Seismic reservoir characterization of the Najmah Formation, Raudhatain field, north Kuwait

Abstract

The Late Jurassic Najmah Formation of Northeast Kuwait consists of a fractured tight carbonate reservoir and organic-rich kerogen layer. The average reservoir unit thickness is about 50 ft at a depth of 14000 ft. It has low porosity and ultra-low permeability. Development of the Najmah reservoir is contingent on recognizing naturally fractured areas. The reservoir section is immediately below thick layers of alternating salts and anhydrite, and the seismic data within the reservoir interval suffers from destructive interference of coherent noise. Coherent noise interference impacts the reliability of seismic attribute extractions for reservoir characterization. Attenuating this interference is vital to exploiting useful information from seismic data. This study investigates the coherent noise sources and offers a cascaded attenuation processing approach to alleviate the noise disturbance. The study reveals some noises related to near-surface condition and multipathing seismic energy above the reservoir layers. According to the model, the major generator of interbed multiple is the first anhydrite layer within the Gotnia Formation. Application of model-based interbed multiple attenuation reduces the noise interference and improves the imaging and characterization of the reservoir section. The multiple attenuated seismic data have better correlation with synthetics from all 10 control wells used in this study, leading to better seismic inversion accuracy. Seismic reservoir characterization is conducted by integrating full-azimuth P-wave seismic analysis, pre-stack simultaneous seismic inversion and rock physics modeling. Rock physics modeling suggests that the seismic variation is most affected by pore geometry. A low Vp/Vs ratio correlates with high porosity, low pore aspect ratio and high brittleness index. The higher organic richness of the kerogen layer correlates with lower acoustic impedance. Azimuthal seismic analysis depicts that multiple fracture sets with multiple orientations exist in the area. The fast interval velocity coincides with the direction of maximum horizontal stress. A combination of brittleness index and fast velocity azimuth is used to identify “sweet spots” for potential future Najmah reservoir development. The seismic analysis results are confirmed by image log interpretation and production test data. Most of the effective fractures coincide with the direction of maximum horizontal stress. The study results provide valuable insights into identifying and attenuating the coherent noises while delineating unconventional Jurassic reservoir potential in the study area. This research impacts not just the Najmah Formation, but deep unconventional reservoirs around the world that deal with similar problems.