Can surface samples be used to infer underground permeability structure? A test case for a Neogene sedimentary basin in Horonobe, Japan

Abstract

We measured gas permeability of Neogene sedimentary rocks from Wakkanai, Koetoi and Yuchi Formations in the Horonobe area in northern Hokkaido, Japan. Measurements were done at room temperature and during pressure cycling to simulate underground conditions. Fresh intact samples from surface outcrops yielded a similar range of permeabilities as did intact samples from drill cores, within lithological variations. Comparison of laboratory and in-situ measurements has revealed that the permeability of Wakkanai Formation (middle to late Miocene, siliceous mudstone) from in-situ measurements is bounded by the permeability of brecciated specimen on the upper side and by the permeability of intact specimens on the lower side. Fragments in fault breccia are smaller than sample size and laboratory data should be applicable to natural fault rocks. Permeability of fractured specimens falls in between those upper and lower bounds, and complex scale issues will not be important in determining upper and lower bounds of permeability. The hydraulic conductivity values from in situ tests for Koetoi Formation (late Miocene to Pliocene, diatomaceous mudstone) are similar to those of the gas permeability of intact specimens. Effects of faults and fractures on permeability seems to be minor for this formation, and this was confirmed by permeability measurements with fractured samples. The hydraulic conductivity values from in situ tests for Yuchi Formation are lower than estimates from laboratory tests by within an order of magnitude. Thus permeability measurements in laboratory on fresh surface samples (intact, fractured and brecciated), combined with geological survey on the development of faults and fractures, leads to quantitative estimates, even if not complete, of underground permeability structures.