Evidence of gas hydrates beneath the continental slope, East Coast, North Island, New Zealand

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Abstract
Organic-rich, terrigenous clays (mainly Late Cretaceous to early Tertiary) are widespread in the East Coast Fold Belt (ECFB), North Island, New Zealand, where generation of hydrocarbons is proven extensively. From seismic evidence the same formations extend across the offshore part of the ECFB, underneath both the continental shelf and slope. Below 1000 m water depth, anomalous seismic reflections which (1) exhibit a high impedance contrast; (2) cut across bedding planes of sediments; (3) are subparallel with the seafloor; and (4) increase their subbottom depth with increasing water depth, can be identified with the base of a gas hydrate layer. These reflections have been found nearly continuously across 10–20 000 km2 of the continental slope. As the hydrate-cemented layer is impervious, free gas may be trapped in the sediments beneath it. This is supported by the anomalously high amplitude reflection that is observed, indicating a pronounced change in acoustic impedam;e. Free gas underneath the hydrate layer is also suggested from the widely observed attenuation of seismic reflections underneath the hydrate layer, which may be the result of velocity inversion caused by the presence of free gas. In either case, the evidence of gas hydrates beneath much of the continental slope indicates that hydrocarbon source rocks are extensively present in the region. Even where normal reservoir rock and trap-forming conditions may not be abundant, vast quantities of gas have been locked in through the formation of hydrates, which in turn may have trapped free gas underneath them. The distribution of gas hydrates suggests that gas has been generated in deeper levels and reached the hydrate stability field by way of normal migration towards structural highs. Gas hydrate distribution, as it appears, is widely controlled by the structure and tectonics of the region, and may thus assist in the correct interpretation of structural configurations.

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