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Application of Log Interpretation Methods to Evaluate Heterogeneity in Diatomaceous Mudstone at Horonobe Area

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原 彰男; 辻 隆司*; 星 一良*; 八木 正彦*; 西村 瑞恵*; 川田 耕司*; Hou, J.-Y.*

Hara, Akio; Takashi, Tsuji,*; Hoshi, Kazuyoshi*; Yagi, Masahiko*; Nishimura, Mizue*; Kawada, Koji*; Hou, J.-Y.*

幌延地域における堆積岩の不均質性を評価するため、自然$$gamma$$線検層とelectrical micro imaging(EMI)検層に着目し、堆積岩の物理特性と岩相の違いとの関係を把握するとともに、孔井対比を行い堆積岩の不均質性の空間分布の把握を試みた。本研究の結果、層理面が明瞭に判読できる堆積相AおよびBは自然$$gamma$$線量(以下GR値)が低い層準に発達し、かつSiO$$_{2}$$含有量が多くなる傾向にあるのに対し、層理面が不明瞭な堆積相DはGR値が高い層準に発達し、SiO$$_{2}$$含有量が少なくなる傾向にある。本解析結果は、原(2004)の指摘、すなわち、GR値の高い地層はAl$$_{2}$$O$$_{3}$$の含有量が高く粘土鉱物など陸源性の砕屑物に富むのに対し、GR値の低い地層はSiO$$_{2}$$の含有量が高く珪藻化石に富むことを支持している。また、自然ガンマ線検層およびEMI検層データに基づく堆積相について、孔井間対比を行うことにより、堆積物の堆積作用や地層の空間分布を推定する上で有意義な層厚分布の情報を得ることができた。

To evaluate heterogeneity of diatomaceous mudstone in Horonobe area, physical properties and lithofacies of sedimentary rocks have been estimated from natural $$gamma$$ ray log data and electrical micro imaging (EMI) log data. Spatial distribution of heterogeneity of sedimentary rocks has been discussed by comparison of log data acquired from several boreholes. Five sedimentary facies, A to E, have been established. Sedimentary facies A and B, which can decipher bedding planes clearly in EMI image, have been developed horizontally with low $$gamma$$ ray and high SiO$$_{2}$$contents while sedimentary facies D, bedding planes are not clear to decipher, have been developed horizontally with high $$gamma$$ ray and low SiO$$_{2}$$contents. The results of this study support the estimation of Hara (2004) namely the strata which characterized with high $$gamma$$ ray have been high Al$$_{2}$$O$$_{3}$$contents and rich in terrigenous sediments such as clay minerals, on the other hand, the strata characterized with low $$gamma$$ ray have high SiO$$_{2}$$contents and rich in diatomaceous fossils. Layered sedimentary facies can correlate between boreholes based on natural $$gamma$$ ray log and EMI log analysis. Sedimentary system and spatial distribution of strata of Horonobe area can estimate from the results of the facies correlation.

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