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Lithology, Biostratigraphy, and Magneto stratigraphy of Gas Hydrate‐Bearing Sediments in the Eastern Nankai Trough

2004; Wiley; Volume: 54; Issue: 1 Linguagem: Inglês

10.1111/j.1751-3928.2004.tb00184.x

1751-3928

Yoshihisa Hiroki, Kazue Watanabe, Ryo Matsumoto,

Hydrocarbon exploration and reservoir analysis

Abstract: Stratigraphic controls on the formation and distribution of gas hydrates were examined for sediments from a BH‐1 well drilled in the landward slope of the Nankai Trough, approximately 60 km off Omaezaki, Japan. Three lithologic units were recognized in the 250 m‐thick sequence of sediments: Unit 1 (0–70 mbsf) consists of calcareous silt and clay with thin volcanic ash layers, Unit 2 (70–150 mbsf) consists of calcareous silt and clay with volcanic ash and thin sand layers, and Unit 3 (150–250 mbsf) consists of weakly consolidated calcareous silt and clay with thick and frequent sand layers. Soupy structures and gas bubbles in the sediments indicate the presence of two hydrate zones between 40 and 130 mbsf and below 195 mbsf. Nannofossil biostratigraphy and magnetostratigraphy indicate that the sequence recovered at the BH‐1 well is mostly continuous and represents sediments deposited from 0 to 1.5 Ma. Calculation of the sedimentation rate reveals a condensed section between 65 and 90 mbsf. The inferred distribution of gas hydrates in the BH‐1 well appears to be strongly controlled by the stratigraphy and lithology of the sediments. Thick, gently inclined sand layers in Unit 3 provide a conduit for the migration of gases from deeper regions, and are considered responsible for the formation of the hydrate zone below 195 mbsf. At shallower levels, thin, gently inclined sand layers are also considered to allow for the migration of gases, leading to the formation of the upper hydrate zone between 40 and 130 mbsf. The overlying sub‐horizontal silt and clay of the condensed section, truncating the underlying gently inclined sand and silt/clay layers, may provide an effective trap for gases supplied through the sand layers, further contributing to hydrate formation in the upper hydrate zone.