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SEISMIC EXPRESSIONS OF HYDROCARBON LEAKAGE

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).

b) 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:
• amplitude anomalies within the zone;
• time pull-up beneath the zone; and
• stack response degradation beneath the zone.

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.

Figure 2. Typical HRDZ seismic signature over the Tahbilk gas discovery in the southern Vulcan Sub-basin, Bonaparte Basin.

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.

d) 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:
• Hydrocarbon charge—has the individual trap or basin segment received charge or not, and if so, what is the likely charge?
• Top seal capacity—is there evidence that the top seal has a high or low capacity?
• Fault seal integrity—what is the likelihood that the charge in the trap has been preserved?

Figure 3. Maps of Australia’s North West Shelf and South-East margin, showing locations of seismic examples presented in this paper.

Table 1. Summary of examples presented in this study.