Page 136 - Basic Well Log Analysis for Geologist

LITHOLOGY LOGGING AND MAPPING TECHNIQUES

cuttings are added to the crossplot. The carbonate rock type wells. This is very important in subsurface facies mapping
and depositional environment of wells without petrographic because of the difficulty in obtaining cores and cuttings
analysis can then be determined by the cluster in which each from every well in an area. Also, because petrographic
occurs on the crossplot chart (see Fig. 51). analysis of every well is unnecessary, you can save a great
In Figure 51, the solid black circles and squares represent deal of time.
data from wells where petrographic analysis was used to However, it should be emphasized that petrographic
determine carbonate rock type and depositional analysis of cores or cuttings from control wells is an
environment. The open circles represent data from a well essential first step to firmly establish the rock-type cluster
without petrographic analysis. The carbonate rock types and used in the crossplots.
depositional environments were determined by the cluster in
which the open circles were plotted (Fig. 51).
Review - Chapter VII
Figure 52 is a crossplot of deep resistivity (R,) versus
sonic porosity (,) for the Lower Permian, Council Grove 1. A Combination Gamma Ray Neutron-Density Log
B-zone in Ochiltree County, Texas. Clusters for the three can be used to determine lithology when a limited number
carbonate rock types (oolite grainstone, oolitic wackestone, of rock types are present.
and argillaceous btoclastic wackestone) were established by 2. Where lithology is more complex, a sonic log plus a
petrographic analysis of cores and cuttings (open circles). Combination Neutron-Density log are both necessary to
The solid circles represent data from wells with only log construct M-N* or MID* lithology tdentification crossplots.
control. Figure 53 is a facies map of the Council Grove 3. The spontaneous potential (SP) and gamma ray logs

B-zone based on the percentage distribution of the three can be used to map shaly versus non-shaly carbonates or
carbonate rock types established by the resistivity/sonic sandstones.
porosity crossplot (Fig. 52), 4. Crossplotung of multiple log reponses can be used to
The advantage of log crossplot techniques 1s that they establish relationships between log responses and rock
maximize use of available information. Cores and cuttings types, provided some petrographic data from cores or
are required from only a few control wells rather than all cuttings ts available.