Controlled-source electromagnetic ͑CSEM͒ field surveys offer a geophysical method to discriminate between high and low hydrocarbon saturations in a potential reservoir. However, the same geological processes that create the possible hydrocarbon reservoir may also create topography and nearsurface variations of resistivity ͑e.g., shallow gas or hydrates͒ that can complicate the interpretation of CSEM data. In this paper, we discuss the interpretation of such data over a thrust belt prospect in deepwater Sabah, Malaysia. We show that detailed modeling of the key scenarios can help us understand the contributions of topography, near-surface hydrates, and possible hydrocarbons at reservoir depth. Complexity at the surface and at depth requires a 3D electromagnetic modeling code that can handle realistic ten-million-cell models. This has been achieved by using an iterative solver based on a multigrid preconditioner, finite-difference approach with frequency-dependent grid adaptation.

We present a CSEM survey case in offshore Sarawak, Malaysia showing how CSEM can help reservoir imaging where seismic struggles due to poor seismic reflection. The Seismic data at the prospect depth shows poor reflection due to low saturated gas seepage The CSEM data was acquired in 2008 along two receiver lines assuring azimuth data coverage between the lines.. EMGS performed 2.5D inversion along both the tow lines and the result showed resistive anomalies in one of the structures in each lines. In 2009 Newfield drilled a well in one of the structure which resulted in a gas discovery. Pre-drill 2.5D inversion result matches well with the well log. In early 2012, Newfield drilled a second well a few Km off the CSEM line in the other structure, which was believed to be oil charged but was dry. CSEM inversion did not show any anomaly in that structure. Unconstrained anisotropic 3D inversion was performed in 2012 and the results were then correlated with the well logs. The result of 3D inversion matches very well with the well logs and also laterally with the structure.

The image of the target in this case study is affected by zones of shallow gas/high Earth attenuation in the overburden. The goal of the study is to determine whether the attenuation of the seismic amplitudes below these areas can be minimized through the application of advanced seismic acquisition designs, e.g. increased azimuth, long offset acquisition.

This paper will focus on a High-Pressure High-Temperature (HPHT) carbonate gas field in Central Luconia, Field X, offshore Sarawak, Malaysia. Discovery of field X was made based on a 2009 3D legacy Pre-Stack Time Migration (PreSTM) seismic data and an exploration well drilled in 2013. The main challenge for this field is to move from the exploration phase into the field development phase, without acquiring additional seismic or well data. One of the ways forward, is to extract new information via seismic data reprocessing. The reasons for this are the cost of seismic reprocessing is marginal compared to drilling an appraisal well and there is significant improvement in the seismic processing and seismic inversion technology over the last decade. Full Waveform Inversion (FWI), combined with other latest processing technology, are the key workflows in this seismic reprocessing that determines the quality of the results. A quantitative interpretation (QI) study was subsequently performed to conclude the seismic study and enhance reservoir understanding in preparation for field development planning.

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