Abstract
Analyzing the development of anomalous high fluid pressure is important for basin analysis and fluid-oil migration analysis at depth. We measured the permeability and the porosity of sedimentary rocks in the Western Foothills of Taiwan where the 1999 Chi-Chi earthquake occurred. Permeabilities of all rocks decreased with increasing Pe (effective pressure), and all of them showed hysteresis. Permeability was generally higher in sandstone (10-15 10-18 m2) than in siltstone (10-1710-20 m2). Sandstones were divided into two groups. One group showed little sensitivity to Pe and the other group decreased moderately with increasing Pe. Porosity also decreased with increasing Pe and showed hysteresis. Porosities decrease to around 4-8% during compaction to 200 MPa. Specific storage decreased with Pe and the values changed from 10-8 Pa-1 to 10-10 Pa-1 at the 200 MPa of Pe.
The fluid pressure distribution of the Western Foothills was calculated by the non-linear one-dimensional compaction flow model in sedimentary basins (Bethke and Corbet, 1988) using the experimental results of permeability and specific storage as a function of Pe. The result indicated that abnormal fluid pressure was generated from 4 km and increased with depth, showing a similar trend to published in-situ data. However, the results calculated from the simplified compaction flow model that permeability and specific storage can be assumed to be constant at these depths showed a strong difference with in-situ data. This indicates that we can not neglect pressure sensitivities to hydraulic properties when we construct a fluid flow model of a basin at depth.
