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IDENTIFICATION
AND INTERPRETATION OF LEAKING R.
Cowley 1
and
G.W. O’Brien 2 The APPEA Journal, 40 (1), 121-150. Australian
Petroleum Production and Exploration Association Limited
ABSTRACT An
extensive volume of 3D seismic data over a number of oil and gas fields in
Australia’s North West Shelf and Gippsland Basin has been examined for
evidence of the effects of hydrocarbon migration and/or leakage. For Direct indicators of hydrocarbon migration and/or leakage are relatively rare throughout the basins studied, though the discoveries themselves characteristically show seismic anomalies attributable to hydrocarbon leakage. The nature and intensity of these hydrocarbon-related seismic effects do, however, vary dramatically between the fields. Over traps such as Skua, Swift, Tahbilk and Macedon, they are intense, whereas over others, for example Chinook and North Rankin, they are quite subtle. Hydrocarbon-related diagenetic zones (HRDZs), which had been identified previously above the reservoir zones of leaky traps within the Bonaparte Basin, have also been recognised within the Browse, Carnarvon, Otway and Gippsland Basins. HRDZs are the most common leakage indicators found and are identified easily via a combination of high seismic amplitudes through the affected zone, time pull-up and degraded stack response of underlying reflectors. In some cases (the Skua and Macedon Fields), the HRDZs actually define the extent of the accumulations at depth. Anomalous, subtle to strong, seismic amplitude anomalies are associated with the majority of the major fields within the Carnarvon Basin. The strength and location of the anomalies are related to a complex interplay between trap type (in particular four-way dip-closed versus fault dependent), top seal capacity, fault seal integrity, and charge history. In some areas within the Carnarvon, Browse and Bonaparte Basins, shallow amplitude anomalies can be related directly to gas chimneys emanating from the reservoir zone itself. In other instances, the continuous migration of gas from the reservoir has produced an assortment of pockmarks, mounds and amplitude anomalies on the present day sea floor, which all provide evidence of hydrocarbon seepage. In the Browse Basin, strong evidence has been found that many of the modern carbonate banks and reefs in the region were initially located over hydrocarbon seeps on the palaeo-seafloor. The
examples and processes presented demonstrate that the analysis of hydrocarbon
leakage indicators on seismic data can help to better understand exploration
risk and locate subtle hydrocarbon accumulations. In mature exploration
provinces, this methodology may lead to the identification of subtle
accumulations previously left undetected by more conventional methods. In
frontier regions, it can help to identify the presence of a viable petroleum
system, typically the principal exploration uncertainty in undrilled regions.
KEYWORDS Seismic
expression of hydrocarbon leakage, HRDZs, amplitude anomalies, gas chimneys,
North West Shelf, pockmarks.
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