Microscopic character of marine sediment containing disseminated gas hydrate examples from he blake ridge and the middle america trench

摘要

The presence of disseminated gas hydrate was inferred based on pore fluid geochemistry and downhole ogging data, but was rarely observed at Ocean Drilling Program (ODP) Leg 164 (Blake Ridge), and Leg 170 (Middle America Trench, offshore from Costa Rica) drilling sites. Gas hydrate nucleation is likely to occur first in larger voids rather than in constricted pore space, where capillary forces depress the temperature-pressure stability field for gas hydrate formation. STraditional macroscopic descriptions of sediment fail to detect the microscopic character of primary and secondary porosity in sediment hosting disseminated gas hydrate. Light transmission and scanning electron microscopy of sediments within and below the depth of gas hydrate occurrences reveral at least four general types of primary and second-ary porosity: (1) microfossils (diatoms, forminifera, and spicules) void of infilling seiment, but commonly containikng small masses of pyrite framboids; (2) infauna burrows filled with unconsilidated sand and or microfossil debris: (3) irregularly shaped pods of nonconsolidated frambodial hydrite; and (4) nonlithified volcanic ash.