Retrieving Amplitude Absorption From Gas Cloud Effect In Deep Water Play, Gulf Of Mottama, Offshore Myanmar

Abstract

The gas cloud effect impacted on marine seismic data is the classical problem had long been known in the seismic data processing. The presence of the gas cloud caused the seismic energy absorption in both amplitude and frequency as well as the subsurface geology geometry distortion. While the accurate velocity plays the important part of the image geometry the Quality Factor (Q) of the anomalous attenuation is the key to compensate the absorbed energy in both phase and amplitude terms. Utilizing variable depth towed streamer broadband technology, the seismic data was acquired in the deep water area located in Gulf of Mottama, Offshore Myanmar in 2016. The seismic data in the study area experiences significant gas cloud phenomenon such as push down effect, amplitude and phase distortion and high frequency drop which lead to high uncertainty in disclosing hydrocarbon reservoirs below the affected media. Based on the study, the amplitude footprint related to the geological media is found consistently in different vintages of seismic data in this area and also outside this survey area. The use of constant Q- compensation prior to seismic imaging doesn’t really optimum handling the issue and therefore a better Q-modeling approach is required. Other than accurate velocity model, high resolution Q-model is also required to obtain reliable QPSDM product. Demanding only on tomography to generate high resolution Q-model is quite difficult especially in complex attenuation with strong wavelet distortion. Centroid Frequency Shift (CFS) overcomes limitation of Q-tomography in fitting seismic wavelet to fix absorption model in more effective way so the Q-model can be more reliable for imaging. In viscoacoustic media, propagation of seismic waves is affected by frequency dependent absorption, causing severe amplitude decay and narrow down frequency bandwidth. Failure in compensating complex attenuation may downgrade amplitude and phase in migration result and hence inaccurate reservoir properties prediction (Best, at al., 1994). Amplitude dependent Q-model is often insufficient providing reliable Q-model since the approach ignores frequency dependency attenuation. CFS takes into account analytical process to adaptively fit amplitude spectrum of seismic wavelets before and after passing through viscoacoustic media. The corrected centroid frequency shift will be back-projected during ray tracing in Q-tomography to estimate attenuation distribution. The CFS based QPSDM technique has proven to be successfully used in deep water play, Gulf of Mottama, Offshore Myanmar. The outcome of Q model is geological reasonable and has better correlation with the seismic imaging velocity especially around anomalous absorption zones corresponding to the area with slow velocity anomaly. Integrating TTI velocity model with CFS based Q-model for imaging of complex attenuation area and their surrounding which was treated as a complex function of Q in QPSDM workflow, an anti-dissipation term can be included in the migration operator for better restoring the seismic images. End result shows the blurred amplitudes underneath the gas cloud and surrounding are restored with shaper wavelets, enhance reflection continuity at target horizons and reduce pseudo-sagging structure. The amplitude interpretation of target play beneath the gas cloud can be made with higher certainty. The technique also provides more reliable Common Image Gathers (CIGs) for further Quantitative Interpretation as it enables more accurate interpretation of the stratigraphy within the deep water hydrocarbon target zone.

AAPG Datapages/Search and Discovery Article #90331©2018 AAPG Asia Pacific Region GTW, Back to the Future – The Past and Future of Oil and Gas Production in the Asia Pacific Region, Bangkok, Thailand, September 26-27, 2018