Large Scale Seismic Modelling of the Pre-Carbonate Geology, Offshore Malaysia, a Multidisciplinary Approach - “Running the Seismic Method Backwards"

Abstract

Abstract Despite proven analogues elsewhere in Malaysia, pre-carbonate hydrocarbon targets offshore Sarawak have met with limited drilling success. This has been mainly attributed to the poor seismic imaging beneath complex velocity carbonate reefs and platforms with low signal-to-noise ratio and inadequate illumination. Seismic imaging is challenging due to inconsistent illumination, multiple energy contamination and complex wave propagation. A collaborative study was undertaken to model these challenges and recommend optimum acquisition and signal processing solutions to produce a step change in seismic imaging of the pre-carbonate targets. The study involved building a detailed anisotropic 3D earth model using borehole and seismic data covering more than 600 km2 area to create a robust synthetic seismic dataset. Geological and petrophysical review preceded seismic interpretation and simultaneous AVO inversion results were used to populate a fine scale model. Testing and validation of the model was performed on a data subset to ensured the kinematics and amplitudes were consistent with the actual seismic response. The model was then used to create over 15 billion prestack traces using an anisotropic finite difference wave propagation modelling algorithm. From this data, different survey designs were extracted including; narrow-azimuth (NAZ), multi-azimuth (MAZ), wide-azimuth (WAZ) and full-azimuth (FAZ) circular shooting streamer geometries. These datasets were processed and analyzed to test optimal demultiple and imaging approaches and to understand the relative benefits of different acquisition geometries. The results indicated that acquiring azimuthally rich data improves illumination of the pre-carbonate section. The demultiple testing successfully demonstrates the workflow needed to attenuate both short and long period multiples. Reverse Time Migration (RTM) produced an improved image compared to Kirchhoff pre stack depth migration. This case study demonstrates that large scale 3D finite difference seismic modelling, aimed at addressing specific geological challenges, is an effective tool in understanding the optimum acquisition design and processing workflow prior to beginning a new seismic acquisition. Based on the findings of this collaborative study, PETRONAS has now acquired a new seismic data set in the area targeting pre-carbonate objectives using a 3D full-azimuth circular acquisition.