Depth conversion and seismic inversion of the Scarborough gas field

Abstract

The Scarborough gas resource is located in the Exmouth Plateau of the North Carnarvon Basin, Western Australia. The reservoir is a deep-water basin floor fan complex. The overburden has multiple layers of polygonal faults and shallow anomalies, which have caused distortions that have affected the depth conversion and inversion of the reservoir section from seismic. This paper shows how the application of advanced seismic processing, innovative seismic to well matching techniques and subsequent seismic inversion have resulted in a significant improvement when compared with seismic reservoir characterisation results which were obtained in 2010. The use of full waveform inversion (FWI) up to 30 Hz provided a better velocity model for seismic imaging and, in conjunction with well ties in depth and well constrained tomography were able to significantly reduce the depth errors with residuals below 1m while preserving the geologic information in the high frequency velocity model. Consideration of the bias versus variance trade off with respect to seismic inversion led to changes in the inversion workflow that resulted in improved depth conformance and frequencies exceeding 100 Hz in the reservoir section. This has led to a reduction in subsurface uncertainty of the geology of the gas resource. In addition to this, it was concluded that only inversion algorithms that contain an appropriate amount of complexity should be used and the number of input angle stacks to the inversion should be maximised where it is feasible to do so.