Case study: Successful application of 3D depth processing in Eromanga Basin, Queensland.

Abstract

The Eromanga Basin is a large Mesozoic sedimentary basin in Central and Northern Australia. It covers an area of 1,000,000 km2 and is the location of important onshore petroleum and natural gas deposits in Australia including the nation's largest onshore oilfield at Jackson. We performed a pre-stack time migration and later on a pre-stack depth migration sequence on a 300sqkm block in the basin with the objective to map out intricate formations for detection of areas with high potential for gas hydrocarbon. The basin contains an ancient meteorite crater (astrobleme) which distorts seismic images, and is made of sandstone, siltstone, mudstone, coal and shale. The oil and gas producing interval is located around 1,200m below the surface. Vibroseis seismic data was processed through an amplitude-preserved and surface-consistent sequence followed by offset-class regularisation and pre-stack migration. Refraction and residual statics were applied to correct for near surface time distortions. The time migrated data illustrates high signal-to-noise ratio with generally flat rock beddings with some noticeably complicated formations at known horizons, forming strong velocity boundaries. These formation oddities generally appear from Cadna-Owie ( C ) Formation and deeper; the Top C horizon is of significance for exploration as the top of the unit approximates a distinctive seismic reflector and is mappable over the entire basin. During depth-velocity modelling it was revealed that there are significant velocity variations (~500m/s) at Top C and other boundaries. Time imaging was not able to image these intervals properly. Depth migration improved seismic image quality especially in fault shadow zones, and areas with complex seismo-geological settings. The benefits of depth imaging on the Eromanga Basin data were demonstrated by significant uplift in structural conformity and increased confidence in interpretation results.