Hydrocarbon leakage can be often be imaged successfully on seismic data, because it has resulted in the acoustic, mechanical or diagenetic alteration of the formations, particularly of those above charged reservoirs, with attendant seismic amplitude and/or velocity effects. Common types of seismic effects observed around offshore Australia include:
a) Prominent chimneys, caused by gas, and flat spots and zones of increased amplitude, produced by shallow, trapped gas. Gas chimneys can provide significant information, and have been studied extensively by some exploration companies. For example, Statoil uses gas chimney mapping to reduce the charge-related risk on undrilled structures. In conjunction with the geophysical company, de Groot - Bril Earth Sciences, Statoil recently developed the Chimney Cube, a method which uses neural networks to automatically detect chimneys on seismic data (Meldahl et al, 1999). The exploration value of this technique has been demonstrated in the North Sea (Heggland et al, 1999).
HRDZs (O’Brien and Woods, 1995; O’Brien et al, 1996) previously described
from the Vulcan Sub-basin, Timor Sea. HRDZs form when hydrocarbons leaking from
charged, fault-reactivated reservoirs migrate upwards and, upon entering a
shallower aquifer sand, are biodegraded. Biological oxidation North
West Shelf and Gippsland Basin: identification and interpretation of leaking
hydrocarbons of the hydrocarbons produces localised, intense carbonate
cementation within otherwise poorly cemented sands (Fig. 1). This cementation
produces sufficient acoustic impedance for a strong seismic response to result,
allowing the HRDZs to be mapped seismically. HRDZs can be identified by one or
more of three main seismic properties:
Figure 1. Schematic showing the formation of HRDZs in the Vulcan
Sub-basin. Hydrocarbons leaking along fault zones during the Neogene are oxidised by bacteria within the Grebe Formation aquifer; the releasedCO2 is incorporated into pervasive carbonate
An HRDZ over the Tahbilk gas accumulation is shown in Figure 2. This example is large and linear, displaying an amplitude anomaly near 700 ms. It has about 100 ms of pull-up and stack response degradation beneath the anomaly. Such strong anomalies usually show the three characteristics listed above. Mild zones of cementation may not exhibit degradation of the stack response, but typically show some combination of amplitude anomaly and pull-up. High resolution, low noise data, with true amplitude preservation, greatly increases the confidence of identifying subtle velocity anomalies.
HRDZs fall into two main categories based on their shape, namely; 1) linear anomalies caused by leakage along a fault; and 2) circular anomalies caused by point leakage, often at the intersection of two faults. Since a seismic line will usually intersect a linear anomaly obliquely, the seismic response may look similar to that of a circular anomaly. 3D seismic or close-spaced 2D are required to map the areal distribution of HRDZs with confidence. Long (1.5 to 5 km in length), fault-controlled HRDZs indicate that the underlying trap has leaked significantly, and may indeed be completely breached, whereas small, effectively point-source anomalies (0.2–1.5 km in length) suggest that only low amounts of leakage have taken place (O’Brien and Woods, 1995). Clearly, HRDZs provide a very useful mapping tool for evaluating risk, particularly in relation to charge and fault seal integrity.
c) High seismic amplitudes at, or near, the seafloor. In shallow strata, and at the sea floor, biological activity promoted and supported by migrating hydrocarbons can result in enhanced induration via carbonate cementation, and seafloor mounding. This induration can produce significant seismic amplitude enhancement, as well as producing a rugose seafloor topography which can produce migration ‘smiles’ over the data.
Pockmarks or other physical disturbances caused by gas escaping explosively at
the seafloor. For example, Heggland (1998) used a 3D data-set from the North Sea
to demonstrate three phases of hydrocarbon leakage in the region, expressed
respectively as buried craters near the sea bed, buried (possibly carbonate)
mounds and Pliocene pockmarks. The advent of 3D seismic data has allowed these
effects to be mapped accurately in three dimensions, providing a vastly improved
understanding of their geometry, the key controls on their distribution, and
their formation. An understanding of these seismic
effects is critical, as it can provide the explorer with powerful predictive
capabilities in relation to the assessment of risks such as:
of examples presented in this study.
The APPEA Journal 2000. © This collection APPEA Limited 2000. Authors retain © in respect of their own contribution.