oa Prospect Identification using AVO Inversion and Lithology Prediction

Identifying prospects within a large study area is traditionally a time consuming process. Schlumberger Reservoir Seismic Services have produced an efficient prospect screening tool in the form of a well log calibrated gas sand probability cube derived from AVO inversion attributes. Pre-stack simultaneous AVO inversion was performed on a 5048 sq/km multi-client seismic survey to compute seismic elastic attributes (acoustic impedance, Poisson’s ratio and density) which can be used to generate lithology and fluid properties. The survey was acquired in 2008 and is located offshore between the giant Io/Jansz and Scarborough gas fields on Australia’s NW shelf, with potential gas reservoir’s situated at multiple levels within a complex geological setting.

Well log data from within the survey area were calibrated to the seismic, from which wavelets were estimated for each angle stack using the angle-dependant reflectivity logs. Low frequency models were derived using log data from wells both inside and outside the survey extents, in conjunction with seismic velocities and guided by interpreted horizons. Elastic attribute logs were generated from the wells and used to produce non-Gaussian lithology probability density functions (PDF), which in turn are utilised to generate a 3D gas sand probability cube from acoustic impedance and Poisson's ratio volumes. The gas sand probability cube can be loaded into a 3D visualisation software package where techniques such as volume rendering and opacity are applied to quickly and efficiently screen for potential prospects from which gas sand geobodies can be extracted for volumetric analysis and further reservoir characterisation.