An Overview: Special Issue on ^|^ldquo;The Slow Subsurface Fluid Flow Processes (Part II): Field Examples and Laboratory Experiments^|^rdquo;

Abstract

Volume 121, Number 1, of the Journal of Geog­ raphy (Chigaku Zasshi) was a Special Issue dedicated to “The Slow Subsurface Fluid Flow Pro ce sses (Part I): Theory, Model, and Physical Properties Estimation.” The current issue is Part II of the Special Issue Series. Here, field examples and laboratory experimental results are presented. As summarized in the overview of Part I, the theory of subsurface fluid flow has been developed and established based mainly on studies of groundwater flow through aquifers and fluid flow processes through reservoirs in oil and gas fields. On the other hand, extremely slow fluid flow processes have become important to address some topics in the subsurface. For example, a structural closure is crucial for the formation of a petroleum field, and in order for the structural closure to function properly, the existence is required of both geological and/or sedimentological structures to prevent significant leaks of oil and gas, and rocks that are tight enough to act as seal rocks encasing a reservoir in a field. The production of a variety of wastes from a highly developed human society also forces us to study the characteristics of subsurface environments. For example, geological disposal of high-level radioactive waste basically expects subsurface environments to behave as long-term barriers to avoid unnecessary exposure of human society to radionuclides (i.e., Yoshida, 2012). Slow fluid flow is also important for understanding geological processes. It is well known that overpressure is often discussed with respect to large-scale thrusting (e.g., Rubey and Hubbert, 1959). The importance of the degree of overpressure on the shape of an accretionary wedge has also been discussed (e.g., Davis et al., 1983). In Part I of this Special Issue, the behavior of geofluids at subduction zones was presented (Iwamori and Nakamura, 2012), and seismological processes along subduction zones (Hasegawa et al., 2012) were studied in relation to the generation and migration of fluids in the deep subsurface. Other examples include significant overpressure generation and related geological phenomena reported off the Mississippi delta (e.g., Flemings et al., 2008), which were explained by the rapid deposition of low permeability muddy sediments. In Part II of this Special Issue, case studies both from field examples and laboratory studies are presented to show new ideas and highlight remaining issues. The main fields treated in Part II relate to petroleum exploration, evaluation of longterm safety of geological environments, and labo-