Hydrocarbon indicators in the Macedon‐Pyrenees Field, Western Australia

Abstract

Summary There are examples of hydrocarbon accumulations in the Exmouth sub-basin that exhibit anomalous amplitude and AVO behaviour which are related to the presence of hydrocarbons. These include the Scarborough gas field, the Jupiter and Vinck gas discoveries, and the Macedon-Pyrenees oil and gas field. In these cases, the pre-drill recognition of anomalous amplitude behaviour aided the decisions to drill the discovery wells. In the case of the Macedon-Pyrenees oil and gas field, high seismic amplitudes caused by gas were recognised pre-drill within the Windalia Radiolarite unit. This unit overlies the Barrow Group reservoir section at the Macedon-Pyrenees field. The mapping of gas-related seismic 'brightening' of the Windalia Radiolarite unit provided an independent check on the depth conversion of the structure and was a key factor in promoting the structure as a drillable prospect. Since drilling the discovery well, seismic ‘brightening’ of the Windalia Radiolarite has been recognised over a number of other fields. This fact provides an indication that valid structural closure exists at the Barrow Group, and indicates the possibility of the presence of hydrocarbons. Further seismic analysis has concentrated on the recognition of gas-related brightening and amplitude variation with offset (AVO) behaviour of the Barrow Group reservoir sands. This work has been carried out to aid exploration in other parts of the WA-155-P exploration permit and in surrounding areas of the Barrow and Exmouth sub-basins. The results of an acoustic impedance study utilising well data have shown that the Macedon sands sub-unit of the Barrow Group should brighten significantly when charged with gas. AVO modelling using recorded P-wave and S-wave data has shown a change in AVO behaviour between Macedon sands filled with gas, and similar sands filled with brine. Detailed reprocessing of seismic data across the Macedon-Pyrenees field has been carried out to see whether the modelled behaviour of gas-filled Macedon sands could be recognised on the seismic data. Seismic data quality has been improved significantly through reprocessing and a strong relationship can now be seen between seismic brightening and gas-filled Macedon sands.