Model-based interbed multiple attenuation: a case study of Najmah reservoir, North Kuwait Jurassic

Abstract

The Late Jurassic Najmah Formation of Northeast Kuwait consists of a fractured tight carbonate reservoir and organic-rich kerogen layer (Figure 1). Development of the Najmah reservoir is contingent on recognizing naturally fractured areas. Prior to seismic reservoir characterization, it is essential to ensure that the data quality is reliable before extracting any information from the seismic data. Noise contamination can obscure the reflections of interest, leading to inaccurate interpretation and the need for further data analysis for reservoir delineation. Data acquired in the study area indicates that P-wave seismic data within the reservoir interval suffers from the interference of coherent noise related to the interbed multiple. The interbed multiple is a serious problem that is common in the land seismic datasets in the Middle East (Al-Khaled et al., 2008; Al-Nahhas et al., 2008; El-Emam et al., 2011, 2005; Lesnikov and Owusu, 2011; Ras et al., 2012; Sonika et al., 2012; Wu et al. 2011). This study utilized a 9 mile2 (23.3 km2) 3D full azimuth P-wave seismic data set provided by Kuwait Oil Company (KOC) to the Reservoir Characterization Project (RCP) at the Colorado School of Mines. The seismic data was a preliminary result of pre-stack time migration from continuing seismic data processing. Short and long period demultiple and interbed demultiples were applied during the original data processing. A synthetic seismogram of well B portrays the reservoir interval with greater amplitude than the seismic data (Figure 2). This anomaly is consistently observed for all seismic well ties of ten available wells within the 3D seismic data. Furthermore, inside-outside corridor stack analysis of zero-offset VSP (Vertical Seismic Profile) depicts similar amplitude difference that indicates the presence of noise interference from multipathing of seismic energy above the reservoir interval. Evidence of the presence of multiples can be observed from the raw stack of down-going wavefields of the zero-offset VSP. Interbed multiple effects on the seismic data can be seen as a periodic repetition of reflection events, either individual events or interference with primaries. In this case, the multiples seem to interfere with the primary reflection. This study reveals interbed multiples related to the first anhydrite layer within the Gotnia Formation above the reservoir layers are the primary cause of the interference. Applying model-based interbed multiple attenuation reduces the noise interference and improves the imaging of the reservoir section. The multiple-attenuated seismic data have a better correlation with synthetics derived from ten control wells used in this study, leading to better seismic inversion accuracy and reservoir characterization.