Alberto Khatchikian
Schlumberger Atlantic
Buenos Aires, Argentina
ABSTRACT
The evaluation of the reservoir parameters - effective porosity, water saturation, clay content of oil-bearing igneous rocks with logs is difficult due to the mineralogical complexity of this type of rocks. The author proposed in 1973 a modified shaly sand method to evaluate tuffs and tuffaceous sandstones in the Golfo San Jorge basin, Argentina, and defined the logging parameters for high and low density tuffs. Since then, several papers on the log responses
and evaluation of igneous rocks have been published, most of them in relation to geothermal wells.
This paper presents log evaluation examples in some igneous rock oil fields of the Neuquen basin in Western Argentina: a rhyolite/rhyolite tuff/shale sequence, a diabase intrusion and an andesite-dacite-zeolite tuff sequence. In each case, the type of igneous rock is recognized from the logs and crossplots and the evaluation is made using the complex lithology GLOBAL computer program. It is shown that the log responses depend on the type of rock (silicic or basic),
the degree of glassiness and the degree of alteration to zeolites. The results are compared with the production from the perforated intervals. Although no oil-bearing basalt was studied, an example of a shallow basalt intrusion is also presented.
It is thought that the log responses for the igneous rocks studied should be similar in other regions and that the evaluation method proposed is of general application.