Vertically Clustered Amplitude Anomalies: Evidence for Vertical Gas Migration in Heterogeneous Sequences

Foschi, Martino¹; Cartwright, Joseph A.¹; Peel, Frank J.²
¹School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom.
²BHP Billiton, BHP Billiton Petroleum, Houston, TX.

We describe a series of unusual DHIs that are characterized by vertical stacking of individual layer/fault bounded amplitude anomalies. More than 50 ‘clusters' of amplitude anomalies have been mapped on 2D seismic data over an area of 280 by 220km of the East Falkland Basin. Each cluster consists of a series of vertically stacked individual reflections that are strongly amplified compared to their ‘background' value. The enhanced amplitude response extends laterally for 100s to 1000s of metres with sharp cut-offs of amplitude at the margins of the anomaly. The amplitude anomalies only in a few cases conform to structure: more typically they occur in homoclinally and gently dipping strata, with no obvious signs of structural or stratigraphic trapping. Attenuation is observed beneath the clusters, and velocity push downs are also seen. Polarity reversal is not observed. The characteristic signature is a simple amplification of an acoustically soft reflection. AVO analysis shows that these anomalies have a Class II response, consistent with the probable presence of gas. The vertical clusters are therefore interpreted as a novel type of direct hydrocarbon indicator. The stratigraphic interval hosting the stacked anomalies is a c.1000m thick pelagic-hemipelagic sedimentary unit characterised by a high continuity, low-moderate amplitude seismic facies. We recognize two end-members: (1) fully stacked DHIs, in which each reflection in the host interval is amplified, and (2) partially stacked DHIs, where only a subset of reflections is amplified. The sharply defined lateral boundaries of the stacked anomalies allow a simple cross-sectional classification of the stacking patterns into triangular, rectangular and oval shapes. Importantly, the shape of the stacking pattern implies clear communication from layer to layer within the stack, but local controls on the geometry such as minor faults and other discontinuities/heterogeneities can perturb what appears to be a systematic arrangement. These extraordinary vertically stacked DHIs are interpreted to result from sequential upward filling of gas within a set of thin, relatively high permeable layers interbedded in the dominantly fine grained hemipelagic succession. The differences between fully and partially stacked clusters may reflect different gas fluxes or subtle differences in sediment physical properties. Short-range vertical migration between layers is likely achieved via small faults and fractures.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012