Physical Properties of Shales and Possible Origin of High Pressures
Authors: Pritchett William C. 
Published: December 2007

Laboratory and borehole measurements of shale properties may be unreliable because of modification during or after drilling or coring. The borehole gravimeter is an ideal tool for measuring the bulk density of thick shale units because of its great depth of investigation and negligible sensitivity to shale in the vicinity of the borehole, which may have been modified in drilling. By contrast, the scatter gamma ray density log has an extremely shallow depth of investigation and its response may be dominated by modified shale surrounding a borehole. A comparison of bulk densities measured by these two methods in both sands and shales was made in three U.S. gulf coast wells. In all three wells the two methods yielded comparable densities for the sands. But in two wells, which were drilled with fresh muds, the density log yielded shale bulk densities significantly less than those shown by the borehole gravimeter. These data indicate that shale adjacent to the borehole in these two wells had been modified by drilling and the modified shale densities had been reduced significantly. In the well drilled with a saline mud, bulk densities from the two methods were in close agreement in both sands and shales, which indicates that shales adjacent to the borehole in this well were not modified significantly.

High-pressure shales are particularly susceptible to modification during drilling since they are relatively permeabble and soft. Sometimes they even flow. Density log data in high-pressure shales are unreliable due to probable shale modification. Unfortunately, we have no borehole gravimeter data in high-pressure shales and, therefore, no reliable measurements of bulk densities in their natural state. The best data available for studying high-pressure shales are some neutron lifetime logs. The effective depth of investigation of the neutron lifetime log is considerably greater than that of the scatter gamma ray density log.1,2 Therefore, neutron lifetime log measurements should be affected much less by any modified shale near the borehole than would measurements made with a density log.

Neutron lifetime logs show a progressive decrease in macroscopic neutron capture cross section with increasing depth for U.S. gulf coast shales with normal pore pressures. This decrease in neutron capture cross section is the result of increased compaction of shales with depth. If high-pressure shales are shales that were not compacted normally with increasing depth of burial, then they should have physical properties comparable with relatively uncompacted shales at much shallower depths of a few thousand feet (1000 to 1500 m). In particular, they should have high neutron capture cross sections as compared with normally pressured shales at slightly shallower depths. However, our logs show neutron capture cross sections for U.S. gulf coast high-pressure shales that range from normal to less than normal for their depths. These data indicate that these high-pressure shales are not shales that were never compacted. The data are consistent with an alternate hypothesis for the generation of high pore pressures in shales.