Introduction

[ Up ] [ Introduction ] [ Seismic Expressions ] [ Results ] [ Bayu-Undan ] [ Tahbilk ] [ Skua ] [ Jabiru ] [ Cornea ] [ Anomaly1 ] [ Anomaly2 ] [ Anomaly3 ] [ North Rankin ] [ Enfield ] [ Macedon ] [ Chinook ] [ Carnarvon ] [ Novara NW ] [ Zeewulf ] [ Jupiter ] [ Scarborough ] [ Otway ] [ Kingfish ] [ Exploration ] [ Skua Detail ] [ Macedon Detail ] [ Implications ] [ Summary ] [ Acknowledgements ] [ References ] [ Authors ] [ Update ]

INTRODUCTION

Sub-aerial and seafloor oil and gas seeps are commonly present within prolific hydrocarbon-producing basins. The presence of seeps may not necessarily indicate the presence nearby of a deeper, commercial accumulation, but it does confirm the presence of a petroleum system within a basin, with associated reduction in exploration uncertainty in relation to source. In fact, the majority of major hydrocarbon accumulations world-wide have experienced some hydrocarbon leakage, either as a result of fault seal failure (often due to tectonic reactivation) or because of insufficient seal capacity in the top seal (which is essentially facies-controlled). Such processes may or may not result in seepage at the surface. Analysis of hydrocarbon seepage and its related effects plays an important exploration role in many parts of the world, including both onshore and offshore areas (Donovan, 1974; Dalziel and Donovan, 1980; Lilburn and Al Shaieb, 1983, 1984; Schumacher, 1996; Saunders et al, 1999). In particular, seeps have been investigated intensively in the US Gulf Coast, the Irish Sea and the North Sea (Aharon et al, 1989, 1990, 1991a and b, 1992a and b; Anderson et al, 1992; Brooks et al, 1990; Graber et al, 1990; Hovland et al, 1987, 1994; Roberts et al, 1987, 1989a, b, and c, 1990, 1991; Sassen et al, 1991a and b, 1993).

Within the context of offshore exploration, the tools used to investigate sub-surface hydrocarbon leakage (i.e. hydrocarbons migrating either from reservoirs or from source horizons) and seepage (either sub-aerial or sub-marine) fall into two broad categories, namely:
1. seismic data; and
2. other data types.

Seismic data provide virtually the only means of mapping hydrocarbon leakage in the sub-surface. They can be used, potentially, to map the route taken by hydrocarbons, all the way from the seep at the surface down to the leaking reservoir or source horizon. Seismic data also have the advantage of being the principal petroleum exploration tool, thereby providing high quality data coverage over large areas and at high density (particularly with 3D data).

Information gained from seismic data can be complemented or enhanced by, and integrated with, data from a number of other techniques which can detect, map and to a greater or lesser extent, characterise hydrocarbon seepage in offshore basins. These techniques also fall into two broad categories, namely those which characterise the physical and/or chemical composition of the seeps, either at, or close to, their source on the seafloor and those which detect the hydrocarbon slicks that the seafloor seepage eventually produces at the sea surface. The former category includes ship-based techniques such as side-scan sonar and swath bathymetry, which can map the physical geometry of gas-related pockmarks on the seafloor, and Water Column Geochemical Sniffer (WaSi). WaSi provides detailed, real-time mapping of the molecular hydrocarbon concentrations and compositions of the seep-related hydrocarbons within the water column, and also allows the subsequent isotopic characterisation of the hydrocarbons within the seeps, which may provide information on source maturity and type etc. The latter category includes tools such as satellite-based Synthetic Aperture Radar (SAR), which indirectly detects oil seeps via the calming effect that the hydrocarbons have on the roughness of the sea’s surface, and Airborne Laser Fluoresensor (ALF). ALF detects aromatic hydrocarbons by firing an aircraft-mounted laser vertically at the sea surface.

In this paper, an extensive volume of 3D seismic data over a number of offshore oil and gas fields in Australia’s North West Shelf and Gippsland Basin are examined for evidence of the effects of hydrocarbon migration and/or leakage. For comparative purposes, seismic data (both 2D and 3D) are presented 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. We catalogue a number of seepage indicators found on seismic data and discuss their exploration value. The common indicators of hydrocarbon seepage found include hydrocarbon related diagenetic zones (HRDZs), similar in appearance to those described by O’Brien and Woods (1995), and gas chimneys. The HRDZs over the Skua oil field and the Macedon gas field are investigated in detail.

The principal goal is to provide a montage of representative examples of the seismic expression of hydrocarbon migration and leakage over known fields, and thereby increase the utility of seismic data in evaluating hydrocarbon leakage. Comparison of these examples with seismic data from frontier basins or undrilled prospects allows an improved evaluation of the exploration risks associated with the issues of hydrocarbon charge and seal integrity.

Where available, complementary data and interpretations from seepage-focussed technologies such as SAR, ALF and WaSi are incorporated into the review.