Quantitative Interpretation of Frequency Decomposition Blends Using Forward Modelling: Thebe Discovery, NW Australia

Geological expression techniques including frequency decomposition are very powerful tools in understanding and risking reservoirs. A cognitive approach in visualising responses of different band-limited frequency volumes is through red-green-blue (RGB) colour blending. The non-unique colour responses are subject to a variety of complex interference patterns that are related to a number of geological factors: bed thickness, lithology, porosity, fluid content. This study presents a joint seismic forward modelling and frequency decomposition workflow on the Thebe gas discovery, offshore NW Australia to isolate and quantify the effects of hydrocarbon saturation on colour blends. Observations from real life blends are compared to equivalent synthetic models created through comprehensive rock physics modelling at two well locations; Thebe-1 and Thebe-2. Primary gas-bearing sand units within the Triassic Mungaroo Formation have been identified and are associated with a unique combination of high intensity frequency responses surrounded by a low frequency zone related to a pronounced gas-water contact. Sensitivity analysis through forward modelling has confirmed that fluid effects play a significant role in the frequency responses, and yield unique interference patterns in gas saturated sands. Frequency responses were used to establish spatial distribution of these gas-bearing sands to identify locations of ‘sweet-spots’ and de-risk development plans.