Abstract
Compressibility at hydrostatic pressure up to 2000 bars were studied on dry samples of about 50 kinds of rocks collected from both surface exposures and exploratory wells in Japanese oil bearing areas. The results indicate that at porosity less than about 30% (for argillaceous rocks) or 15-20% (for arenaceous rocks), compressibility changes regularly according to porosity, while at more than the above porosity compressibility becomes as large as that of liquid. Meantime, porosity of the cores from recent deep drillings in centre of Niigata sedimentary basin were studied.
In conclusion, following three stages of compaction were recognized. Stage 1: The mineral grains within the rocks are not touched each other. The rocks behave like soil and physically rather liquid-like. Stage 2: The mineral grains come into contact. The rocks consist of the mechanically stable framework of these grains. Physically, they are plastic solid. Stage 3: Authigenic minerals appear among the mineral grains. Cementation is common in every part of the rocks.
In argillaceous rocks, primary porosity is around 80%, and from 80 to 30% belongs to stage 1. At 5 to 15% porosity, it changes from stage 2 to stage 3. In arenaceous rocks, primary porosity is approximately 50%. Stage 1 is considered to be between 15 or 20% and 50%. From 15 or 20% it becomes stage 3. Stage 2 is possibly absent in arenaceous rocks.
