IDENTIFICATION AND INTERPRETATION OF LEAKING HYDROCARBONS USING SEISMIC DATA:A COMPARATIVE MONTAGE OF EXAMPLES FROM THE MAJOR FIELDS IN AUSTRALIA'S NORTHWEST SHELF AND GIPPSLAND BASIN

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 comparative purposes, 2D and 3D data have also been studied over a number of adjacent traps, including dry traps and partially to completely breached accumulations. Fields and traps investigated include Bayu-Undan, Jabiru, Skua, Swift and Tahbilk in the Bonaparte Basin, Cornea in the Browse Basin, North Rankin, Chinook, Macedon, Enfield and Zeewulf in the Carnarvon Basin, and Kingfish in the Gippsland Basin. The principal goal of the study is to provide representative case studies from known fields so that, in undrilled regions, the exploration uncertainties associated with the issues of hydrocarbon charge and trap integrity might be reduced.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.

Structurally controlled hydrocarbon seeps on a glaciated continental margin, Hecate Strait, offshore British Columbia

“…accumulations, especially when observed with other direct hydrocarbon indicators such as flat spots, dim spots, low frequency shadow zones and gas chimneys (Cowley and O'Brien, 2000;Brown, 2004). At the study site, the alignment of bright spots with local structure, in addition to the observation of low frequency zones indicates they are the result of gas accumulations.…”

Fluid flow features in hydrocarbon plumbing systems: What do they tell us about the basin evolution?

“…These types of structures are often recognized as high-amplitude anomalies in the seismic data (e.g. Cowley and O'Brien, 2000;Cunningham and Lindholm, 2000;Bünz et al, 2003;Berndt et al, 2004;Davies and Cartwright, 2007;. Table 1 Case studies with varying descriptions of chimneys and pipes.…”

“…During the biodegradation biological oxidation of the hydrocarbons produces a strong cementation with high acoustic impedance which causes HRDZs to appear as high-amplitude anomalies associated with velocity pull-up and attenuation in the section below (e.g. Cowley and O'Brien, 2000;González et al, 2009). …”

“…Several components or elements have however been recognized as constituting whole or parts of a fluid flow conduit including: gas chimneys, pipes, fractures including hydro-fractures, tectonic induced fractures and micro-fractures, irreversible deformations of the strata (e.g. bending, folding and faulting), diatremes, calderas, downward tapering cones, dikes and low-angled sills, leakage zones and hydrocarbon related diagenetic zones (HRDZ) (Brown, 1990;Cowley and O'Brien, 2000;Hurst et al, 2003;Stewart and Davies, 2006;Cartwright et al, 2007;Løseth et al, 2009). Apparent fluid flow conduits may in reality be seismic artifacts related to either migration anomalies, scattering artifacts, lateral velocity anomalies, or attenuation artifacts related to shallower high-amplitude features (e.g.…”

The CO2CRC Otway Project: quantitative risk assessment with newly acquired data and updated interpretation.

“…Leakage via fault reactivation as a result of enhanced natural or anthropogenic mechanisms is handled separately in the pages below. An evaluation of 3D seismic was undertaken to assess if there was any evidence that could be attributed to previous gas migration, including hydrocarbon related diagenetic zones (HRDZs), or gas chimneys, such as those described from elsewhere in the basin by Cowley and O'Brien (2000). No features relating to gas migration were detected above the Naylor structure or in the region of the Naylor structure.…”