Page 135 - Basic Well Log Analysis for Geologist

shale in a fornmiation deercases the SP response. is composed of clean carbonate relative to the non-bioherm
The alpha method can be extremely valuable in mapping facies, the gamma ray log can be used to map the bioherm
because it can allow you to more narrowly define desirable facies. The procedure for obtaining a clean carbonate
zones. Alpha values from nearby wells can be used to cut-off from a gamma ray log is described in Figure 49.
construct clean sand (high energy) maps (in effect, you are A gamma ray API value of 20 on the gamma ray log (Fig.
mapping iso-alpha values). 49) will represent clean carbonate with a volume of shale
To construct an Alpha map, first calculate the static (V.,) equal to or less than 5%. By drawing a vertical line on
spontaneous potential (SSP) that a sand would have, if it the gamma ray log equal to 20 API units (Fig. 49), the
was 100% shale-free and unaffected by bed thickness. The geologist can identify and map the clean carbonate (or
equation for SSP is: sand).
Figure 50 is an isopach map of clean carbonate for the
SSP = -K « log( Rin Ry) upper Mission Canyon Formation in Roosevelt County,
Where: Montana. Because the relationship between clean carbonate
SSP = static spontaneous potential and the crinoid-fenestrate bryozoan bioherm facies 1s
K = 60 + (0.133 X formation temperature} already established, the map (Fig. 50) delineates the
Ring = resistivity of mud filtrate at formation distribution of the bioherm facies. Clean carbonate niaps
temperature have also been used to map the Pennsylvanian banks
R, = resistivity of formation water at formation (bioherms) of north central Texas (Wermund, 1975).
tem perature
Rock Typing and Facies Mapping
The SSP must be calculated for the formation in each
well, so that variations in Ry, and Ry, can be corrected. An important contribution to subsurface analysis of
Next, determine alpha values by the method shown in carbonate rocks has been the attempt to establish
Figure 48. The alpha cutoff (50%, 75%, whatever) is relationships between log responses and carbonate facies.
arbitrary. but should be based on production histories in the Pickett (1977), Asquith (1979), and Watney (1979; 1980)
area. uscd crossplots to identify log response/rock type
The resulting alpha (@) map delineates clean sand relationships. Table 12 is a list of the crossplots applied by
environments. In the above example (in Fig. 48), the these authors.
greater alpha thickness for a given alpha cut-off (i.e. 75%
a, or 50% c) indicates a greater thickness of higher energy,
low-shale sandstones. Also, because the presence of shale Table 12. Types of Carbonate Rock Type
in a sandstone can cause a loss of permeability, an alpha Identification Crossplots (after Pickett, 1977; Asquith,
map is indicative of better reservoir conditions. 1979; and Watney, 1979 and 1980).

The probleni with alpha mapping from an SP log ts that At (interval transit time) vs. @x (neutron porosity)
SP response is decreased, not only by shale, but also by thin Py (bulk density) vs. @y (neutron porosity)
beds (<10 feet) and the presence of hydrocarbons (Chapter
Pp (bulk density) vs. At (interval transit time)
II). Bed thickness problems are minimized by making an SP R, (deep resistivity) vs. @y (neutron porosity)
log bed thickness correction (Chapter Ef). But, the SP log GR (gamma ray) vs. @x (neutron porosity)T
can’t be corrected for hydrocarbons. R, (deep resistivity) vs. @, (sonic porosity)

Formation in Roosevelt County, Montana. fn this interval of available from cores or cuttings in selected wells.
7 Watney (1979 and 1980) also uses neutron log readings
Clean Sand or Carbonate Maps from
measured in counts/sec.
Gamma Ray Log
The gamnia ray log can be used to map clean (shale-free)
sandstones or carbonates versus shaly sandstones and
‘To date, crossplots have been used to establish log versus
carbonates. Because shales are more radioactive than clean lithology relationships only when petrographic data is
sandstones or carbonates (Chapter V), when the percentage
of shale increases in these rock types, the gamma ray
Petrographic analysis from selected wells is essential to
reading also increases.
firmly establish rock type.
Figure 49 is from a Gamma Ray Neutron-Density Log
When establishing log/lithology relationships, log
through the Mississippian, upper Mission Canyon
analysis) are crossplotted. Next, areas that delineate
rock-type clusters are outlined (see Fig. 51) on the
the Mission Canyon Formation, crinoid-fenestrate bryozoan responses from control wells (i.e. wells with petrographic
bioherms are often devcloped. Because the bioherm facies crossplot. Finally, log responses from wells without cores or

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