology varying from dolomite to
Porous and permeable zones. which occur intermittently dolomitic limestone.

over the interval from 9,308 to 9,415 ft, are identified by Another crossplot (Fig. 73) ts useful for establishing
analyzing the Combination Neutron-Density Log (Fig. 62). grain size. A plot of water saturation (S,) versus porosity
the density log (Fig. 63), and the sonic log (Fig. 64). () shows grain size variations from coarse-grained to
Approximately eleven different porous and permeable fine-grained. However, data which cluster in the area of
zones can be identified on these logs from a depth of 9,308 coarse or larger grain sizes probably don’t reflect the grain
to 9.415 ft. On the Combination Neutron-Density Log (Fig. size of the intererystalline porosity, Rather, this data
62), the zones of porosity and permeability are seen by an clustering in the larger size areas may, instead, reflect
increase In both neutron and density porosity. They appear vuggy porosity, Data which cluster above very fine-grained

TThe value for (Atmala is obtained by crossplotting (Fig. 69) interval transit

7Of course, the reverse would be true in a salt saturated mud system if all time (AQ) with neutron porosity (dN). A crossplot (Pig. 70) of bulk density
three resistivity curves—MSFL*, LLS, and LLD—had essentially the (pp) Versus neutron porosity (PN) provides a value of (pmada. See Chapter
same Vidues and separation did not occur. You would then conclude either VIP. or in tthe book. Log farerpretation Mantal Applications,
Invasion hadn‘L occured or hydrocarbons weren‘t present. ‘Schlumberger, 1974).