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.
This study summarizes the efforts taken to provide reliable reservoir characterizations products to mitigate seismic interpretation challenges and delineation of the reservoirs. ADNOC has conducted seismic exploration activities to assess Miocene to Upper Cretaceous aged reservoirs in East Onshore Abu Dhabi. The Oligo-Miocene section comprises of interbedded salt (mainly halite), anhydrite, limestones and marls. Deposited in the foreland basin related to the Oman thrust-belt. Ranging in thickness from nearly 1.5 km in the depocenter to almost nil on the forebulge located to the west of the studied area. The well data based geological model suggests that initially porous rocks (presumably grain-supported carbonates) encompassed polyphase sulfate cementation during recurrent subaerial exposure in which pores and grains were recrystallized sometimes completely too massive, tight anhydrite beds. This heterogeneity of the complex shallow section showing high variation of velocity impact seismic imaging, and interpretation to model the stratigraphic/structural framework and link it with reservoir characterization. Hence, ADNOC decided to conduct a trial on state-of-art technique Litho-Petro-Elastic (LPE) AVA Inversion to mitigate the seismic interpretation challenges and delineate the reservoirs. The LPE AVA inversion provides a single-loop approach to reservoir characterization based on rock physics models and compaction trends, reducing the dependency on a detailed prior the low frequency model, Where the rock modelling and lithology classification are not separate steps but interact directly with the seismic AVO inversion for optimal estimates of lithologies and elastic properties. The LPE inversion scope requires seismic data conditioning such as CMP gathers de-noising, de-multiple, flattening and amplitude preservation, in addition to detailed log conditioning, petro-elastic and rock physics analysis to maximize the quality and value of the results. The study proved that the LPE AVA Inversion can be used to guide seismic interpreters in mapping the structural framework in challenging seismic data, as it managed to improve the prospect evaluation.
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