Quantitative analysis of anomalous seismic amplitudes caused by fluid migration

Two- (2D) and three- (3D) dimensional pre-stack and post-stack seismic reflection data are used to investigate the processes which have led to the development of amplitude anomalies on reflections in the faulted, Cenozoic overburden on the Laminaria High, Northwest Shelf of Australia. The integration of amplitude and seismic attribute maps for four key horizons (the seabed, Horizon H9, Horizon H10 and Horizon H13) with the corresponding two-way time (TWT) structure maps has identified the structural controls on the distribution of seismic anomalies. On the seabed, the main anomaly is located on the up-dip side of the fault trace, and is elongated parallel to the local time structure contours. These observations are consistent with the anomalies having developed in response to structurally-controlled fluid seepage along, and up-dip migration away from the fault trace. Amplitude anomalies associated with the deeper H9 reflector are also located adjacent to fault traces but are discordant to the local time structure contours. This observation suggests that the anomalies may be due to cemented hardgrounds that formed due to seepage when the faults intersected the palaeo-seafloor but were subsequently buried and deformed during ongoing sedimentation and fault growth/linkage. Reprocessing of the 2D and 3D seismic pre-stack data supports the seismic interpretation of amplitude anomalies at the seabed. It is concluded that these anomalies are robust – that is, they are likely to reflect geological processes and are not simply a function of the chosen seismic processing workflow – and are caused by localised changes in acoustic impedence in the subsurface. More important is that using processed data without the knowledge of the background processing sequence for the data could be an issue in any 2D or 3D seismic interpretation. For this reason the veracity of processing of any seismic data needs to be questioned, and should not be taken for granted especially if different surveys produce conflicting interpretations. 2D hydrocarbon migration modelling combined with fault slip- and dilation-tendency analyses were undertaken in order to investigate the impact of faults and host-rock lithologies on hydrocarbon seepage at the present-day sea floor. Results show that some active faults associated with amplitude anomalies (e.g. Fault F10) are critically stressed, assuming a static, and spatially homogeneous regional stress field. However, other faults associated with amplitude anomalies (e.g. Fault F11) appear not to be critically stressed. These results suggests that the “regional” stress field could, in fact, vary spatially and temporally allowing faults in different parts of the study area to become critically stressed – hence act as fluid migration pathways – at different times. The migration models show that hydrocarbon migration pathways are strongly influenced by fault-zone properties, specifically the capillary entry pressure (CEP) along faults. The dip of the sediment layers also influences hydrocarbon leakage from the subsurface to the seabed. In general, the migration models show vertical hydrocarbon migration along faults coupled with lateral migration below the seal layers and between faults. Fluids migrate along faults with two patterns of flow based on the CEP values along the faults: 1) focused – fluids migrate as a linear pattern along faults when the capillary entry pressure along the fault is within the lower range of the “background” CEP values; 2) diffuse – fluids are guided by faults when the capillary entry pressure along the fault is within the higher range of the “background” CEP values.