常磐沖堆積盆における下部中新統のシーケンス層序解析  [in Japanese] Sequence stratigraphic analysis of Lower Miocene strata in the Offshore Joban Basin, Northeast Japan  [in Japanese]

常磐沖堆積盆において, 坑井及び震探データを用いて下部中新統のシーケンス層序解析を行い, 堆積環境の変遷を探った。その結果, 下部中新統において下位よりDS-C2〜C6の5つの堆積シーケンスを認定した。DS-C2は陸成から汽水成の堆積物を主とし, DS-C3〜C5は沿岸成から浅海成の海進期堆積体と高海水準期堆積体の繰り返しからなる。DS-C6は汽水成層に始まって外側陸棚の環境に至り, また浅海成層に戻る大きなサイクルを示す。DS-C6の最大海氾濫面付近から石灰質ナンノ化石群集のNN4帯に相当する微化石を得ており, DS-C6は陸域の高久層群に対比される。これらの堆積シーケンスは各々第3オーダーサイクルであり, 下部中新統全体でより低次オーダーの堆積シーケンスを構成するとともに, 時代とともにデポセンターがより陸側に移動した。石油地質学的には, 各堆積シーケンスの海進期堆積体の泥岩が帽岩となり, その下位の砂岩が貯留岩となっている。

A sequence stratigraphic analysis of the well and seismic data for the Lower Miocene strata in the Offshore Joban Basin has enabled the identification of a series of correlatable sequence boundaries and maximum flooding surfaces. The Lower Miocene strata can be subdivided into five regionally correlatable depositional sequences; DS-C2 to-C6 in ascending order. The DS-C2 is made up of continental to paralic deposits. The DS-C3 to-C5 are composed of coastal to shallow marine transgressive deposits and progradational highstand deposits. The DS-C6 is a large sedimentary cycle which begins from paralic deposits, goes through outer shelf deposits of a maximum nooding surface, and then ends in highstand progradational shallow marine deposits. The DS-C6 can be correlated to the onshore Takaku Group in the Joban Coal Field, because the NN4 zone equivalents of the calcareous nanno-fossil assemblages were found around the maximum nooding surface in the DS-C6. These depositional sequences are identified as third-order cycles of Vail et al. (1991), and constitute a lower-order large cycle through out the Lower Miocene. In the lower-order cycle, the DS-C2 is a lowstand systems tract, the aggradational sequence-set of the DS-C3 to-C5 and the retrogradational part of the DS-C6 are a transgressive systems tract, and the progradational part of the DS-C6 is a highstand systems tract. Depocenters of each depositional sequence shad moved from east (sea-ward) to west (land-ward) through the time. These sequence stratigraphic cycles control the gas reservoir/seal distribution in the Lower Miocene in this area, and the reservoirs and seals are recognized in the transgressive systems tracts in most cases.