Karstification is one of the major culprits for drilling issues (e.g., mud loss/blowout/wellbore collapse) in carbonate fields. Hence, it is crucial to precisely identify and predicate paleokarst distribution before formalizing a drilling plan. In this study, an isolated Miocene carbonate buildup in the offshore of Malaysia which is covered by a 3D marine seismic survey is selected for Karst mapping. Geological study shows that the carbonate buildup is bounded by two boundary faults and carbonate bodies were altered by multiple episodes of faulting and subaerial exposure related karstification. Extensive karstification is also evident by total mud losses in wells penetrating carbonate succession. Two in-house technologies: Advanced high-resolution Spectral Decomposition (ASD) and Hessian Filter-enhanced Variance (HFV) were used to illustrate the discontinuities within carbonate bodies and map Karst distribution. Well calibration with ASD and HFV results demonstrates that 1) Carbonate buildup can be clearly distinguished from surrounding clastics by specific (bright orange-colored) boundaries against dull colored clastics in RGB blending images; 2) Karst-related cavernous systems can be depicted by dendritic pattern variation in ASD images and HFV anomalies; 3) Linear discontinuities caused by fractures/faults are also highlighted by the results of ASD and HFV. It can be concluded that the combination of discontinuities revealed by ASD and HFV and with appropriate well and core calibration is a reliable way to predicate karstified reservoirs within the offshore carbonate buildup.
Imaging complex subsurface like the gas clouds has always been challenging. Gas clouds are gas accumulation, trapped as an overburden that lowers P-wave velocities (Vp) and frequencies, disrupting transmitted energy and obscures events beneath it. Attenuation of seismic amplitude due to multiple scattering in thin layering of heterogeneous media was first reported by Anstey (1971). A nonlinear full waveform redatuming method proposed by Ghazali (2011) utilized multiple scattering phenomena which described the overburden as complex scattering and translated it to a transmission correction operator. Since it is a nonlinear full waveform inversion (FWI) method, it is important to characterize the rock properties in the gas clouds to identify factors that caused the formation of shallow overburden and help to constraint non-linear inversion results.
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