北海道天北地域における新第三系珪質岩中の炭化水素トラップ

Abstract

A possible mechanism of a hydrocarbon trap in the Neogene porcelanites in northern Hokkaido, Japan is discussed through petrographic and pore throat size analyses. In the studied two wells, quartzose porcelanites with lesser amount of clay and located at shallower depth have a higher reservoir quality associated with hydrocarbon shows and gas production. It indicates that matrix pores as well as fractures play an important role in the hydrocarbon trap in siliceous rocks. In the Yurihara Oil and Gas Field in northeastern Honshu, another field with quartzose porcelanites reservoirs in Japan, opaline porcelanites just above the silica mineral opal-CT/quartz transformation boundary act as cap rocks. In this studied area, however, the boundary is too shallow for opaline porcelanites to behave as cap rocks because the confining pressure is insufficient, and hydrocarbon traps are formed in the quartz zone substitutively; porcelanites with much clay minerals and smaller pore throat are cap rocks and those with lesser amount of clay minerals and larger pore throat form hydrocarbon reservoirs.