Eromanga Basin seismic stratigraphy and tectonic modelling – keys to exploration success in ATP 299P

Within the Eromanga Basin, stratigraphic variations in the Jurassic Birkhead Formation are recognizable on seismic sections as lateral character changes. Understanding these character changes and developing representative depositional and structural models results in lower risk and hence more favourable economics when exploring for Birkhead Formation oil.

Authority to Prospect (ATP) 299P is located in the Eromanga Basin, on the southeastern flank of the Cooper Basin. It is 120 km northwest of the Jackson Oil Field. Over 1.5 million barrels of oil have been produced from the permit, of which 500,000 barrels have been produced from the Birkhead Formation, highlighting it as an important exploration target.

By late July 1989 Cranstoun-1, a Birkhead Formation oil well, had produced more than the mapped proved and probable reserves and was still producing 203 barrels of clean oil per day. To investigate this anomaly the existing seismic data was reprocessed with additional 80 m uphole control and a seismic stratigraphic interpretation was carried out. The mapped data identified a Devonian wrench system overlain by a large low relief structure known as the Greater Cranstoun Structure (GCS). Endeavour-1 was drilled on this structure 3 km north of Cranstoun-1 and was completed as a Birkhead oil producer. Post drill analysis revealed that the interpreted top intra-Birkhead seal seismic marker crossed a geologically correlated boundary — from the top Birkhead ('A' sand) seal at the Endeavour wells switching down to the top 'B' sand seal at the Cranstoun wells. A further three wells were drilled based on the mapping of this seismic marker resulting in limited success. Detailed 2D seismic modelling, using the sonic logs from the five wells on the GCS as input, indicated that an increase in the frequency bandwidth of the seismic data was needed to resolve those geologically correlated boundaries within the Birkhead Formation which have an acoustic impedance contrast.

Future seismic acquisition and processing should use good statics together with optimum recording parameters (defined from the 2D modelling results) to produce the required seismic resolution for accurate stratigraphic interpretation and mapping.

Quantitative understanding of the limitations and possible resolution of seismic data is crucial to accurate seismic stratigraphy, which, combined with a valid tectonic model are keys to exploration success in the Birkhead Formation in ATP 299P and throughout the Eromanga Basin.