ABSTRACT

We describe a spectrum of seismic high-amplitude anomalies from the Great South and Canterbury Basins (offshore New Zealand) that have many characteristics that make them distinct from previously described hydrocarbon-related amplitude anomalies. We propose a new classification scheme that is based on the specific vertical stacking of the anomalies and thereby reflects their genetic interrelationships. We demonstrate by combining amplitude versus offset and other attribute analyses that the anomalies are the product of gas migration across thick sequences of low-permeability sediments and identify specific units where migration is focused through discontinuities (e.g., faults) and storage units where the gas spreads laterally. Based on our observations we argue that fluid flow phenomena in which Darcy flow must have occurred can be hosted within low-permeability layers that would normally be regarded as high-quality sealing sequences. The documentation of a wide range of gas-related anomalies developed in a relatively uniform lithostratigraphy and in similar basinal contexts allows us to infer a migration sequence based on the morphostructural and geophysical characteristics of the anomalies. We suggest that the shape of the composite anomalies is directly controlled by gas flux and by probably quite subtle variations in physical properties of the host sediments. The distribution of the anomalies is generally correlated with maximum burial regions of the most prospective source rock intervals that are currently in the gas maturation window. This spatial coincidence strongly suggests that the anomalies result from vertical migration of thermogenic gas from these underlying source kitchens.

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