秋田・山形地域の坑井における女川層と岩相と有機炭素量

Abstract

Subsurface geology of the Onnagawa Formation, in the Akita-Yamagata Sedimentary Basin, Northeastern Honshu, Japan, is studied using well logs and geochemical data. This study includes the distribution of lithology and total organic carbon content.Gamma ray (GR) log is a useful indicator of detritus content. Rocks with lower GR value contain less detritus whereas those with higher GR value contain more detritus. In the Ayukawa, Yurihara and Fukubezawa fields, oil and gas reservoirs are composed of porcelanite, chert and dolomite, which contain little detritus. They are characterized by low GR and high resistivity on the log (they will be called ‘LG/HR rocks’ hereafter). It is also observed from the ordinal source rock analyses using cutting samples that rocks with lower GR value tend to have higher organic carbon content.Reservoir-forming LG/HR rocks, which also have high potential as source rocks, mainly occur in the middle part of the Onnagawa Formation. It is observed that they develop in the areas where the formation is moderately thick and contains less amount of tuff.In the areas where basaltic rocks occur in the uppermost part of the underlying Nishikurosawa Formation, we observe the Onnagawa Formation is thin if we exclude volcanic and intrusive rocks. These areas are considered to have formed a palaeogeographic high. It indicates that deposition should be restricted in the latest Nishikurosawa and the early Onnagawa Stages.In the Onnagawa Formation, the higher average total organic carbon content is observed in the areas where LG/HR rocks content is high. High average total organic carbon content is also observed in their surrounding areas where the Onnagawa Formation is relatively thick.